FIDO Security Reference

The user registers a FIDO authentication key with a fraudulent web site instead of the genuine Relying Party.

Consequences: The fraudulent site may convince the user to disclose a set of non-FIDO credentials sufficient to allow the attacker to register a FIDO Authenticator under its own control, at the genuine Relying Party, on the users' behalf, violating [SG-1] Strong User Authentication.

Mitigations: Disclosure of non-FIDO credentials is outside of the scope of the FIDO security measures, but Relying Parties should be aware that the initial strength of an authentication key is no better than the identity-proofing applied as part of the registration process.

T-1.1.2	Homograph Mis-Authentication	Violates
AC3	The user accidentally browses to a fraudulent web site. The attacker tries to act as man-in-the-middle (MITM) and requests the user to authenticate. In the case of username/password based authentication this is a typical phishing attack. Consequences: The FIDO subsystem will determine that either (a) no FIDO authenticator has been registered with the fraudulent site or (b) it will use the FIDO Uauth key registered to the fraudulent site - which is different from the Uauth key for the relying party's site. Mitigations: FIDO inherently ties keys to the relying party (formally identified by the AppID, and authenticated by TLS and the CA infrastructure).	SG-1, SG-4

T-1.1.2

Homograph Mis-Authentication

Violates

The user accidentally browses to a fraudulent web site. The attacker tries to act as man-in-the-middle (MITM) and requests the user to authenticate. In the case of username/password based authentication this is a typical phishing attack.

Consequences: The FIDO subsystem will determine that either (a) no FIDO authenticator has been registered with the fraudulent site or (b) it will use the FIDO Uauth key registered to the fraudulent site - which is different from the Uauth key for the relying party's site.

Mitigations: FIDO inherently ties keys to the relying party (formally identified by the AppID, and authenticated by TLS and the CA infrastructure).

SG-1, SG-4

7.1.2 Threats to the User Device, FIDO Client and Relying Party Client Applications

T-1.2.1	FIDO Client Corrpution	Violates
AC3	Attacker gains ability to execute code in the security context of the FIDO Client. Consequences: Violation of [SA-4]. Mitigations: When the operating environment on the FIDO user device allows, the FIDO Client should operate in a privileged and isolated context under [SA-2] to protect itself from malicious modification by anything outside of the Trusted Computing Base.	SA-4

T-1.2.1

FIDO Client Corrpution

Violates

Attacker gains ability to execute code in the security context of the FIDO Client.

Consequences: Violation of [SA-4].

Mitigations: When the operating environment on the FIDO user device allows, the FIDO Client should operate in a privileged and isolated context under [SA-2] to protect itself from malicious modification by anything outside of the Trusted Computing Base.

T-1.2.2	Logical/Physical User Device Attack	Violates
AC3 / AC5	Attacker gains physical access to the FIDO user device but not the FIDO Authenticator. Consequences: Possible violation of [SA-4] by installing malicious software or otherwise tampering with the FIDO user device. Mitigations: [SM-1] Key Protection prevents the disclosure of authentication keys or other assets during a transient compromise of the FIDO user device. A persistent compromise of the FIDO user device can lead to a violation of [SA-4] unless additional protection measures outside the scope of FIDO are applied to the FIDO user device. (e.g. whole disk encryption and boot-chain integrity).	SA-4

T-1.2.2

Logical/Physical User Device Attack

Violates

AC3 / AC5

Attacker gains physical access to the FIDO user device but not the FIDO Authenticator.

Consequences: Possible violation of [SA-4] by installing malicious software or otherwise tampering with the FIDO user device.

Mitigations: [SM-1] Key Protection prevents the disclosure of authentication keys or other assets during a transient compromise of the FIDO user device.

A persistent compromise of the FIDO user device can lead to a violation of [SA-4] unless additional protection measures outside the scope of FIDO are applied to the FIDO user device. (e.g. whole disk encryption and boot-chain integrity).

T-1.2.3	User Device Account Access	Violates
AC3 / AC4	Attacker gains access to a user's login credentials on the FIDO user device. Consequences: Authenticators might be remotely abused, or weakly-verifying authenticators might be locally abused, violating [SG-1] Strong User Authentication and [SG-13] Transaction Non-Repudiation. Possible violation of [SA-4] by the installation of malicious software. Mitigations: Relying Parties can use [SM-9] Authenticator Certification and [SM-3] Authenticator Class Attestation to determine the nature of authenticators and not rely on weak, or weakly-verifying authenticators for high value operations.	SG-1, SG-13; SA-4

T-1.2.3

User Device Account Access

Violates

AC3 / AC4

Attacker gains access to a user's login credentials on the FIDO user device.

Consequences: Authenticators might be remotely abused, or weakly-verifying authenticators might be locally abused, violating [SG-1] Strong User Authentication and [SG-13] Transaction Non-Repudiation.

Possible violation of [SA-4] by the installation of malicious software.

Mitigations: Relying Parties can use [SM-9] Authenticator Certification and [SM-3] Authenticator Class Attestation to determine the nature of authenticators and not rely on weak, or weakly-verifying authenticators for high value operations.

SG-1, SG-13; SA-4

T-1.2.4	App Server Verification Error	Violates
AC3	A client application fails to properly validate the remote sever identity, accepts forged or stolen credentials for a remote server, or allows weak or missing cryptographic protections for the secure channel. Consequences: An active network adversary can modify the Relying Party's authenticator policy and downgrade the client's choice of authenticator to make it easier to attack. An active network adversary can intercept or view FIDO messages intended for the Relying Party. It may be able to use this ability to violate [SG-12] Parallel Session Resistance, [SG-11] Forgery Resistance or [SG-13] Forwarding Resistance. Mitigations: The server can verify [SM-8] Protocol Nonces to detect replayed messages and protect from an adversary that can read but not modify traffic in a secure channel. The server can mandate a channel with strong cryptographic protections to prevent message forgery and can verify a [SM-12] Channel Binding to detect forwarded messages.	SG-11, SG-12, SG-13

T-1.2.4

App Server Verification Error

Violates

A client application fails to properly validate the remote sever identity, accepts forged or stolen credentials for a remote server, or allows weak or missing cryptographic protections for the secure channel.

Consequences: An active network adversary can modify the Relying Party's authenticator policy and downgrade the client's choice of authenticator to make it easier to attack.

An active network adversary can intercept or view FIDO messages intended for the Relying Party. It may be able to use this ability to violate [SG-12] Parallel Session Resistance, [SG-11] Forgery Resistance or [SG-13] Forwarding Resistance.

Mitigations: The server can verify [SM-8] Protocol Nonces to detect replayed messages and protect from an adversary that can read but not modify traffic in a secure channel.

The server can mandate a channel with strong cryptographic protections to prevent message forgery and can verify a [SM-12] Channel Binding to detect forwarded messages.

T-1.2.5	RP App Corruption	Violates
	An attacker is able to obtain malicious execution in the security context of the Relying Party client application (e.g. via Cross-Site Scripting (XSS)) or abuse the secure channel or session identifier after the user has successfully authenticated. This is a client side attack. Consequences: The attacker is able to control the users' session, violating [SG-14] Transaction Non-Repudiation. Mitigations: The server can employ [SM-10] Transaction Confirmation to gain additional assurance for high value operations.	SG-14

T-1.2.5

RP App Corruption

Violates

An attacker is able to obtain malicious execution in the security context of the Relying Party client application (e.g. via Cross-Site Scripting (XSS)) or abuse the secure channel or session identifier after the user has successfully authenticated. This is a client side attack.

Consequences: The attacker is able to control the users' session, violating [SG-14] Transaction Non-Repudiation.

Mitigations: The server can employ [SM-10] Transaction Confirmation to gain additional assurance for high value operations.

SG-14

T-1.2.6	Fingerprinting Authenticators	Violates
	A remote adversary is able to uniquely identify a FIDO user device using the fingerprint of discoverable configuration of its FIDO Authenticators. Consequences: The exposed information violates [SG-8] Limited PII, allowing an adversary to violate [SG-7] User Consent by strongly identfying the user without their knowledge and [SG-4] Unlinkablity by sharing that fingerprint. Mitigations: [SM-3] Authenticator Class Attestation ensures that the fingerprint of an Authenticator will not be unique. For web browsing situations where this threat is most prominent, user agents may provide additional user controls around the discoverability of FIDO Authenticators.	SG-4, SG7, SG-8

T-1.2.6

Fingerprinting Authenticators

Violates

A remote adversary is able to uniquely identify a FIDO user device using the fingerprint of discoverable configuration of its FIDO Authenticators.

Consequences: The exposed information violates [SG-8] Limited PII, allowing an adversary to violate [SG-7] User Consent by strongly identfying the user without their knowledge and [SG-4] Unlinkablity by sharing that fingerprint.

Mitigations: [SM-3] Authenticator Class Attestation ensures that the fingerprint of an Authenticator will not be unique.

For web browsing situations where this threat is most prominent, user agents may provide additional user controls around the discoverability of FIDO Authenticators.

SG-4, SG7, SG-8

T-1.2.7	App to FIDO Client full MITM attack	Violates
AC3	Malicious software on the FIDO user device is able to read, tamper with, or spoof the endpoint of inter-process communication channels between the FIDO Client and browser or Relying Party application. Consequences: Adversary is able to subvert [SA-2]. Mitigations: On platforms where [SA-2] is not strong the security of the system may depend on preventing malicious applications from being loaded onto the FIDO user device. Such protections, e.g. app store policing, are outside the scope of FIDO. When using [SM-10] Transaction Confirmation, the user will be presented with the relevant AppID and transaction text and will be able to evaluate whether or not to consent to the transaction.	SA-2

T-1.2.7

App to FIDO Client full MITM attack

Violates

Malicious software on the FIDO user device is able to read, tamper with, or spoof the endpoint of inter-process communication channels between the FIDO Client and browser or Relying Party application.

Consequences: Adversary is able to subvert [SA-2].

Mitigations: On platforms where [SA-2] is not strong the security of the system may depend on preventing malicious applications from being loaded onto the FIDO user device. Such protections, e.g. app store policing, are outside the scope of FIDO.

When using [SM-10] Transaction Confirmation, the user will be presented with the relevant AppID and transaction text and will be able to evaluate whether or not to consent to the transaction.

SA-2

T-1.2.8	Authenticator to App Read-Only MITM attack	Violates
AC3	An adversary is able to obtain an authenticator's signed protocol response message. Consequences: The attacker attempts to replay the message to authenticate as the user, violating [SG-1] Strong User Authentication, [SG-13] Forwarding Resistance and [SG-12] Parallel Session Resistance. Mitigations: The server can use [SM-8] Protocol Nonces to detect replay of messages and verify [SM-11] Round Trip Integrity to detect modified messages.	SG-1, SG-12, SG-13

T-1.2.8

Authenticator to App Read-Only MITM attack

Violates

An adversary is able to obtain an authenticator's signed protocol response message.

Consequences: The attacker attempts to replay the message to authenticate as the user, violating [SG-1] Strong User Authentication, [SG-13] Forwarding Resistance and [SG-12] Parallel Session Resistance.

Mitigations: The server can use [SM-8] Protocol Nonces to detect replay of messages and verify [SM-11] Round Trip Integrity to detect modified messages.

SG-1, SG-12, SG-13

T-1.2.9	Malicious App	Violates
AC3	A user installs an application that represents itself as being associated with to one Relying Party application but actually initiates a protocol conversation with a different Relying Party and attempts to abuse previously registered authentication keys at that Relying Party. Consequences: Adversary is able to violate [SG-7] User Consent by misrepresenting the target of authentication. Other consequences equivalent to [T-1.2.5] Mitigations: If a [SM-5] Transaction Confirmation Display is present, the user may be able to verify the true target of an operation. If the malicious application attempts to communicate directly with an Authenticator that uses [SM-13] KeyHandleAccessToken, it should not be able to access keys registered by other FIDO Clients. If the operating environment on the FIDO user device supports it, the FIDO client may be able to determine the application's identity and verify if it is authorized to target that Relying Party using a [SM-14] AppID Separation.	SG-7

T-1.2.9

Malicious App

Violates

A user installs an application that represents itself as being associated with to one Relying Party application but actually initiates a protocol conversation with a different Relying Party and attempts to abuse previously registered authentication keys at that Relying Party.

Consequences: Adversary is able to violate [SG-7] User Consent by misrepresenting the target of authentication.

Other consequences equivalent to [T-1.2.5]

Mitigations: If a [SM-5] Transaction Confirmation Display is present, the user may be able to verify the true target of an operation.

If the malicious application attempts to communicate directly with an Authenticator that uses [SM-13] KeyHandleAccessToken, it should not be able to access keys registered by other FIDO Clients.

If the operating environment on the FIDO user device supports it, the FIDO client may be able to determine the application's identity and verify if it is authorized to target that Relying Party using a [SM-14] AppID Separation.

SG-7

T-1.2.10	Phishing Attack	Violates
AC2	A Phisher convinces the user to enter his PIN used for user verification into an application / web site disclosing the PIN to the Phisher. In the traditional username/password world this enables the attacker to successfully impersonate the user (to the relying party). Consequences: None as the phisher additionally would need access to the Authenticator in order to pass user verification [SM-1]. In FIDO, the user verification PIN (if user verification is done via PIN) is not known to the relying party and hence isn't sufficient for user impersonation. If user verification is done using an alternative user verification method, this applies accordingly. Mitigations: In FIDO, the Uauth.priv key is used to sign a relying party supplied challenge. without (use) access to that key, no impersonation is possible.	SG-1

T-1.2.10

Phishing Attack

Violates

A Phisher convinces the user to enter his PIN used for user verification into an application / web site disclosing the PIN to the Phisher. In the traditional username/password world this enables the attacker to successfully impersonate the user (to the relying party).

Consequences: None as the phisher additionally would need access to the Authenticator in order to pass user verification [SM-1]. In FIDO, the user verification PIN (if user verification is done via PIN) is not known to the relying party and hence isn't sufficient for user impersonation. If user verification is done using an alternative user verification method, this applies accordingly.

Mitigations: In FIDO, the Uauth.priv key is used to sign a relying party supplied challenge. without (use) access to that key, no impersonation is possible.

T-1.2.11	Malicious FIDO Client	Violates
AC3	Attacker convinces users to install and use a malicious FIDO Client. Consequences: Violation of [SA-4]. The malicious FIDO Client could trigger the use of any Uauth key in the context of the related AppID. User Verification / User Presence Check will be performed by the Authenticator as indicated. Mitigations: Mitigating malicious software installation is outside the scope of FIDO. If an authenticator implements [SM-1] Key Protection, the user may be able to recover full control of their registered authentication keys by removing the malicious software from their user device. If it is a bound authenticator, then [SM-13] might help distinguishing the original FIDO Client (i.e. the one used at registration time) from the malicious one. When using [SM-10] Transaction Confirmation, the user sees the claimed AppIDs and transaction text and can decide to accept or reject the action. Platforms implementing FIDO can filter communication to FIDO Authenticators. Depending on the platform such filter mechanisms may limit access to white-listed Apps.	SA-4

T-1.2.11

Malicious FIDO Client

Violates

Attacker convinces users to install and use a malicious FIDO Client.

Consequences: Violation of [SA-4]. The malicious FIDO Client could trigger the use of any Uauth key in the context of the related AppID. User Verification / User Presence Check will be performed by the Authenticator as indicated.

Mitigations: Mitigating malicious software installation is outside the scope of FIDO.

If an authenticator implements [SM-1] Key Protection, the user may be able to recover full control of their registered authentication keys by removing the malicious software from their user device.

If it is a bound authenticator, then [SM-13] might help distinguishing the original FIDO Client (i.e. the one used at registration time) from the malicious one.

When using [SM-10] Transaction Confirmation, the user sees the claimed AppIDs and transaction text and can decide to accept or reject the action.

Platforms implementing FIDO can filter communication to FIDO Authenticators. Depending on the platform such filter mechanisms may limit access to white-listed Apps.

7.1.3 Threat to the communication between Client and FIDO Authenticator

T-1.3.1	Malicious App with direct communication access to FIDO Authenticator	Violates
AC3	Attacker convinces users to install and use a malicious App that gained direct communication access to a FIDO Authenticator. Consequences: Violation of [SA-4]. The malicious App could trigger the use of any Uauth key in the context of the AppID (related to that key). User Verification / User Presence Check will be performed by the Authenticator as indicated. Mitigations: Mitigating malicious software installation is outside the scope of FIDO. If an authenticator implements [SM-1] Key Protection, the user may be able to recover full control of their registered authentication keys by removing the malicious software from their user device. If it is a bound authenticator, then [SM-13] might help distinguishing the original FIDO Client (i.e. the one used at registration time) from the malicious one. When using [SM-10] Transaction Confirmation, the user sees the claimed AppIDs and transaction text and can decide to accept or reject the action. Platforms implementing FIDO can filter communication to FIDO Authenticators. Depending on the platform such filter mechanisms may limit access to white-listed Apps.	SA-4

T-1.3.1

Malicious App with direct communication access to FIDO Authenticator

Violates

Attacker convinces users to install and use a malicious App that gained direct communication access to a FIDO Authenticator.

Consequences: Violation of [SA-4]. The malicious App could trigger the use of any Uauth key in the context of the AppID (related to that key). User Verification / User Presence Check will be performed by the Authenticator as indicated.

Mitigations: Mitigating malicious software installation is outside the scope of FIDO.

If an authenticator implements [SM-1] Key Protection, the user may be able to recover full control of their registered authentication keys by removing the malicious software from their user device.

If it is a bound authenticator, then [SM-13] might help distinguishing the original FIDO Client (i.e. the one used at registration time) from the malicious one.

When using [SM-10] Transaction Confirmation, the user sees the claimed AppIDs and transaction text and can decide to accept or reject the action.

Platforms implementing FIDO can filter communication to FIDO Authenticators. Depending on the platform such filter mechanisms may limit access to white-listed Apps.

T-1.3.2	Malicious Device with direct communication access to FIDO Authenticator	Violates
AC3	Attacker operates a malicious device that can intercept and modify (full MITM attack) the communication between Client and FIDO Authenticator (e.g. through some wireless transport protocol supported by the FIDO Authenticator). Consequences: Violation of [SA-4]. The malicious device could trigger the use of any Uauth key in the context of the AppID (related to that key). User Verification / User Presence Check will be performed by the Authenticator as indicated. Mitigations: FIDO expects each transport protocol (i.e. the communication between client and FIDO Authenticator) to provide a secure link (e.g. see section 8.3.3 in [FIDOCTAP]. When using [SM-10] Transaction Confirmation, the user sees the claimed AppIDs and transaction text and can decide to accept or reject the action.	SA-4

T-1.3.2

Malicious Device with direct communication access to FIDO Authenticator

Violates

Attacker operates a malicious device that can intercept and modify (full MITM attack) the communication between Client and FIDO Authenticator (e.g. through some wireless transport protocol supported by the FIDO Authenticator).

Consequences: Violation of [SA-4]. The malicious device could trigger the use of any Uauth key in the context of the AppID (related to that key). User Verification / User Presence Check will be performed by the Authenticator as indicated.

Mitigations: FIDO expects each transport protocol (i.e. the communication between client and FIDO Authenticator) to provide a secure link (e.g. see section 8.3.3 in [FIDOCTAP].

When using [SM-10] Transaction Confirmation, the user sees the claimed AppIDs and transaction text and can decide to accept or reject the action.

T-1.3.3	Hostile ASM / Client	Violates
AC3, AC5, AC6	In this threat, the Authenticator support infrastructure is hostile, and can feed arbitrary data to the Authenticator. For example some SW running on the User Device and having direct access to the Authenticator (e.g. through CTAP). Or some software running on a separate device that also has direct access to the Authenticator (e.g. through CTAP via BLE transport). Consequences: May undermine [SG-4], [SG-5], [SG-7], [SG-8]. Mitigations: This threat is mitigated by [SM-10], [SM-13].	SG-4, SG-5, SG-7, SG-8

T-1.3.3

Hostile ASM / Client

Violates

AC3, AC5, AC6

In this threat, the Authenticator support infrastructure is hostile, and can feed arbitrary data to the Authenticator. For example some SW running on the User Device and having direct access to the Authenticator (e.g. through CTAP). Or some software running on a separate device that also has direct access to the Authenticator (e.g. through CTAP via BLE transport).

Consequences: May undermine [SG-4], [SG-5], [SG-7], [SG-8].

Mitigations: This threat is mitigated by [SM-10], [SM-13].

SG-4, SG-5, SG-7, SG-8

7.2 Threats to FIDO Authenticator

T-1.4.1	Malicious Authenticator	Violates
AC2, AC3	Attacker convinces users to use a maliciously implemented authenticator. Consequences: The fake authenticator does not implement any appropriate security measures and is able to violate all security goals of FIDO. Mitigations: A user may be unable to distinguish a malicious authenticator, but a Relying Party can use [SM-3] Authenticator Class Attestation to identify and only allow registration of reliable authenticators that have passed [SM-9] Authenticator Certification. A Relying Party can additionally rely on [SM-4] Authenticator Status Checking to check if an attestation presented by a malicious authenticator has been marked as compromised.	SG-1

T-1.4.1

Malicious Authenticator

Violates

AC2, AC3

Attacker convinces users to use a maliciously implemented authenticator.

Consequences: The fake authenticator does not implement any appropriate security measures and is able to violate all security goals of FIDO.

Mitigations: A user may be unable to distinguish a malicious authenticator, but a Relying Party can use [SM-3] Authenticator Class Attestation to identify and only allow registration of reliable authenticators that have passed [SM-9] Authenticator Certification.

A Relying Party can additionally rely on [SM-4] Authenticator Status Checking to check if an attestation presented by a malicious authenticator has been marked as compromised.

T-1.4.2	Uauth.priv Key Compromise	Violates
AC2	Attacker succeeds in extracting a user's cryptographic authentication private key for use in a different context. Consequences: The attacker could impersonate the user with a cloned authenticator that does not do trustworthy user verification, violating [SG-1]. Mitigations: [SM-1] Key Protection measures are intended to prevent this. Each authentication private key is only used for one relying party. Relying Parties can check [SM-9] Authenticator Certification attributes to determine the type of key protection in use by a given authenticator class. Relying Parties can additionally verify the [SM-15] Signature Counter and detect that an authenticator has been cloned if it ever fails to advance relative to the prior operation.	SG-1

T-1.4.2

Uauth.priv Key Compromise

Violates

Attacker succeeds in extracting a user's cryptographic authentication private key for use in a different context.

Consequences: The attacker could impersonate the user with a cloned authenticator that does not do trustworthy user verification, violating [SG-1].

Mitigations: [SM-1] Key Protection measures are intended to prevent this.

Each authentication private key is only used for one relying party.

Relying Parties can check [SM-9] Authenticator Certification attributes to determine the type of key protection in use by a given authenticator class.

Relying Parties can additionally verify the [SM-15] Signature Counter and detect that an authenticator has been cloned if it ever fails to advance relative to the prior operation.

T-1.4.3	User Verification By-Pass	Violates
AC3, AC5	Attacker could use the cryptographic authentication key (inside the authenticator) either with or without being noticed by the legitimate user. Consequences: Attacker could impersonate user, violating [SG-1]. Mitigations: A user can only register and a Relying Party only allow authenticators that perform [SM-1] Key Protection with an appropriately secure user verification process. Does not apply to Silent Authenticators (see [FIDOGlossary]).	SG-1

T-1.4.3

User Verification By-Pass

Violates

AC3, AC5

Attacker could use the cryptographic authentication key (inside the authenticator) either with or without being noticed by the legitimate user.

Consequences: Attacker could impersonate user, violating [SG-1].

Mitigations: A user can only register and a Relying Party only allow authenticators that perform [SM-1] Key Protection with an appropriately secure user verification process.

Does not apply to Silent Authenticators (see [FIDOGlossary]).

T-1.4.4	Physical Authenticator Attack	Violates
AC2, AC5, AC6	Attacker could get physical access to FIDO Authenticator (e.g. by stealing it). Consequences: Attacker could bring the authenticator in a lab in order to use the authentication key (e.g. by-passing user verification and knowing the RP related to this key). If this physical attack succeeds, the attacker could successfully impersonate the user, violating [SG-1] Strong User Authentication. Attacker can introduce a low entropy situation to recover an ECDSA signature key (or optherwise extract the Uauth.priv key), violating [SG-9] Attestable Properties if the attestation key is targeted or [SG-1] Strong User Authentication if a user key is targeted. Mitigations: [SM-1] Key Protection includes requirements to implement strong protections for key material, including resilience to offline attacks and low entropy situations. Relying Parties should use [SM-3] Authenticator Class Attestation to only accept Authenticators implementing a sufficiently strong user verification method.	SG-1

T-1.4.4

Physical Authenticator Attack

Violates

AC2, AC5, AC6

Attacker could get physical access to FIDO Authenticator (e.g. by stealing it).

Consequences: Attacker could bring the authenticator in a lab in order to use the authentication key (e.g. by-passing user verification and knowing the RP related to this key). If this physical attack succeeds, the attacker could successfully impersonate the user, violating [SG-1] Strong User Authentication.

Attacker can introduce a low entropy situation to recover an ECDSA signature key (or optherwise extract the Uauth.priv key), violating [SG-9] Attestable Properties if the attestation key is targeted or [SG-1] Strong User Authentication if a user key is targeted.

Mitigations: [SM-1] Key Protection includes requirements to implement strong protections for key material, including resilience to offline attacks and low entropy situations.

Relying Parties should use [SM-3] Authenticator Class Attestation to only accept Authenticators implementing a sufficiently strong user verification method.

T-1.4.6	Fake Authenticator	Violates
AC2	Attacker is able to extract the authenticator attestation key from an authenticator, e.g. by neutralizing physical countermeasures in a laboratory setting. Consequences: Attacker can violate [SG-9] Attestable Properties by creating a malicious hardware or software device that represents itself as a legitimate one. Mitigations: Relying Parties can use [SM-4] Authenticator Status Checking to identify known-compromised keys. Identification of such compromise is outside the strict scope of the FIDO protocols.	SG-9

T-1.4.6

Fake Authenticator

Violates

Attacker is able to extract the authenticator attestation key from an authenticator, e.g. by neutralizing physical countermeasures in a laboratory setting.

Consequences: Attacker can violate [SG-9] Attestable Properties by creating a malicious hardware or software device that represents itself as a legitimate one.

Mitigations: Relying Parties can use [SM-4] Authenticator Status Checking to identify known-compromised keys. Identification of such compromise is outside the strict scope of the FIDO protocols.

T-1.4.7	Transaction Confirmation Display Overlay Attack	Violates
AC6	Attacker is able to subvert [SM-5] Secure Display functionality (WYSIWYS), perhaps by overlaying the display with false information. Consequences: Violation of [SG-14] Transaction Non-Repudiation. Mitigations: Authenticator implementations must take care to protect in their implementation of a secure display, e.g. by implementing a distinct hardware display or employing appropriate privileges in the operating environment of the user device to protect against spoofing and tampering. [SM-9] Authenticator Certification will provide Relying Parties with metadata about the nature of a transaction confirmation display information that can be used to assess whether it matches the assurance level and risk tolerance of the Relying Party for that particular transaction.	SG-14

T-1.4.7

Transaction Confirmation Display Overlay Attack

Violates

AC6

Attacker is able to subvert [SM-5] Secure Display functionality (WYSIWYS), perhaps by overlaying the display with false information.

Consequences: Violation of [SG-14] Transaction Non-Repudiation.

Mitigations: Authenticator implementations must take care to protect in their implementation of a secure display, e.g. by implementing a distinct hardware display or employing appropriate privileges in the operating environment of the user device to protect against spoofing and tampering.

[SM-9] Authenticator Certification will provide Relying Parties with metadata about the nature of a transaction confirmation display information that can be used to assess whether it matches the assurance level and risk tolerance of the Relying Party for that particular transaction.

SG-14

T-1.4.8	Signature Algorithm Attack	Violates
AC1, AC2, AC3, AC5	A cryptographic attack is discovered against the public key cryptography system used to sign data by the FIDO authenticator. See also T-1.4.10. Consequences: Attacker is able to use messages generated by the client to violate [SG-2] Credential Guessing Resistance. Mitigations: [SM-8] Protocol Nonces, including client-generated entropy, limit the amount of control any adversary has over the internal structure of an authenticator. [SM-1] Key Protection for non-silent authenticators requires user interaction to authorize any operation performed with the authentication key, severely limiting the rate at which an adversary can perform adaptive cryptographic attacks.	SG-2

T-1.4.8

Signature Algorithm Attack

Violates

AC1, AC2, AC3, AC5

A cryptographic attack is discovered against the public key cryptography system used to sign data by the FIDO authenticator. See also T-1.4.10.

Consequences: Attacker is able to use messages generated by the client to violate [SG-2] Credential Guessing Resistance.

Mitigations: [SM-8] Protocol Nonces, including client-generated entropy, limit the amount of control any adversary has over the internal structure of an authenticator.

[SM-1] Key Protection for non-silent authenticators requires user interaction to authorize any operation performed with the authentication key, severely limiting the rate at which an adversary can perform adaptive cryptographic attacks.

SG-2

T-1.4.9	Abuse Functionality	Violates
AC2, AC3, AC5, AC6	It might be possible for an attacker to abuse the Authenticator functionality by sending commands with invalid parameters or invalid commands to the Authenticator. Consequences: This might lead to e.g., user verification by-pass or potential key extraction. Mitigations: Proper robustness (e.g. due to testing) of the Authenticator firmware.	SG-1

T-1.4.9

Abuse Functionality

Violates

AC2, AC3, AC5, AC6

It might be possible for an attacker to abuse the Authenticator functionality by sending commands with invalid parameters or invalid commands to the Authenticator.

Consequences: This might lead to e.g., user verification by-pass or potential key extraction.

Mitigations: Proper robustness (e.g. due to testing) of the Authenticator firmware.

T-1.4.10	Random Number prediction	Violates
AC2, AC3, AC5, AC6	It might be possible for an attacker to get access to information allowing the prediction of RNG data. Consequences: This might lead to key compromise situation [T-1.4.2] when using ECDSA (if the k value is used multiple times or if it is predictable). Mitigations: Proper robustness of the Authenticator's RNG and verification of the relevant operating environment parameters (e.g. temperature, ...).	SG-1

T-1.4.10

Random Number prediction

Violates

AC2, AC3, AC5, AC6

It might be possible for an attacker to get access to information allowing the prediction of RNG data.

Consequences: This might lead to key compromise situation [T-1.4.2] when using ECDSA (if the k value is used multiple times or if it is predictable).

Mitigations: Proper robustness of the Authenticator's RNG and verification of the relevant operating environment parameters (e.g. temperature, ...).

T-1.4.11	Firmware Rollback	Violates
	Attacker might be able to install a previous and potentially buggy version of the firmware. Consequences: This might lead to successful attacks, e.g. T-1.4.9. Mitigations: Proper robustness firmware update and verification method.	SG-1

T-1.4.11

Firmware Rollback

Violates

Attacker might be able to install a previous and potentially buggy version of the firmware.

Consequences: This might lead to successful attacks, e.g. T-1.4.9.

Mitigations: Proper robustness firmware update and verification method.

T-1.4.12	User Verification Data Injection	Violates
AC3, AC6	Attacker might be able to inject pre-captured user verification data into the Authenticator. For example, if a password is used as user verification method, the attacker could capture the password entered by the user and then send the correct password to the Authenticator (by-passing the expected keyboard/PIN pad). Passwords could be captured ahead of the attack e.g. by convincing the user to enter the password into a malicious app (“phishing”) or by spying directly or indirectly the password data. In another example, some malware could play an audio stream which would be recorded by the microphone and used by a Speaker-Recognition based Authenticator. Consequences: This might lead to successful user impersonation (if the attacker has access to valid user verification data). Mitigations: Use a physically secured user verification input method, e.g. Fingerprint Sensor or Trusted-User-Interface for PIN entry which cannot be by-passed by malware.	SG-1

T-1.4.12

User Verification Data Injection

Violates

Attacker might be able to inject pre-captured user verification data into the Authenticator. For example, if a password is used as user verification method, the attacker could capture the password entered by the user and then send the correct password to the Authenticator (by-passing the expected keyboard/PIN pad). Passwords could be captured ahead of the attack e.g. by convincing the user to enter the password into a malicious app (“phishing”) or by spying directly or indirectly the password data.

In another example, some malware could play an audio stream which would be recorded by the microphone and used by a Speaker-Recognition based Authenticator.

Consequences: This might lead to successful user impersonation (if the attacker has access to valid user verification data).

Mitigations: Use a physically secured user verification input method, e.g. Fingerprint Sensor or Trusted-User-Interface for PIN entry which cannot be by-passed by malware.

T-1.4.13	Verification Reference Data Modification	Violates
AC3, AC6	An attacker gains logical or physical access to the Authenticator and modifies Verification Reference Data (e.g. hashed PIN value, fingerprint templates) stored in the Authenticator and adds reference data known to or reproducible by the attacker. Consequences: The attacker would be recognized as the legitimate User and could impersonate the user. Mitigations: [SM-27] Proper protection of the verification reference data and biometric data in the Authenticator.	SG-1

T-1.4.13

Verification Reference Data Modification

Violates

An attacker gains logical or physical access to the Authenticator and modifies Verification Reference Data (e.g. hashed PIN value, fingerprint templates) stored in the Authenticator and adds reference data known to or reproducible by the attacker.

Consequences: The attacker would be recognized as the legitimate User and could impersonate the user.

Mitigations: [SM-27] Proper protection of the verification reference data and biometric data in the Authenticator.

T-1.4.14	Read access to captured user verification data	Violates
AC3, AC6	The Attacker gained read access to the captured user verification data (e.g. PIN, fingerprint image, ...). Consequences: The attacker gets access to PII and could disclose it violating [SG-8]. Mitigations: Limiting access to the user verification data to the Authenticator exclusively.	SG-8

T-1.4.14

Read access to captured user verification data

Violates

The Attacker gained read access to the captured user verification data (e.g. PIN, fingerprint image, ...).

Consequences: The attacker gets access to PII and could disclose it violating [SG-8].

Mitigations: Limiting access to the user verification data to the Authenticator exclusively.

SG-8

T-1.4.15	Compromised the internal PRNG state and the entropy source	Violates
AC1, AC2, AC5	In this threat, an attacker compromises the entropy source prior to the Authenticator initially seeding the PRNG during initialization or otherwise compromises the internal PRNG state, and the attacker is able to know or specify all future entropy inputs to the PRNG. No PRNG is able to recover to a secure status under this threat, but it serves as a useful point for comparison. Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-11], [SG-14]. Mitigations: This constitutes a complete compromise of the RNG, with no ability to recover, so mitigation for this threat involves reducing the impact of a compromised RNG. This is partially mitigated by using an allowed random number generator that allows secure integration of additional input [SM-16] and introduction of data derived from the RP challenge additional input to the PRNG, which can help so long as the attacker has not additionally compromised the TLS session or the ASM / Authenticator link. Using the deterministic signature generation methods (e.g., RFC 6979) can reduce the risk of compromise of existing keys during the signature process, as can using the private key and hash of the signed message as additional input to the PRNG during signature generation. Prevention of non-scalable versions of this style of attack is at least partially addressed by [SM-17] and [SM-18].	SG-1, SG-2, SG-3, SG-4, SG-11, SG-14

T-1.4.15

Compromised the internal PRNG state and the entropy source

Violates

AC1, AC2, AC5

In this threat, an attacker compromises the entropy source prior to the Authenticator initially seeding the PRNG during initialization or otherwise compromises the internal PRNG state, and the attacker is able to know or specify all future entropy inputs to the PRNG. No PRNG is able to recover to a secure status under this threat, but it serves as a useful point for comparison.

Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-11], [SG-14].

Mitigations: This constitutes a complete compromise of the RNG, with no ability to recover, so mitigation for this threat involves reducing the impact of a compromised RNG. This is partially mitigated by using an allowed random number generator that allows secure integration of additional input [SM-16] and introduction of data derived from the RP challenge additional input to the PRNG, which can help so long as the attacker has not additionally compromised the TLS session or the ASM / Authenticator link. Using the deterministic signature generation methods (e.g., RFC 6979) can reduce the risk of compromise of existing keys during the signature process, as can using the private key and hash of the signed message as additional input to the PRNG during signature generation. Prevention of non-scalable versions of this style of attack is at least partially addressed by [SM-17] and [SM-18].

SG-1, SG-2, SG-3, SG-4, SG-11, SG-14

T-1.4.16	Compromised entropy source after successful seeding during initialization	Violates
AC1, AC2, AC5	In this threat, an attacker gains the ability to influence the Authenticator’s entropy source, but only after the initial seeding has been conducted (e.g., if initial seeding occurred prior to the attack and / or as per-Authenticator factory injection of entropy). Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-11], [SG-14]. Mitigations: This is mitigated by using an allowed PRNG which retains PRNG state between power cycles; i.e., which conserves PRNG state even when being reseeded [SM-16]. Prevention of non-scalable versions of this style of attack is at least partially addressed by [SM-17] and [SM-18].	SG-1, SG-2, SG-3, SG-4, SG-11, SG-14

T-1.4.16

Compromised entropy source after successful seeding during initialization

Violates

AC1, AC2, AC5

In this threat, an attacker gains the ability to influence the Authenticator’s entropy source, but only after the initial seeding has been conducted (e.g., if initial seeding occurred prior to the attack and / or as per-Authenticator factory injection of entropy).

Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-11], [SG-14].

Mitigations: This is mitigated by using an allowed PRNG which retains PRNG state between power cycles; i.e., which conserves PRNG state even when being reseeded [SM-16]. Prevention of non-scalable versions of this style of attack is at least partially addressed by [SM-17] and [SM-18].

SG-1, SG-2, SG-3, SG-4, SG-11, SG-14

T-1.4.17	Compromised the internal PRNG state, but not the entropy source	Violates
AC1, AC2, AC5	In this threat, an attacker compromises the entropy source prior to seeding the PRNG or otherwise compromises the internal PRNG state, but then at some point, the attacker no longer can access / control the entropy source. Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-11], [SG-14] Mitigations: This can be mitigated by Authenticators reseeding periodically from an internal entropy source [SM-16]. As a note, this imposes a total number of random number generator requests prior to a required reseed event; in the event that the Authenticator does not have an entropy source internally, this may act as a hard limit on the number of registrations / authentications that such an Authenticator can perform. Prevention of non-scalable versions of this style of attack is at least partially addressed by [SM-17] and [SM-18].	SG-1, SG-2, SG-3, SG-4, SG-11, SG-14

T-1.4.17

Compromised the internal PRNG state, but not the entropy source

Violates

AC1, AC2, AC5

In this threat, an attacker compromises the entropy source prior to seeding the PRNG or otherwise compromises the internal PRNG state, but then at some point, the attacker no longer can access / control the entropy source.

Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-11], [SG-14]

Mitigations: This can be mitigated by Authenticators reseeding periodically from an internal entropy source [SM-16]. As a note, this imposes a total number of random number generator requests prior to a required reseed event; in the event that the Authenticator does not have an entropy source internally, this may act as a hard limit on the number of registrations / authentications that such an Authenticator can perform. Prevention of non-scalable versions of this style of attack is at least partially addressed by [SM-17] and [SM-18].

SG-1, SG-2, SG-3, SG-4, SG-11, SG-14

T-1.4.18	Bad Key Generation	Violates
AC1, AC2, AC5	In this threat, random chance or active attack causes the key generated to be cryptographically flawed; e.g., a RSA key that can be factored using the Pollard p-1 algorithm more quickly than with the General Number Field Sieve. See also T-1.4.21. Consequences: May undermine [SG-1], [SG-2], [SG-4], [SG-11], [SG-14] Mitigations: This is mitigated by requiring use of an allowed random number generator (in the case of certified authenticators), requiring that keys be generated in the way required in the relevant standard specified in the Allowed Cryptography List [SM-16], and making the key generation process resistant to tampering by the attacker [SM-18].	SG-1, SG-2, SG-4, SG-11, SG-14

T-1.4.18

Bad Key Generation

Violates

AC1, AC2, AC5

In this threat, random chance or active attack causes the key generated to be cryptographically flawed; e.g., a RSA key that can be factored using the Pollard p-1 algorithm more quickly than with the General Number Field Sieve. See also T-1.4.21.

Consequences: May undermine [SG-1], [SG-2], [SG-4], [SG-11], [SG-14]

Mitigations: This is mitigated by requiring use of an allowed random number generator (in the case of certified authenticators), requiring that keys be generated in the way required in the relevant standard specified in the Allowed Cryptography List [SM-16], and making the key generation process resistant to tampering by the attacker [SM-18].

SG-1, SG-2, SG-4, SG-11, SG-14

T-1.4.19	Local external side channel attacks	Violates
AC2 (associated with shared keys), AC5	In this threat, an attacker with possession of the Authenticator may be able to extract keys using timing, power, RF, or near-field analysis. The impact depends on the key or secret recovered. Consequences: May undermine [SG-1], [SG-2], [SG-4], [SG-11], [SG-14]. Mitigations: This is mitigated by the side channel resistance security measure [SM-17].	SG-1, SG-2, SG-4, SG-11, SG-14

T-1.4.19

Local external side channel attacks

Violates

AC2 (associated with shared keys), AC5

In this threat, an attacker with possession of the Authenticator may be able to extract keys using timing, power, RF, or near-field analysis. The impact depends on the key or secret recovered.

Consequences: May undermine [SG-1], [SG-2], [SG-4], [SG-11], [SG-14].

Mitigations: This is mitigated by the side channel resistance security measure [SM-17].

SG-1, SG-2, SG-4, SG-11, SG-14

T-1.4.20	Internal side channel attacks	Violates
AC2 (associated with shared keys), AC5	In this threat, an attacker controlling a process running on the same hardware environment as the Authenticator may be able to recover keys by using information leaked by hardware or operating system characteristics (e.g., how often the attacker’s process is scheduled, the state of the L1, L2 caches, etc.). Consequences: May undermine [SG-1], [SG-4], [SG-11], [SG-14]. Mitigations: This is mitigated by the side channel resistance security measure [SM-17].	SG-1, SG-4, SG-11, SG-14

T-1.4.20

Internal side channel attacks

Violates

AC2 (associated with shared keys), AC5

In this threat, an attacker controlling a process running on the same hardware environment as the Authenticator may be able to recover keys by using information leaked by hardware or operating system characteristics (e.g., how often the attacker’s process is scheduled, the state of the L1, L2 caches, etc.).

Consequences: May undermine [SG-1], [SG-4], [SG-11], [SG-14].

Mitigations: This is mitigated by the side channel resistance security measure [SM-17].

T-1.4.21	Error injection during key or signature generation	Violates
AC2 (associated with shared keys), AC5	In this threat, an attacker is able to inject an error in the key or signature generation process that leaks part or all of the private key. Consequences: May undermine [SG-1], [SG-4], [SG-11], [SG-14]. Mitigations: This is mitigated by [SM-18] and [SM-28].	SG-1, SG-4, SG-11, SG-14

T-1.4.21

Error injection during key or signature generation

Violates

AC2 (associated with shared keys), AC5

In this threat, an attacker is able to inject an error in the key or signature generation process that leaks part or all of the private key.

Consequences: May undermine [SG-1], [SG-4], [SG-11], [SG-14].

Mitigations: This is mitigated by [SM-18] and [SM-28].

T-1.4.22	Birthday Paradox Collision	Violates
AC3, AC6	In this threat, a set of randomly generated parameters collide. The probability of this occurrence can be bounded using analysis similar to that associated with the classical Birthday Paradox. Consequences: May undermine [SG-1], [SG-11], [SG-14]. Mitigations: Establishing a bounded number of allowable outputs based on the size of the randomly generated value [SM-19].	SG-1, SG-11, SG-14

T-1.4.22

Birthday Paradox Collision

Violates

In this threat, a set of randomly generated parameters collide. The probability of this occurrence can be bounded using analysis similar to that associated with the classical Birthday Paradox.

Consequences: May undermine [SG-1], [SG-11], [SG-14].

Mitigations: Establishing a bounded number of allowable outputs based on the size of the randomly generated value [SM-19].

SG-1, SG-11, SG-14

T-1.4.23	Privacy Reduction	Violates
AC1	In this threat, a small number of Authenticators share an attestation key which leaks information about the user across Relying Parties. Consequences: May undermine [SG-4]. Mitigations: This is mitigated by [SM-20].	SG-4

T-1.4.23

Privacy Reduction

Violates

In this threat, a small number of Authenticators share an attestation key which leaks information about the user across Relying Parties.

Consequences: May undermine [SG-4].

Mitigations: This is mitigated by [SM-20].

SG-4

T-1.4.24	Covert Channel	Violates
AC1	In this threat, an Authenticator is malicious (either by design, or after having been independently compromised) and it is configured to leak secret or identifying data within apparently normal exchanges, or to other processes on the same hardware platform as the Authenticator. Consequences: May undermine [SG-1], [SG-4], [SG-5], [SG-6], [SG-8], [SG-11], [SG-14]. Mitigations: Note: This is an interesting thought experiment; use of random nonces and other non-deterministic elements make protection against this threat problematic.	SG-1, SG-4, SG-5, SG-6, SG-8, SG-11, SG-14

T-1.4.24

Covert Channel

Violates

In this threat, an Authenticator is malicious (either by design, or after having been independently compromised) and it is configured to leak secret or identifying data within apparently normal exchanges, or to other processes on the same hardware platform as the Authenticator.

Consequences: May undermine [SG-1], [SG-4], [SG-5], [SG-6], [SG-8], [SG-11], [SG-14].

Mitigations: Note: This is an interesting thought experiment; use of random nonces and other non-deterministic elements make protection against this threat problematic.

SG-1, SG-4, SG-5, SG-6, SG-8, SG-11, SG-14

T-1.4.25	Substitution of Protected Information	Violates
AC1, AC3, AC5, AC6	In this threat, an attacker substitutes protected information, either by modifying it piecemeal, or by completely substituting it with another value. (Some encryption modes allow an attacker to target bit-level changes to the plaintext. Authenticated data may also have been replaced with data that had previously been authenticated in the same way.) Consequences: May undermine [SG-1], [SG-4], [SG-11], [SG-14]. Mitigations: This threat is mitigated by [SM-1], [SM-16], [SM-21].	SG-1, SG-4, SG-11, SG-14

T-1.4.25

Substitution of Protected Information

Violates

AC1, AC3, AC5, AC6

In this threat, an attacker substitutes protected information, either by modifying it piecemeal, or by completely substituting it with another value. (Some encryption modes allow an attacker to target bit-level changes to the plaintext. Authenticated data may also have been replaced with data that had previously been authenticated in the same way.)

Consequences: May undermine [SG-1], [SG-4], [SG-11], [SG-14].

Mitigations: This threat is mitigated by [SM-1], [SM-16], [SM-21].

T-1.4.26	Compromise of Protected Information	Violates
AC1, AC2, AC5, AC6	In this threat, an attacker recovers data that should be protected by the Authenticator. Consequences: May undermine [SG-1], [SG-2], [SG-4], [SG-5], [SG-7], [SG-8], [SG-11], [SG-14]. Mitigations: This threat is mitigated by using allowed cryptographic primitives [SM-1], [SM-16].	SG-1, SG-2, SG-4, SG-5, SG-7, SG-8, SG-11, SG-14

T-1.4.26

Compromise of Protected Information

Violates

AC1, AC2, AC5, AC6

In this threat, an attacker recovers data that should be protected by the Authenticator.

Consequences: May undermine [SG-1], [SG-2], [SG-4], [SG-5], [SG-7], [SG-8], [SG-11], [SG-14].

Mitigations: This threat is mitigated by using allowed cryptographic primitives [SM-1], [SM-16].

SG-1, SG-2, SG-4, SG-5, SG-7, SG-8, SG-11, SG-14

T-1.4.27	Signature or registration counter non-monotonicity	Violates
AC1	In this threat, an attacker may be able to cause these counters to be reset, to roll over, or otherwise to decrease in value. Consequences: May undermine [SG-1], [SG-12], [SG-14]. Mitigations: This threat is mitigated by [SM-15].	SG-1, SG-12, SG-14

T-1.4.27

Signature or registration counter non-monotonicity

Violates

In this threat, an attacker may be able to cause these counters to be reset, to roll over, or otherwise to decrease in value.

Consequences: May undermine [SG-1], [SG-12], [SG-14].

Mitigations: This threat is mitigated by [SM-15].

SG-1, SG-12, SG-14

T-1.4.28	Unexpected use of User Verification Caching	Violates
AC3, AC6	In this threat the FIDO Authenticator support User Verification Caching (e.g. section 5.5. in [UAFRegistry]), and the RP App triggers a signature assertions to be created by the authenticator in which malware convinces the FIDO Authenticator to use User Verification Caching (i.e. not verifying the user if the FIDO Authenticator has verified the user within a specified time frame) and specifies a large timeframe so that the user is not freshly verified. Consequences: May undermine [SG-4], [SG-5], [SG-7], [SG-8]. Mitigations: Conforming Authenticators will add a signed extension to the signature assertion indicating that User Verification Caching was indeed activated and also indicating the maximum acceptable caching time. Conforming FIDO Servers will only accept such signature assertion if the use of user verification caching was triggered by the FIDO Server - indicating that the FIDO Server is aware of the implications (e.g. no explicit user consent).	SG-1, SG-7

T-1.4.28

Unexpected use of User Verification Caching

Violates

In this threat the FIDO Authenticator support User Verification Caching (e.g. section 5.5. in [UAFRegistry]), and the RP App triggers a signature assertions to be created by the authenticator in which malware convinces the FIDO Authenticator to use User Verification Caching (i.e. not verifying the user if the FIDO Authenticator has verified the user within a specified time frame) and specifies a large timeframe so that the user is not freshly verified.

Consequences: May undermine [SG-4], [SG-5], [SG-7], [SG-8].

Mitigations: Conforming Authenticators will add a signed extension to the signature assertion indicating that User Verification Caching was indeed activated and also indicating the maximum acceptable caching time. Conforming FIDO Servers will only accept such signature assertion if the use of user verification caching was triggered by the FIDO Server - indicating that the FIDO Server is aware of the implications (e.g. no explicit user consent).

SG-1, SG-7

T-1.4.29	Debug Interface	Violates
AC2 (associated with shared keys), AC3 (associated with shared keys), AC5, AC6	In this threat, the Authenticator has a hardware or software debugging interface that is not completely disabled prior to distribution of the Authenticator (e.g., pads for a JTAG port). Consequences: May undermine [SG-1], [SG-4], [SG-5], [SG-6], [SG-8], [SG-11], [SG-14]. Mitigations: This threat is mitigated by [SM-18], [SM-22], and [SM-28].	SG-1, SG-4, SG-5, SG-6, SG-8, SG-11, SG-14

T-1.4.29

Debug Interface

Violates

AC2 (associated with shared keys), AC3 (associated with shared keys), AC5, AC6

In this threat, the Authenticator has a hardware or software debugging interface that is not completely disabled prior to distribution of the Authenticator (e.g., pads for a JTAG port).

Consequences: May undermine [SG-1], [SG-4], [SG-5], [SG-6], [SG-8], [SG-11], [SG-14].

Mitigations: This threat is mitigated by [SM-18], [SM-22], and [SM-28].

SG-1, SG-4, SG-5, SG-6, SG-8, SG-11, SG-14

T-1.4.30	Fault induced by malformed input	Violates
AC2, AC3, AC5, AC6	In this threat, the Authenticator behaves in an unexpected fashion due to an error in processing malformed input. The result of this style of attack is poorly controllable, absent strong internal segmentation of the Authenticator. Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-6], [SG-7], [SG-8], [SG-11], [SG-14], [SG-16]. Mitigations: This threat is mitigated by [SM-1], [SM-2], [SM-4], [SM-5], [SM-10], [SM-5], [SM-23], [SM-13], [SM-26].	SG-1, SG-2, SG-3, SG-4, SG-6, SG-7, SG-8, SG-11, SG-14, SG-16

T-1.4.30

Fault induced by malformed input

Violates

AC2, AC3, AC5, AC6

In this threat, the Authenticator behaves in an unexpected fashion due to an error in processing malformed input. The result of this style of attack is poorly controllable, absent strong internal segmentation of the Authenticator.

Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-6], [SG-7], [SG-8], [SG-11], [SG-14], [SG-16].

Mitigations: This threat is mitigated by [SM-1], [SM-2], [SM-4], [SM-5], [SM-10], [SM-5], [SM-23], [SM-13], [SM-26].

SG-1, SG-2, SG-3, SG-4, SG-6, SG-7, SG-8, SG-11, SG-14, SG-16

T-1.4.31	Fault Injection Attack	Violates
AC2 (associated with shared keys), AC5, AC6	In this threat, an attacker subjects the Authenticator to conditions that induce hardware faults (e.g., exposure to photons or charged particles, inducing variations in supply voltage or external clock, altering the temperature, etc.) in an attempt to subvert some logical or physical protection. The result of this style of attack is poorly controllable, absent active detection and response functionality within the Authenticator. This is related to T-1.4.21, but applies more broadly. Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-6], [SG-7], [SG-8], [SG-11], [SG-14], [SG-16]. Mitigations: Mitigated by [SM-1], [SM-2], [SM-4], [SM-5], [SM-10], [SM-5], [SM-18], [SM-23], [SM-13], [SM-26], [SM-28].	SG-1, SG-2, SG-3, SG-4, SG-6, SG-7, SG-8, SG-11, SG-14, SG-16

T-1.4.31

Fault Injection Attack

Violates

AC2 (associated with shared keys), AC5, AC6

In this threat, an attacker subjects the Authenticator to conditions that induce hardware faults (e.g., exposure to photons or charged particles, inducing variations in supply voltage or external clock, altering the temperature, etc.) in an attempt to subvert some logical or physical protection. The result of this style of attack is poorly controllable, absent active detection and response functionality within the Authenticator. This is related to T-1.4.21, but applies more broadly.

Consequences: May undermine [SG-1], [SG-2], [SG-3], [SG-4], [SG-6], [SG-7], [SG-8], [SG-11], [SG-14], [SG-16].

Mitigations: Mitigated by [SM-1], [SM-2], [SM-4], [SM-5], [SM-10], [SM-5], [SM-18], [SM-23], [SM-13], [SM-26], [SM-28].

SG-1, SG-2, SG-3, SG-4, SG-6, SG-7, SG-8, SG-11, SG-14, SG-16

T-1.4.32	Remote Timing Attacks	Violates
AC2, AC5	In this threat, an attacker may be able to extract keys using a timing attack from a remote location. The impact depends on the key or secret recovered. Consequences: May undermine [SG-1], [SG-2], [SG-4], [SG-11], [SG-14]. Mitigations: This threat is mitigated by the remote timing attack resistance security measure [SM-29].	SG-1, SG-2, SG-4, SG-11, SG-14

T-1.4.32

Remote Timing Attacks

Violates

AC2, AC5

In this threat, an attacker may be able to extract keys using a timing attack from a remote location. The impact depends on the key or secret recovered.

Consequences: May undermine [SG-1], [SG-2], [SG-4], [SG-11], [SG-14].

Mitigations: This threat is mitigated by the remote timing attack resistance security measure [SM-29].

SG-1, SG-2, SG-4, SG-11, SG-14

7.3 Threats to Relying Party

7.3.1 Threats to FIDO Server Data

T-2.1.1	FIDO Server DB Read Attack	Violates
	Attacker could obtains read-access to FIDO Server registration database. Consequences: Attacker can access all cryptographic key handles and authenticator characteristics associated with a username. If an authenticator or combination of authenticators is unique, they might use this to try to violate [SG-2] Unlinkability. Attacker attempts to perform factorization of public keys by virtue of having access to a large corpus of data, violating [SG-5] Verifier Leak Resilience and [SG-2] Credential Guessing Resilience. Mitigations: [SM-2] Unique Authentication Keys help prevent disclosed key material from being useful against any other Relying Party, even if successfully attacked. The use of an [SM-6] Cryptographically Secure Verifier Database helps assure that it is infeasible to attack any leaked verifier keys. [SM-9] Authenticator Certification along with [SM-16] should help prevent authenticators with poor entropy from entering the market, reducing the likelihood that even a large corpus of key material will be useful in mounting attacks.	SG-2, SG-5

T-2.1.1

FIDO Server DB Read Attack

Violates

Attacker could obtains read-access to FIDO Server registration database.

Consequences: Attacker can access all cryptographic key handles and authenticator characteristics associated with a username. If an authenticator or combination of authenticators is unique, they might use this to try to violate [SG-2] Unlinkability.

Attacker attempts to perform factorization of public keys by virtue of having access to a large corpus of data, violating [SG-5] Verifier Leak Resilience and [SG-2] Credential Guessing Resilience.

Mitigations: [SM-2] Unique Authentication Keys help prevent disclosed key material from being useful against any other Relying Party, even if successfully attacked.

The use of an [SM-6] Cryptographically Secure Verifier Database helps assure that it is infeasible to attack any leaked verifier keys.

[SM-9] Authenticator Certification along with [SM-16] should help prevent authenticators with poor entropy from entering the market, reducing the likelihood that even a large corpus of key material will be useful in mounting attacks.

SG-2, SG-5

T-2.1.2	FIDO Server DB Modification Attack	Violates
AC1	Attacker gains write-access to the FIDO Server registration database. Consequences: Violation of [SA-6] The attacker may inject a key registration under its control, violating [SG-1] Strong User Authentication. Mitigations: Mitigating such attacks is outside the scope of the FIDO specifications. The Relying Party must maintain the integrity of any information it relies up on to identify a user as part of [SA-6].	SA-6

T-2.1.2

FIDO Server DB Modification Attack

Violates

Attacker gains write-access to the FIDO Server registration database.

Consequences: Violation of [SA-6]

The attacker may inject a key registration under its control, violating [SG-1] Strong User Authentication.

Mitigations: Mitigating such attacks is outside the scope of the FIDO specifications. The Relying Party must maintain the integrity of any information it relies up on to identify a user as part of [SA-6].

SA-6

T-2.2.1

Web App Malware

Violates

Attacker gains ability to execute code in the security context of the Relying Party web application or FIDO Server.

Consequences: Attacker is able to violate [SG-1], [SG-10], [SG-9] and any other Relying Party controls.

Mitigations: The consequences of such an incident are limited to the relationship between the user and that particular Relying Party by [SM-1], [SM-2], and [SM-5].

Even within the Relying Party to user relationship, a user can be protected by [SM-10] Transaction Confirmation if the compromise does not include the users' computing environment.

SG-1, SG-9, SG-10

T-2.2.2

Linking through compromised Relying Party database

Violates

In this threat, a Relying Party is able to access another Relying Party’s database (either because the Relying Parties are collaborating or because of the compromise of another Relying Party’s database). The malicious party then sends Key Handles (which may contain a wrapped private key) from the other Relying Party’s database in an attempt to link the two separate accounts to the same Authenticator (thus user).

Consequences: May undermine [SG-1], [SG-4].

Mitigations: This threat is mitigated by [SM-1], [SM-2], [SM-5], [SM-23].

SG-1, SG-4

7.4 Threats to the Secure Channel between Client and Relying Party

7.4.1 Exploiting Weaknesses in the Secure Transport of FIDO Messages

FIDO takes as a base assumption that [SA-3] applications on the user device are able to establish secure channels that provide trustworthy server authentication, and confidentiality and integrity for messages. e.g. through TLS. [T-1.2.4] Discusses some consequences of violations of this assumption due to implementation errors in a browser or client application, but other threats exist in different layers.

T-3.1.1

TLS Proxy

Violates

The FIDO user device is administratively configured to connect through a proxy that terminates TLS connections. The client trusts this device, but the connection between the user and FIDO server is no longer end-to-end secure.

Consequences: Any such proxies introduce a new party into the protocol. If this party is untrustworthy, consequences may be as for [T-1.2.4].

Mitigations: Mitigations for [T-1.2.4] apply, except that the proxy is considered trusted by the client, so certain methods of [SM-12] Channel Binding may indicate a compromised channel even in the absence of an attack. Servers should use multiple methods and adjust their risk scoring appropriately. A trustworthy client that reports a server certificate that is unknown to the server and does not chain to a public root may indicate a client behind such a proxy. A client reporting a server certificate that is unknown to the server but validates for the server's identity according to commonly used public trust roots is more likely to indicate [T-3.1.2].

T-3.1.2

Fraudulent TLS Server Certificate

Violates

An attacker is able to obtain control of a certificate credential for a Relying Party, perhaps from a compromised Certification Authority or poor protection practices by the Relying Party.

Consequences: As for [T-1.2.4].

Mitigations: As for [T-1.2.4].

T-3.1.3

Protocol level real-time MITM attack

Violates

An adversary can intercept and manipulate network packets sent from the relying party to the client. The adversary uses this capability to (a) terminate the underlying TLS session from the client at the adversary and to (b) simultaneously use another TLS session from the adversary to the relying party. In the traditional username/password world, this allows the adversary to intercept the username and the password and then successfully impersonate the user at the relying party.

Consequences: None if FIDO channelBinding [SM-12] or transaction confirmation [SM-10] are used.

Mitigations: In the case of channelBinding [SM-12], the FIDO server will detect the MITM in the TLS channel by comparing the channel binding information provided by the client and the channel binding information retrieved locally by the server.

In the case of transaction confirmation [SM-10], the user verifies and approves a particular transaction. The adversary could modify the transaction before approval. This would lead to rejection by the user. Alternatively, the adversary could modify the transaction after approval. This will break the signature in the transaction confirmation response. The FIDO Server will not accept it as a consequence.

HTTP Public Key Pinning (RFC7469) can also be used to mitigate this attack (outside the FIDO stack).

7.5 Threats to the Infrastructure

7.5.1 Threats to FIDO Authenticator Manufacturers

T-4.1.1

Manufacturer Level Attestation Key Compromise

Violates

Attacker obtains control of an attestation key or attestation key issuing key.

Consequences: Same as [T-1.4.6]: Attacker can violate [SG-9] Attestable Properties by creating a malicious hardware or software device that represents itself as a legitimate one.

Mitigations: Same as [T-1.4.6]: Relying Parties can use [SM-4] Authenticator Status Checking to identify known-compromised keys. Identification of such compromise is outside the strict scope of the FIDO protocols.

T-4.1.2

Malicious Authenticator HW

Violates

AC1, AC2, AC3, AC5, AC6

FIDO Authenticator manufacturer relies on hardware or software components that generate weak cryptographic authentication key material or contain backdoors.

Consequences: Effective violation of [SA-1] in the context of such an Authenticator.

Mitigations: The process of [SM-9] Authenticator Certification may reveal a subset of such threats, but it is not possible that all such can be revealed with black box testing and white box examination may be is economically infeasible. Users and Relying Parties with special concerns about this class of threat must exercise their own necessary caution about the trustworthiness and verifiability of their vendors and supply chain. [SM-24] builds confidence that an Authenticator is not malicious or poorly implemented.

SA-1

7.5.2 Threats to FIDO Server Vendors

T-4.2.1 Vendor Level Trust Anchor Injection Attack Violates

Attacker adds malicious trust anchors to the trust list shipped by a FIDO Server vendor.

Consequences: Attacker can deploy fake Authenticators which Relying Parties cannot detect as such, which do not implement any appropriate security measures, and is able to violate all security goals of FIDO.

Mitigations: This type of supply chain threat is outside the strict scope of the FIDO protocols and violates [SA-6]. Relying Parties can verify their trust list against the data published by the FIDO Alliance Metadata Service [FIDOMetadataService] (see https://fidoalliance.org/mds).

SA-6

7.5.3 Threats to FIDO Metadata Service Operators

T-4.3.1

Metadata Service Signing Key Compromise

Violates

The attacker gets access to the private Metadata TOC signing key.

Consequences: The attacker could sign invalid Metadata. The attacker could

make trustworthy authenticators look less trustworthy (e.g. by increasing FAR).
make weak authenticators look strong (e.g. by changing the key protection method to a more secure one)
inject malicious attestation trust anchors, e.g. root certificates which cross-signed the original attestation trust anchor and the cross-signed original attestation root certificate. This malicious trust anchors could be used to sign attestation certificates for fraudulent authenticators, e.g. authenticators using the AAID of trustworthy authenticators but not protecting their keys as stated in the metadata.

Mitigations: The Metadata Service operator should protect the Metadata signing key appropriately, e.g. using a hardware protected key storage.

Relying parties could use out-of-band methods to cross-check Metadata Statements with the respective vendors and cross-check the revocation state of the Metadata signing key with the provider of the Metadata Service.

T-4.3.2

Metadata Statement Data Injection

Violates

An attacker injects malicious Authenticator data into the Metadata Statement.

Consequences: The attacker could make the Metadata Service operator sign invalid Metadata Statements. The attacker could

make trustworthy authenticators look less trustworthy (e.g. by increasing FAR).
make weak authenticators look strong (e.g. by changing the key protection method to a more secure one)
inject malicious attestation trust anchors, e.g. root certificates which cross-signed the original attestation trust anchor and the cross-signed original attestation root certificate. This malicious trust anchors could be used to sign attestation certificates for fraudulent authenticators, e.g. authenticators using the AAID of trustworthy authenticators but not protecting their keys as stated in the metadata.

Mitigations: The Metadata Service operator could carefully review the delta between the old and the new Metadata Statements. Authenticator vendors could verify the published Metadata Statements related to their Authenticators.

7.6 Threats Specific to Second Factor Authenticators (U2F /UAF / WebAuthn)

T-5.1.1

Error Status Side Channel

Violates

Relying parties issues an authentication challenge to an authenticator and can infer from error status if it is already registered.

Consequences: UAF Silent authenticators / U2F authenticators not requiring user interaction for generating a signed response may be used to track users without their consent by issuing a pre-authentication challenge to them, revealing the identity of an otherwise anonymous user. Users would be identifiable by relying parties without their knowledge, violating [SG-7].

Mitigations: The U2F specification recommends that browsers prompt users whether to allow this operation using mechanisms similar to those defined for other privacy sensitive operations like Geolocation.

SG-7

T-5.1.2

Malicious RP

Violates

Malicious relying party mounts a cryptographic attack on a key handle it is storing.

Consequences: If the Relying Party is able to recover the contents of the key handle, it might forge logs of protocol exchanges to associate the user with actions he or she did not perform.

If the Relying Party is able to recover the key used to wrap a key handle, that key is likely used for all key handles, and hence might be used to decrypt key handles stored with other Relying Parties and violate [SG-1] Strong User Authentication.

Mitigations: None. Some U2F Authenticators and some Authenticators implementing Web Authentication (i.e. the ones without "resident keys") depend on [SA-1] to hold for key wrapping operations.

T-5.1.3

Physical Attack on a User Presence Authenticator

Violates

AC5

Attacker gains physical access to an authenticator with only user presence check (e.g., by stealing it).

Consequences: Same as for [T-1.4.4].

Such authenticators have weak local user verification. If the attacker can guess the username and password/PIN, they can impersonate the user, violating [SG-1] Strong User Authentication.

Mitigations: Relying Parties can use strong additional factors.

Relying Parties should provide users a means to revoke keys associated with a lost device.

T-5.1.4

Physical Attack

Violates

AC2 (associated with shared keys), AC5

In this threat, keys or other sensitive information is read out by directly accessing it from the authenticator that the attacker has physically compromised.

Consequences: May undermine [SG-1], [SG-4], [SG-11], [SG-14].

Authenticator with user presence check have weak local user verification. If the attacker can guess the username and password/PIN, they can impersonate the user, violating [SG-1] Strong User Authentication.

Mitigations: Mitigated by resistance to injected faults [SM-18] and [SM-28].