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CVE Record

CVE-2021-31924: Yubico pam-u2f before 1.1.1 has a logic issue that, depending on the pam-u2f configuration and the applicat...

Yubico pam-u2f before 1.1.1 has a logic issue that, depending on the pam-u2f configuration and the application used, could lead to a local PIN bypass. This issue does not allow user presence (touch) or cryptographic signature verification to be bypassed, so an attacker would still need to physically possess and interact with the YubiKey or another enrolled authenticator. If pam-u2f is configured to require PIN authentication, and the application using pam-u2f allows the user to submit NULL as the PIN, pam-u2f will attempt to perform a FIDO2 authentication without PIN. If this authentication is successful, the PIN requirement is bypassed.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This flaw affects Yubico pam-u2f before 1.1.1. In a narrow configuration, a local user could bypass the PIN part of authentication. It does not bypass the required authenticator touch or cryptographic verification, so the attacker still needs the enrolled YubiKey or authenticator and physical interaction with it.

Executive priority

Treat as a targeted local-authentication risk. It is not remotely exploitable from the provided evidence, but it can weaken MFA assurance on sensitive systems where physical authenticators protect administrative access.

Technical view

If pam-u2f is configured to require PIN authentication, and the consuming application allows a NULL PIN submission, pam-u2f may perform FIDO2 authentication without PIN. Successful authentication then bypasses the PIN requirement. The issue is described as fixed after versions before 1.1.1.

Likely exposure

Exposure is likely limited to systems using Yubico pam-u2f before 1.1.1 with PIN-required configuration and applications that can submit NULL PIN values. Linux distributions referenced by Fedora and Gentoo advisories may be relevant.

Exploitation context

The source bundle does not show active exploitation or KEV listing. Practical abuse requires local access plus physical possession and interaction with an enrolled authenticator. It is a control weakening, not a full bypass of user presence or signature verification.

Researcher notes

Key limiting conditions are explicit: pam-u2f before 1.1.1, PIN-required configuration, NULL PIN submission by the application, and possession of an enrolled authenticator. Sources do not provide CVSS, CWE, CPEs, or active exploitation evidence.

Mitigation direction

  • Upgrade Yubico pam-u2f to version 1.1.1 or later.
  • Apply relevant Fedora, Gentoo, or other distribution security updates.
  • Review Yubico advisory YSA-2021-03 for vendor-specific guidance.
  • Check applications using pam-u2f for NULL PIN handling.
  • Prioritize systems where pam-u2f enforces PIN for privileged access.

Validation and detection

  • Inventory installed pam-u2f versions across Linux hosts.
  • Confirm whether pam-u2f is configured to require PIN authentication.
  • Identify applications or PAM flows that consume pam-u2f.
  • Verify distribution security advisories are applied on managed hosts.
  • Document systems that rely on pam-u2f for privileged local access.
Prepared
Confidence
high
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
6Source links

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Source materials

Affected products

Products and packages named in the record

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Weakness

CWE details

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