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FedoraOperating system · Fedoraproject

CVE-2021-31924

MEDIUM · 6.8 CVSS v3.1 Published 2021-05-26
Fix available
A fix is available. Upgrade to 1.1.1 or later.
See remediation →
73/100
Remediation priority · Elevated
No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

pam-u2f before 1.1.1 contains a logic flaw where submitting NULL as the PIN to a pam-u2f configuration requiring PIN authentication causes the module to attempt FIDO2 authentication without a PIN. If successful, the PIN requirement is bypassed while user presence (touch) and cryptographic verification remain enforced.

MitigationUpgrade pam-u2f to version 1.1.1 or later. Additionally, ensure applications using pam-u2f do not permit NULL PIN submissions to prevent this bypass.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FedoraOperating system
Affected:= 34= 35
Pam U2fApplication
Affected:< 1.1.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Physical
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify pam-u2f installation
    Query the system package manager for the pam-u2f package (e.g., using 'rpm -qa | grep -i u2f' on Fedora/RHEL or 'dpkg -l | grep -i u2f' on Debian-based systems)
    Affected if The pam-u2f package is installed
  2. Determine installed pam-u2f version
    Use the package manager to query the installed version (e.g., 'rpm -q pam_u2f' on Fedora or 'dpkg -s libpam-u2f' on Debian)
    Affected if The installed version is before 1.1.1 (e.g., 1.0.0 through 1.1.0)
  3. Locate pam-u2f configuration files
    Examine PAM configuration files in /etc/pam.d/ for any entries referencing 'pam_u2f.so' or 'pam-u2f'
    Affected if A PAM configuration file references pam-u2f with auth or account control
  4. Check if PIN authentication is configured
    Review the pam-u2f configuration lines in the PAM files - look for 'pinverification=1', 'pin=1', or similar PIN requirement settings in the module arguments
    Affected if The configuration explicitly requires PIN verification (e.g., contains 'pinverification=1' or equivalent setting)
  5. Verify PIN bypass vulnerability exists
    Submit a NULL/empty PIN to the pam-u2f authentication prompt in an affected configuration - if the module proceeds to FIDO2 authentication without rejecting the empty PIN, the bypass is present
    Affected if The module accepts an empty/NULL PIN and proceeds to FIDO2 authentication without a PIN while still enforcing user presence (touch)

A user is affected if pam-u2f version is below 1.1.1 AND the configuration requires PIN authentication AND submitting an empty PIN bypasses the PIN requirement while still requiring user touch.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Upgrade available Upgrade to 1.1.1 or later
Fixed in 1.1.1
Interim mitigation

Upgrade pam-u2f to version 1.1.1 or later. Additionally, ensure applications using pam-u2f do not permit NULL PIN submissions to prevent this bypass.

Fix this in Fedora Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-31924 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
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