CVE-2021-31924

Local PIN bypass via NULL conversation result

Published May 25, 2021 · Updated Aug 3, 2024

Authentication bypass in Yubico pam-u2f 1.1.0 allows physically proximate attackers to log in without the configured PIN. In do_authentication in util.c, a NULL result from converse() clears the PIN requirement and continues FIDO2 authentication without a PIN instead of aborting. Exploitation requires an application that can return a NULL PIN plus physical possession of and interaction with an enrolled authenticator; touch and signature verification remain enforced.

CVSS severity6.8
Medium
EPSS probability0.33%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Authentication bypass in Yubico pam-u2f 1.1.0 allows physically proximate attackers to log in without the configured PIN. In do_authentication in util.c, a NULL result from converse() clears the PIN requirement and continues FIDO2 authentication without a PIN instead of aborting. Exploitation requires an application that can return a NULL PIN plus physical possession of and interaction with an enrolled authenticator; touch and signature verification remain enforced.

The record

CVE
CVE-2021-31924
Published
May 25, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-287
Attack vector
physical
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 25, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectorphysical
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

4.6NIST National Vulnerability DatabaseAV:L/AC:L/Au:N/C:P/I:P/A:P
6.8NIST National Vulnerability DatabaseCVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorphysical
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-287Improper Authentication

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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