---
title: CVE-2021-31924
description: Ubuntu is an open source software operating system that runs from the
  desktop, to the cloud, to all your internet connected things.
url: https://ubuntu.com/security/CVE-2021-31924
---

# CVE-2021-31924

Publication date 26 May 2021

Last updated 29 August 2025

---

Ubuntu priority

**Medium**

[Why this priority?](https://ubuntu.com/security/cves/about#priority )

## Cvss 3 Severity Score

**6.8 · Medium**

[Score breakdown](https://ubuntu.com/security/CVE-2021-31924#impact-score)

Toggle side navigation

## Description

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.

[Read the notes from the security team](https://ubuntu.com/security/CVE-2021-31924#notes)

## Status

Show unmaintained releases

| Package | Ubuntu Release | Status |
| --- | --- | --- |
| pam-u2f | 25.04 plucky | Not affected |
| 24.10 oracular | Ignored end of life, was needs-triage |
| 24.04 LTS noble | Not affected |
| 23.10 mantic | Ignored end of life, was needs-triage |
| 23.04 lunar | Ignored end of life, was needs-triage |
| 22.10 kinetic | Ignored end of life, was needs-triage |
| 22.04 LTS jammy | Not affected |
| 21.10 impish | Ignored end of life |
| 21.04 hirsute | Ignored end of life |
| 20.10 groovy | Ignored end of life |
| 20.04 LTS focal | Not affected |
| 18.04 LTS bionic | Not affected |
| 16.04 LTS xenial | Not affected |
| 14.04 LTS trusty | Not in release |

---

* [How can I get the fixes?](https://ubuntu.com/security/cves/about#security)
* [What do statuses mean?](https://ubuntu.com/security/cves/about#statuses)

## Notes

---

### [john-breton](https://launchpad.net/~john-breton)

Per Debian, PIN verfication was only introduced in 1.1.0.
Only versions 1.1.0 to 1.1.1 are vulnerable.
Debian also fixed this in 1.1.0-1.1

## Severity score breakdown

CVSS version:
CVSS v3.0

**Base score**

6.8 · Medium

* Base metrics

  | Parameter | Value |
  | --- | --- |
  | Attack vector | Physical |
  | Attack complexity | Low |
  | Privileges required | None |
  | User interaction | None |
  | Scope | Unchanged |
  | Confidentiality impact | High |
  | Integrity impact | High |
  | Availability impact | High |
* Scores

  | Parameter | Value |
  | --- | --- |
  | Base score | 6.8 · Medium |
  | Exploitability score | - |
  | Impact score | - |

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

## References

* [MITRE](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-31924)
* [NVD](https://nvd.nist.gov/vuln/detail/CVE-2021-31924)
* [Launchpad](https://launchpad.net/bugs/cve/CVE-2021-31924)
* [Debian](https://security-tracker.debian.org/tracker/CVE-2021-31924)

### Other references

* <https://www.yubico.com/support/security-advisories/ysa-2021-03>
* <https://developers.yubico.com/pam-u2f/>
* <https://www.cve.org/CVERecord?id=CVE-2021-31924>