# CVE-2021-31924: Auth bypass in n/a

> Local attackers can bypass PIN authentication in Yubico pam-u2f if configured to require PIN and the application allows NULL input, though physical key possession is still required.

- **Severity:** Medium 6.8 (`CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 May 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-31924, GHSA-v2qj-ch59-r5g4

## What is CVE-2021-31924?

CVE-2021-31924 is a medium-severity authentication bypass vulnerability in Fedoraproject Fedora, published on 25 May 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official 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.

Weakness: CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2021-31924?

It affects 2 products.


Products: Fedoraproject Fedora, Yubico Pam-U2f.

## How do I fix CVE-2021-31924?

Upgrade to a fixed version.

## Is CVE-2021-31924 being exploited?

There is no known exploitation of CVE-2021-31924 in the wild and no public exploit code.

## Other vulnerabilities in Fedoraproject Fedora

- [CVE-2024-4577](https://insights.sbomify.com/CVE-2024-4577/): PHP-CGI OS Command Injection (critical 9.8, exploited in the wild)
- [CVE-2021-39226](https://insights.sbomify.com/CVE-2021-39226/): Grafana Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2019-5544](https://insights.sbomify.com/CVE-2019-5544/): VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2024-4947](https://insights.sbomify.com/CVE-2024-4947/): Google Chromium V8 Type Confusion (critical 9.6, exploited in the wild)
- [CVE-2024-4671](https://insights.sbomify.com/CVE-2024-4671/): Google Chromium Visuals Use-After-Free (critical 9.6, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-31924 and fix it if it is affected.

What it is: CVE-2021-31924 is a medium-severity authentication bypass vulnerability in Fedoraproject Fedora, published on 25 May 2021. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-31924 in the wild and no public exploit code.

Affected product: Fedoraproject Fedora. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-31924/ (Markdown: https://insights.sbomify.com/CVE-2021-31924/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-31924
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-31924
- OSV: https://osv.dev/vulnerability/CVE-2021-31924

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-31924/
Summaries are generated and may contain mistakes; the official description above is the record.