# CVE-2021-3996: Missing authorization in util-linux

> A logic error in libmount allows local users to unmount other users' filesystems, causing denial of service for affected applications.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 23 Aug 2022
- **Updated:** 15 Oct 2024
- **Also known as:** ALPINE-CVE-2021-3996, CLEANSTART-2026-GD03162, DEBIAN-CVE-2021-3996, JLSEC-2025-193, UBUNTU-CVE-2021-3996

## What is CVE-2021-3996?

CVE-2021-3996 is a medium-severity missing authorization vulnerability in Fedoraproject Fedora, published on 23 Aug 2022. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A logic error was found in the libmount library of util-linux in the function that allows an unprivileged user to unmount a FUSE filesystem. This flaw allows a local user on a vulnerable system to unmount other users' filesystems that are either world-writable themselves (like /tmp) or mounted in a world-writable directory. An attacker may use this flaw to cause a denial of service to applications that use the affected filesystems.

Weakness: CWE-552 (Files or Directories Accessible to External Parties)

## Which versions are affected by CVE-2021-3996?

It affects 2 products.


Products: Fedoraproject Fedora, Kernel Util-Linux.

## How do I fix CVE-2021-3996?

Apply the vendor's patch.

## Is CVE-2021-3996 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-3996 exists.

## 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-2021-3996 and fix it if it is affected.

What it is: CVE-2021-3996 is a medium-severity missing authorization vulnerability in Fedoraproject Fedora, published on 23 Aug 2022. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-3996 exists.

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-2021-3996/ (Markdown: https://insights.sbomify.com/CVE-2021-3996/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3996
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3996
- OSV: https://osv.dev/vulnerability/CVE-2021-3996

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3996/
Summaries are generated and may contain mistakes; the official description above is the record.