# CVE-2021-3493: Linux Kernel Privilege Escalation

> Attackers can gain elevated privileges via overlayfs capability validation flaws in the Linux kernel when unprivileged user namespaces and overlay mounts are enabled.

- **Severity:** High 8.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Yes, since 20 Oct 2022 (CISA KEV)
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Apr 2021
- **Updated:** 21 Oct 2025
- **Also known as:** DEBIAN-CVE-2021-3493, GHSA-2fj2-4h38-3c72, UBUNTU-CVE-2021-3493

## What is CVE-2021-3493?

CVE-2021-3493 is a high-severity vulnerability in Canonical Ubuntu Linux, published on 17 Apr 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

The overlayfs implementation in the linux kernel did not properly validate with respect to user namespaces the setting of file capabilities on files in an underlying file system. Due to the combination of unprivileged user namespaces along with a patch carried in the Ubuntu kernel to allow unprivileged overlay mounts, an attacker could use this to gain elevated privileges.

Weakness: CWE-863 (Incorrect Authorization), CWE-270 (Privilege Context Switching Error)

## Which versions are affected by CVE-2021-3493?

It affects 1 product.


Products: Canonical Ubuntu Linux.

## How do I fix CVE-2021-3493?

Apply the vendor's patch. US federal agencies must fix it by 10 Nov 2022.

## Is CVE-2021-3493 being exploited?

Yes. Attackers have exploited CVE-2021-3493 in the wild since at least 20 Oct 2022.

Public exploit code:

- Ubuntu OverlayFS Local Privesc (github_poc): https://github.com/briskets/CVE-2021-3493
- CVE-2021-3493 Ubuntu OverlayFS Local Privesc (Interactive Bash Shell & Execute Command Entered) (github_poc): https://github.com/inspiringz/CVE-2021-3493
- https://github.com/oneoy/CVE-2021-3493 (github_poc): https://github.com/oneoy/CVE-2021-3493
- https://github.com/puckiestyle/CVE-2021-3493 (github_poc): https://github.com/puckiestyle/CVE-2021-3493
- A vulnerable Boot-to-Root CTF lab machine simulating a hospital environment. Features a realistic 17-step attack chain including SQL Injection, XSS, Session Hijacking, SSH access, password cracking, privilege escalation via CVE-2021-3493, and full root compromise. Developed as a graduation project at Ajloun National University (ANU), awarded 97%. (github_poc): https://github.com/Ayham-Megdadi/Zero-Day-Legacy
- 2021 kernel vulnerability in Ubuntu. (github_poc): https://github.com/cerodah/overlayFS-CVE-2021-3493

## Other vulnerabilities in Canonical Ubuntu Linux

- [CVE-2020-11651](https://insights.sbomify.com/CVE-2020-11651/): SaltStack Salt Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2020-7247](https://insights.sbomify.com/CVE-2020-7247/): OpenSMTPD Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2019-16928](https://insights.sbomify.com/CVE-2019-16928/): Exim Out-of-bounds Write (critical 9.8, exploited in the wild)
- [CVE-2018-6789](https://insights.sbomify.com/CVE-2018-6789/): Exim Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2015-2590](https://insights.sbomify.com/CVE-2015-2590/): Oracle Java SE and Java SE Embedded Remote Code Execution (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-3493 and fix it if it is affected.

What it is: CVE-2021-3493 is a high-severity vulnerability in Canonical Ubuntu Linux, published on 17 Apr 2021. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2021-3493 in the wild since at least 20 Oct 2022.

Affected product: Canonical Ubuntu Linux. 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-2021-3493/ (Markdown: https://insights.sbomify.com/CVE-2021-3493/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3493
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3493
- OSV: https://osv.dev/vulnerability/CVE-2021-3493

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-3493/
Summaries are generated and may contain mistakes; the official description above is the record.