# CVE-2021-20181: Use after free in qemu

> A race condition in QEMU's 9pfs server allows malicious clients to trigger a use-after-free, leading to privilege escalation and potential code execution.

- **Severity:** High 7.5 (`CVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 May 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-20181, GHSA-mq7v-47j3-rc24, OESA-2021-1218, UBUNTU-CVE-2021-20181

## What is CVE-2021-20181?

CVE-2021-20181 is a high-severity use after free vulnerability in Debian Debian Linux, published on 13 May 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A race condition flaw was found in the 9pfs server implementation of QEMU up to and including 5.2.0. This flaw allows a malicious 9p client to cause a use-after-free error, potentially escalating their privileges on the system. The highest threat from this vulnerability is to confidentiality, integrity as well as system availability.

Weakness: CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization), CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition)

## Which versions are affected by CVE-2021-20181?

It affects 2 products.


Products: Debian Debian Linux, Qemu Qemu.

## How do I fix CVE-2021-20181?

Apply the vendor's patch.

## Is CVE-2021-20181 being exploited?

There is no known exploitation of CVE-2021-20181 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-20181 and fix it if it is affected.

What it is: CVE-2021-20181 is a high-severity use after free vulnerability in Debian Debian Linux, published on 13 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-2021-20181 in the wild and no public exploit code.

Affected product: Debian Debian 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-2021-20181/ (Markdown: https://insights.sbomify.com/CVE-2021-20181/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20181
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20181
- OSV: https://osv.dev/vulnerability/CVE-2021-20181

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-20181/
Summaries are generated and may contain mistakes; the official description above is the record.