# CVE-2021-20263: Privilege escalation in QEMU

> A flaw in QEMU's virtiofsd allows guest users to retain security capabilities on file writes, potentially enabling privilege escalation within the guest.

- **Severity:** Low 3.3 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N`)
- **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:** 9 Mar 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-20263, GHSA-349h-phx6-ghfj

## What is CVE-2021-20263?

CVE-2021-20263 is a low-severity vulnerability in Qemu Qemu, published on 9 Mar 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A flaw was found in the virtio-fs shared file system daemon (virtiofsd) of QEMU. The new 'xattrmap' option may cause the 'security.capability' xattr in the guest to not drop on file write, potentially leading to a modified, privileged executable in the guest. In rare circumstances, this flaw could be used by a malicious user to elevate their privileges within the guest.

Weakness: CWE-281 (Improper Preservation of Permissions)

## Which versions are affected by CVE-2021-20263?

It affects 1 product.


Products: Qemu Qemu.

## How do I fix CVE-2021-20263?

Apply the vendor's patch.

## Is CVE-2021-20263 being exploited?

There is no known exploitation of CVE-2021-20263 in the wild and no public exploit code.

## Other vulnerabilities in Qemu Qemu

- [CVE-2022-36648](https://insights.sbomify.com/CVE-2022-36648/): Null dereference in n/a (critical 10.0)
- [CVE-2015-8556](https://insights.sbomify.com/CVE-2015-8556/): Race condition in n/a (critical 10.0)
- [CVE-2009-3616](https://insights.sbomify.com/CVE-2009-3616/): Use after free in n/a (critical 9.9)
- [CVE-2019-12928](https://insights.sbomify.com/CVE-2019-12928/): Command injection in n/a (critical 9.8)
- [CVE-2019-12929](https://insights.sbomify.com/CVE-2019-12929/): Command injection in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-20263 and fix it if it is affected.

What it is: CVE-2021-20263 is a low-severity vulnerability in Qemu Qemu, published on 9 Mar 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-2021-20263 in the wild and no public exploit code.

Affected product: Qemu Qemu. 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-2021-20263/ (Markdown: https://insights.sbomify.com/CVE-2021-20263/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20263
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20263
- OSV: https://osv.dev/vulnerability/CVE-2021-20263

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-20263/
Summaries are generated and may contain mistakes; the official description above is the record.