# CVE-2021-3939: Free of static data in accountsservice

> A use-after-free vulnerability in Ubuntu's accountsservice allows attackers to execute arbitrary code or cause a denial of service via the SetLanguage D-Bus function.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/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:** 17 Nov 2021
- **Updated:** 16 Sep 2024
- **Also known as:** GHSA-jxh7-q7rm-43ww, UBUNTU-CVE-2021-3939, USN-5149-1

## What is CVE-2021-3939?

CVE-2021-3939 is a high-severity use after free vulnerability in Canonical Ubuntu Linux, published on 17 Nov 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Ubuntu-specific modifications to accountsservice (in patch file debian/patches/0010-set-language.patch) caused the fallback_locale variable, pointing to static storage, to be freed, in the user_change_language_authorized_cb function. This is reachable via the SetLanguage dbus function. This is fixed in versions 0.6.55-0ubuntu12~20.04.5, 0.6.55-0ubuntu13.3, 0.6.55-0ubuntu14.1.

Weakness: CWE-763 (Release of Invalid Pointer or Reference), CWE-590 (Free of Memory not on the Heap)

## Which versions are affected by CVE-2021-3939?

It affects 2 products.


Products: Canonical Accountsservice, Canonical Ubuntu Linux.

## How do I fix CVE-2021-3939?

Apply the vendor's patch.

## Is CVE-2021-3939 being exploited?

There is no known exploitation of CVE-2021-3939 in the wild and no public exploit code.

## Other vulnerabilities in Canonical Ubuntu Linux

- [CVE-2023-3297](https://insights.sbomify.com/CVE-2023-3297/): Use after free in AccountService (high 8.1)
- [CVE-2022-1804](https://insights.sbomify.com/CVE-2022-1804/): Accountsservice incorrectly drops privileges (medium 5.5)
- [CVE-2011-4406](https://insights.sbomify.com/CVE-2011-4406/): Privilege escalation in n/a (low 3.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-3939 and fix it if it is affected.

What it is: CVE-2021-3939 is a high-severity use after free vulnerability in Canonical Ubuntu Linux, published on 17 Nov 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-2021-3939 in the wild and no public exploit code.

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-2021-3939/ (Markdown: https://insights.sbomify.com/CVE-2021-3939/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3939
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3939
- OSV: https://osv.dev/vulnerability/CVE-2021-3939

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3939/
Summaries are generated and may contain mistakes; the official description above is the record.