# CVE-2021-3492: Ubuntu linux kernel shiftfs file system double free vulnerability

> An attacker can exploit a double-free vulnerability in the Ubuntu shiftfs file system to cause denial of service or execute arbitrary code with kernel privileges.

- **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:** Not known
- **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:** 17 Sep 2024
- **Also known as:** GHSA-hr5j-7qj8-mpp3, LSN-0077-1, UBUNTU-CVE-2021-3492

## What is CVE-2021-3492?

CVE-2021-3492 is a high-severity use after free 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

Shiftfs, an out-of-tree stacking file system included in Ubuntu Linux kernels, did not properly handle faults occurring during copy_from_user() correctly. These could lead to either a double-free situation or memory not being freed at all. An attacker could use this to cause a denial of service (kernel memory exhaustion) or gain privileges via executing arbitrary code. AKA ZDI-CAN-13562.

Weakness: CWE-415 (Double Free), CWE-401 (Missing Release of Memory after Effective Lifetime)

## Which versions are affected by CVE-2021-3492?

It affects 1 product.


Products: Canonical Ubuntu Linux.

## How do I fix CVE-2021-3492?

Apply the vendor's patch.

## Is CVE-2021-3492 being exploited?

There is no known exploitation of CVE-2021-3492 in the wild and no public exploit code.

Public exploit code:

- PoC for CVE-2021-3492 used at Pwn2Own 2021 (github_poc): https://github.com/synacktiv/CVE-2021-3492

## 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-2021-3492 and fix it if it is affected.

What it is: CVE-2021-3492 is a high-severity use after free 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: There is no known exploitation of CVE-2021-3492 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-2021-3492/ (Markdown: https://insights.sbomify.com/CVE-2021-3492/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3492
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3492
- OSV: https://osv.dev/vulnerability/CVE-2021-3492

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-3492/
Summaries are generated and may contain mistakes; the official description above is the record.