# CVE-2026-89551

## Summary

- **CVE ID:** CVE-2026-89551
- **Severity:** CRITICAL
- **CVSS Score:** 9.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 14, 2026

## 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.

## Affected Products

- Linux — Linux (4c190e2f913f038c9c91ee63b59cd037260ba353)
- Linux — Linux (3.9)
- Linux — Linux (0)
- Linux — Linux (6.12.109)
- Linux — Linux (6.18.50)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)
- Linux — Linux (5.10.270)
- Linux — Linux (5.15.221)
- Linux — Linux (6.1.188)
- Linux — Linux (6.6.157)

## References

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

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.52%
- **EPSS Percentile:** 43.0

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._