# CVE-2026-89551: SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow

> An integer underflow in the Linux kernel SUNRPC module can cause buffer length corruption, potentially leading to denial of service or memory safety issues.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100338, BELL-CVE-2026-89551, DEBIAN-CVE-2026-89551, GHSA-7gqj-379q-cvhq, UBUNTU-CVE-2026-89551

## What is CVE-2026-89551?

CVE-2026-89551 is a critical vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

## Which versions are affected by CVE-2026-89551?


## How do I fix CVE-2026-89551?

Apply the vendor's patch.

## Is CVE-2026-89551 being exploited?

There is no known exploitation of CVE-2026-89551 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89551 and fix it if it is affected.

What it is: CVE-2026-89551 is a critical vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89551 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89551/ (Markdown: https://insights.sbomify.com/CVE-2026-89551/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89551
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89551
- OSV: https://osv.dev/vulnerability/CVE-2026-89551

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89551/
Summaries are generated and may contain mistakes; the official description above is the record.