# CVE-2026-89550: SUNRPC: svcauth_gss: enforce krb5 token minimum length

> A missing minimum length check in the Linux kernel SUNRPC module allows remote attackers to cause a denial of service via division by zero.

- **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-100776, BELL-CVE-2026-89550, DEBIAN-CVE-2026-89550, GHSA-72qv-m5jw-m97f, UBUNTU-CVE-2026-89550

## What is CVE-2026-89550?

CVE-2026-89550 is a critical denial of service 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: svcauth_gss: enforce krb5 token minimum length

svcauth_gss_unwrap_priv() validates only an upper bound on the
wire-supplied opaque length before handing the buffer to
gss_unwrap():

    if (len > xdr_stream_remaining(xdr))
            goto unwrap_failed;
    offset = xdr_stream_pos(xdr);
    ...
    maj_stat = gss_unwrap(ctx, offset, offset + len, buf);

The wire value `len` flows unchanged as the upper bound into the
krb5 unwrap path, so a len in [0, 16] passes this check and is
handed to gss_unwrap(). For a krb5 v2 context that lands in
gss_krb5_unwrap_v2(), which reads the 16-byte RFC 4121 token
header fields at ptr+4 and ptr+6 and then calls rotate_left()
before any integrity check. With a sub-header length the header
reads run past the token, and _rotate_left()'s `shift %= buf->len`
path can divide by zero when buf->len has been driven to zero by
the truncated token. A header-only token (len == 16) is equally
invalid: with a non-zero RRC field and the opaque blob ending at
the XDR buffer boundary, rotate_left() builds a zero-length
subbuffer, reaching the same division.

Reject the token at the server entry point before it reaches the
krb5 unwrap core. A valid sealed RFC 4121 token must contain
the 16-byte header plus at least some encrypted payload.

Fix by adding a minimum-length check immediately after the
existing upper-bound check:

    if (len <= GSS_KRB5_TOK_HDR_LEN)
            goto unwrap_failed;

## Which versions are affected by CVE-2026-89550?


## How do I fix CVE-2026-89550?

Apply the vendor's patch.

## Is CVE-2026-89550 being exploited?

There is no known exploitation of CVE-2026-89550 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89550 and fix it if it is affected.

What it is: CVE-2026-89550 is a critical denial of service 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-2026-89550 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-2026-89550/ (Markdown: https://insights.sbomify.com/CVE-2026-89550/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89550
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89550
- OSV: https://osv.dev/vulnerability/CVE-2026-89550

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89550/
Summaries are generated and may contain mistakes; the official description above is the record.