# CVE-2026-89538: SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

> An attacker with a valid GSS context can cause a denial of service by sending malformed Kerberos v2 tokens that leave the XDR buffer in an inconsistent state.

- **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-100070, BELL-CVE-2026-89538, DEBIAN-CVE-2026-89538, GHSA-3r47-q2hr-fjxp, UBUNTU-CVE-2026-89538

## What is CVE-2026-89538?

CVE-2026-89538 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: Reject krb5 v2 wrap tokens with oversized ec field

gss_krb5_unwrap_v2() sets buf->len to a logical
length, which can be much smaller than head[0].iov_len
(the allocated receive-page capacity).  It then calls
xdr_buf_trim() with a trim length derived from the 16-bit
"extra count" (ec) field in the Kerberos v2 token header.

The ec field is authenticated by the post-decrypt memcmp()
against the encrypted header copy, so a randomly-mutated
value is rejected.  However, any peer holding a valid GSS
context can legitimately encrypt a token whose ec exceeds
the plaintext length.  Per RFC 4121, such a token is
structurally malformed.

Although xdr_buf_trim() now clamps the buf->len subtraction
to avoid unsigned underflow, the buffer is still left in a
semantically invalid state (zero length, inconsistent iov
lengths) when ec is oversized.

Reject these tokens before calling xdr_buf_trim(), giving
callers a well-defined GSS_S_DEFECTIVE_TOKEN error and
keeping the xdr_buf internally consistent.  The wrapped blob
begins at a nonzero offset -- both callers pass len as
offset + opaque_len -- so buf->len still counts the offset
bytes that precede the blob.  Compare the trim length
against the remaining wrapped segment, buf->len - offset,
rather than the whole buffer; comparing against buf->len
alone leaves an offset-wide window in which an oversized ec
passes the test and xdr_buf_trim() cuts into the bytes ahead
of the blob.

## Which versions are affected by CVE-2026-89538?


## How do I fix CVE-2026-89538?

Apply the vendor's patch.

## Is CVE-2026-89538 being exploited?

There is no known exploitation of CVE-2026-89538 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89538 and fix it if it is affected.

What it is: CVE-2026-89538 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-2026-89538 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-2026-89538/ (Markdown: https://insights.sbomify.com/CVE-2026-89538/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89538
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89538
- OSV: https://osv.dev/vulnerability/CVE-2026-89538

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89538/
Summaries are generated and may contain mistakes; the official description above is the record.