# CVE-2026-89537: SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2

> A malicious NFS server can trigger a buffer over-read in the Linux kernel's SUNRPC module by sending short Kerberos MIC tokens, potentially causing a crash or code execution.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/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:** 21 Oct 2026
- **Also known as:** AZL-100097, BELL-CVE-2026-89537, DEBIAN-CVE-2026-89537, GHSA-8jwh-2pr5-8fq7, UBUNTU-CVE-2026-89537

## What is CVE-2026-89537?

CVE-2026-89537 is a critical memory corruption 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 short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2

gss_krb5_verify_mic_v2() reads the token ID at ptr[0..1], the flags
byte at ptr[2], and padding at ptr[3..7], then passes
ptr + GSS_KRB5_TOK_HDR_LEN and cksum_len to gss_krb5_mic_build_sg().
None of these accesses check read_token->len first.

The minimum safe token size is GSS_KRB5_TOK_HDR_LEN (16) plus
ctx->krb5e->cksum_len (12-24, depending on the enctype).  All callers
accept shorter tokens from the wire:

 - gss_unwrap_resp_integ() enforces only an upper bound
   (offset + len <= rcv_buf->len) before allocating
   mic.data = kmalloc(len) and passing it to gss_verify_mic().
   A malicious NFS server can therefore supply a short checksum
   opaque, producing a small slab allocation that the Kerberos MIC
   verifier reads past.

 - gss_validate() enforces only len <= RPC_MAX_AUTH_SIZE (400)
   before passing the wire-supplied length to
   gss_validate_seqno_mic(), which constructs a mic xdr_netobj
   and calls gss_verify_mic().

 - svcauth_gss_verify_header() enforces only
   checksum.len >= XDR_UNIT (4 bytes) before dispatching to
   gss_verify_mic().

 - svcauth_gss_unwrap_integ() checks only that the checksum fits
   in gsd->gsd_scratch.

Add a length guard at the top of gss_krb5_verify_mic_v2(), before any
ptr[] access or scatterlist construction.  Well-formed MIC tokens from
gss_krb5_get_mic_v2() already have exactly GSS_KRB5_TOK_HDR_LEN +
cksum_len bytes, so valid traffic is unaffected.

## Which versions are affected by CVE-2026-89537?


## How do I fix CVE-2026-89537?

Apply the vendor's patch.

## Is CVE-2026-89537 being exploited?

There is no known exploitation of CVE-2026-89537 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89537 and fix it if it is affected.

What it is: CVE-2026-89537 is a critical memory corruption 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-2026-89537 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-2026-89537/ (Markdown: https://insights.sbomify.com/CVE-2026-89537/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89537
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89537
- OSV: https://osv.dev/vulnerability/CVE-2026-89537

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89537/
Summaries are generated and may contain mistakes; the official description above is the record.