# CVE-2026-89651: ceph: bound MDSCapAuth path and fs_name decode in handle_session()

> A malicious Ceph Metadata Server can trigger an out-of-bounds read in the Linux kernel client during session setup, causing a crash.

- **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:** BELL-CVE-2026-89651, DEBIAN-CVE-2026-89651, GHSA-vqxr-6xh4-gccq, UBUNTU-CVE-2026-89651

## What is CVE-2026-89651?

CVE-2026-89651 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:

ceph: bound MDSCapAuth path and fs_name decode in handle_session()

handle_session() decodes the MDSCapAuth records carried by a
CEPH_SESSION_OPEN message (msg_version >= 6). For each record the
match.path and match.fs_name byte strings are read by first decoding a
32-bit length and then copying that many bytes with the bare
ceph_decode_copy(). Unlike the surrounding fields, which all use the
_safe decode variants, these two copies are not preceded by a
ceph_decode_need() bounds check, and the enclosing MDSCapAuth and
MDSCapMatch struct_len fields are skipped rather than enforced as an
upper bound. A length larger than the bytes remaining in the message
front makes ceph_decode_copy() read past the end of the front buffer.

The message front is a dedicated allocation (ceph_msg_new2() ->
kvmalloc), so the over-read runs off that object. A malicious or
compromised MDS can trigger this with the first post-connect message on
mount, with no client-side user interaction; under KASAN it is reported
as a slab-out-of-bounds read in handle_session().

Impact: a malicious MDS can force the kernel client to read up to 4 GiB
past the message front allocation during session setup, crashing the
client (out-of-bounds read).

Switch both copies to ceph_decode_copy_safe(), which performs the
ceph_decode_need() bounds check before the copy and branches to the
existing bad label, matching the rest of the decoder and the error path
that frees the partially decoded cap_auths array.

## Which versions are affected by CVE-2026-89651?


## How do I fix CVE-2026-89651?

Apply the vendor's patch.

## Is CVE-2026-89651 being exploited?

There is no known exploitation of CVE-2026-89651 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89651 and fix it if it is affected.

What it is: CVE-2026-89651 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-2026-89651 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-2026-89651/ (Markdown: https://insights.sbomify.com/CVE-2026-89651/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89651
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89651
- OSV: https://osv.dev/vulnerability/CVE-2026-89651

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89651/
Summaries are generated and may contain mistakes; the official description above is the record.