# CVE-2026-89656: libceph: reject buckets with mismatched CRUSH ids

> A malformed CRUSH map in the Linux kernel can cause a slab out-of-bounds write, potentially leading to code execution or denial of service.

- **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-100506, BELL-CVE-2026-89656, DEBIAN-CVE-2026-89656, GHSA-qwcm-8gh9-xc6h, UBUNTU-CVE-2026-89656

## What is CVE-2026-89656?

CVE-2026-89656 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:

libceph: reject buckets with mismatched CRUSH ids

crush_decode() stores bucket data by array slot, and the mapper later
derives the per-bucket workspace index from the decoded bucket id. A
malformed map can therefore make one bucket reuse another bucket's
workspace by encoding an id different from -1 - slot.

For uniform buckets, the second replica selection expands the source
bucket's permutation into that aliased workspace buffer. If the source
bucket is larger than the aliased bucket, the write runs past the smaller
permutation array and can escape the kvmalloc'd CRUSH workspace. KASAN
reports a slab OOB write of 4 bytes in bucket_perm_choose().

Reject buckets whose encoded id does not match their array slot. Valid
CRUSH maps already use the canonical negative id corresponding to the
bucket slot, so this restores the invariant expected by
work->work[-1 - in->id] without changing valid map behavior.

## Which versions are affected by CVE-2026-89656?


## How do I fix CVE-2026-89656?

Apply the vendor's patch.

## Is CVE-2026-89656 being exploited?

There is no known exploitation of CVE-2026-89656 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89656 and fix it if it is affected.

What it is: CVE-2026-89656 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-2026-89656 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-2026-89656/ (Markdown: https://insights.sbomify.com/CVE-2026-89656/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89656
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89656
- OSV: https://osv.dev/vulnerability/CVE-2026-89656

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89656/
Summaries are generated and may contain mistakes; the official description above is the record.