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

Critical 9.8 Published 11 Sep 2026 Updated 19 Oct 2026

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.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

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.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2026-89656 being exploited?

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

Sources

Also known as AZL-100506, BELL-CVE-2026-89656, DEBIAN-CVE-2026-89656, GHSA-qwcm-8gh9-xc6h, UBUNTU-CVE-2026-89656.