# CVE-2026-80557: libceph: fix OOB read in decode_watchers() via missing bounds check

> A malicious OSD can trigger an out-of-bounds read in the Linux kernel Ceph client, potentially leaking memory or 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:** 26 Aug 2026
- **Updated:** 16 Oct 2026
- **Also known as:** AZL-97763, BELL-CVE-2026-80557, DEBIAN-CVE-2026-80557, GHSA-j5wf-cpjp-5vx9, UBUNTU-CVE-2026-80557

## What is CVE-2026-80557?

CVE-2026-80557 is a critical memory corruption vulnerability in Linux, published on 26 Aug 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: fix OOB read in decode_watchers() via missing bounds check

ceph_start_decoding() validates that struct_len bytes remain in the
buffer after the encoding header, but accepts struct_len=0 as valid:
ceph_decode_need(p, end, 0, bad) always passes. When a malicious or
compromised OSD sends an obj_list_watch_response_t reply with
struct_len=0, ceph_start_decoding() returns success with p == end,
leaving zero bytes guaranteed for subsequent reads.

The immediately following ceph_decode_32(p) in decode_watchers() has
no preceding bounds check. With p == end this is a 4-byte read past
the validated buffer boundary. The garbage value is then passed
directly to kzalloc_objs() as the watcher count.

The sibling function decode_watcher() already uses the safe variants
(ceph_decode_copy_safe, ceph_decode_64_safe, ceph_decode_skip_32)
after its own ceph_start_decoding() call. decode_watchers() is the
only site that uses the bare variant, confirming an oversight.

Fix by replacing ceph_decode_32(p) with ceph_decode_32_safe(p, end,
*num_watchers, bad), consistent with the established pattern.

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph
deployment (e.g. cloud) can trigger this against any kernel client
that calls CEPH_OSD_OP_LIST_WATCHERS, without any further privileges
beyond OSD session establishment.

[ idryomov: trim changelog ]

## Which versions are affected by CVE-2026-80557?


## How do I fix CVE-2026-80557?

Apply the vendor's patch.

## Is CVE-2026-80557 being exploited?

There is no known exploitation of CVE-2026-80557 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80557 and fix it if it is affected.

What it is: CVE-2026-80557 is a critical memory corruption vulnerability in Linux, published on 26 Aug 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-2026-80557 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-2026-80557/ (Markdown: https://insights.sbomify.com/CVE-2026-80557/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80557
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80557
- OSV: https://osv.dev/vulnerability/CVE-2026-80557

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80557/
Summaries are generated and may contain mistakes; the official description above is the record.