# CVE-2026-89637

## Summary

- **CVE ID:** CVE-2026-89637
- **Severity:** CRITICAL
- **CVSS Score:** 9.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 13, 2026

## 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.

## Affected Products

- Linux — Linux (316cf94a910f6f93d43cc574359d163ccae098a3)
- Linux — Linux (3.6)
- Linux — Linux (0)
- Linux — Linux (6.18.51)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)

## References

- [CNA](https://git.kernel.org/stable/c/9eed72e9534b10a6d9f8f5146feff3db53aebdba)
- [CNA](https://git.kernel.org/stable/c/5e6533a683f6a851158d9f33fb4ea8f4f25d7f84)
- [CNA](https://git.kernel.org/stable/c/730d0bb19507b9e19c2fe5343109ac618e2fbce5)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.59%
- **EPSS Percentile:** 46.6

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._