CVE-2026-89631 smb: client: reject a tree connect response whose byte count is too small
A malicious SMB server can trigger an out-of-bounds read in the Linux kernel SMB client, leaking kernel memory to userspace via debug data.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89631?
CVE-2026-89631 is a critical vulnerability in Linux, published on 11 Sep 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:
smb: client: reject a tree connect response whose byte count is too small
CIFSTCon() bounds its strnlen() over the byte area with the server's ByteCount minus two, which for ByteCount 0 or 1 goes negative as an int and converts to a huge size_t. The later subtraction wraps the __u16 bytes_left, and that is what bounds cifs_strndup_from_utf16(): a bound of up to 65535 against a ~16 KB cifs_req_poolp object runs off the end of the slab object, and the bytes reach userspace through tcon->nativeFileSystem in /proc/fs/cifs/DebugData.
Reject a byte area too small for what the parser consumes. Two bytes is the least it can consume, and no conformant response carries fewer. The new trace point is the 129th smb_eio_trace entry, which __mode(byte) cannot represent, so the attribute goes with it.
Which versions are affected by CVE-2026-89631?
How do I fix CVE-2026-89631?
Apply the vendor's patch.
Is CVE-2026-89631 being exploited?
There is no known exploitation of CVE-2026-89631 in the wild and no public exploit code.
Sources
Also known as AZL-100496, BELL-CVE-2026-89631, DEBIAN-CVE-2026-89631, GHSA-95qf-fgcq-pcrc, UBUNTU-CVE-2026-89631.