CVE-2026-89631

/// Technical 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.

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References