CVE-2026-89631
Summary
by MITRE • 09/11/2026
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.
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Analysis
by VulDB Data Team • 09/11/2026
The Linux kernel SMB client implementation contains a critical buffer over-read vulnerability within the CIFSTCon function, specifically triggered during the processing of tree connect responses from remote servers. This flaw stems from an improper handling of the ByteCount field in the Server Message Block protocol header. The code attempts to bound its string length calculation using strnlen by subtracting two from the server-provided ByteCount value. When a malicious or malformed SMB response contains a ByteCount of zero or one, this arithmetic operation results in a negative integer due to signed integer underflow. This negative value is then implicitly converted to an unsigned size_t type, resulting in a massive positive number that effectively disables any bounds checking on the subsequent memory read operations.
The operational impact of this vulnerability allows for out-of-bounds reads from kernel slab objects allocated for SMB request processing. Specifically, the miscalculated length leads to a wraparound in the bytes_left variable, which is defined as an unsigned 16-bit integer. This results in a bound check that permits reading up to sixty-five thousand five hundred thirty-five bytes against a cifs_req_poolp object that is significantly smaller, typically around sixteen kilobytes or less depending on configuration and kernel version. Consequently, the vulnerability enables the extraction of arbitrary kernel memory contents beyond the intended buffer boundaries. This leaked data can include sensitive information such as stack pointers, cryptographic keys, or other process-specific data residing in adjacent slab caches.
The extracted kernel memory is subsequently exposed to user space through a specific debug interface within the CIFS filesystem driver. The over-read bytes are written into the nativeFileSystem field of the tree connection structure and become accessible via the /proc/fs/cifs/DebugData file system entry. This pathway provides an unprivileged local attacker with a reliable mechanism to read kernel memory, potentially facilitating further exploitation steps such as information disclosure or bypassing security mechanisms that rely on unpredictable kernel addresses. The vulnerability is classified under CWE-125 Out-of-bounds Read and aligns with ATT&CK technique T1083 File and Directory Discovery when considering the method of data exfiltration through debug interfaces, although its primary classification remains an information disclosure flaw due to the nature of the memory access.
Mitigation for this vulnerability requires applying kernel updates that include patches addressing the SMB client parsing logic. The fix involves rejecting tree connect responses where the ByteCount is less than two bytes, as no conformant SMB response should carry fewer bytes in this field. This validation ensures that the arithmetic operation remains within safe positive integer bounds before proceeding with string length calculations. System administrators should ensure their Linux distributions are updated to versions containing these security patches and monitor for any unusual activity related to access of debug data files if patching is not immediately possible, although disabling such interfaces entirely may impact legitimate debugging efforts.