CVE-2026-99275 in Linux
Summary
by MITRE • 11/07/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: reject short SESSION_SETUP responses
smb2_check_message() accepts frames shorter than struct smb2_pdu when the SMB2 status is non-zero, before the minimum-PDU check added by b4694f269e66d. A 64-byte STATUS_MORE_PROCESSING_REQUIRED SESSION_SETUP response is therefore accepted even though it cannot contain the SecurityBufferOffset and SecurityBufferLength fields.
The parser then reads those fields from stale bytes left in the reused receive buffer by an earlier frame. With a long stale SecurityBufferLength, decode_ntlmssp_challenge() copies up to about 64 KB from beyond the receive object and sends the data back to the server in the NTLMv2 AUTHENTICATE target info.
Reject short MPR SESSION_SETUP responses before the short-error path, and validate SecurityBufferOffset + SecurityBufferLength against the received response length before decoding the challenge.
The ordering bypass is not new: the short-error early return predates b4694f269e66d, and the unvalidated SecurityBufferLength read predates the 2016 raw NTLMSSP split in 166cea4dc3a4, so all supported stable trees are affected. Reproduced on 6.6.87.2; the path is unchanged on mainline.
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Analysis
by VulDB Data Team • 11/07/2026
The Linux kernel SMB client implementation contains a critical logic flaw within the message validation sequence that allows for out-of-bounds memory reads and potential information disclosure. The vulnerability resides in the smb2_check_message function, which is responsible for validating incoming Server Message Block version 2 protocol frames before they are processed by higher-level parsing routines. Specifically, when an SMB2 server responds with a non-zero status code indicating an error condition such as STATUS_MORE_PROCESSING_REQUIRED during the SESSION_SETUP phase, the client incorrectly accepts response frames that are shorter than the minimum required size for a standard struct smb2_pdu. This behavior stems from an early exit path designed to handle short error responses quickly, which bypasses subsequent length validation checks introduced in earlier kernel revisions. Consequently, a malicious or misconfigured server can send a minimal 64-byte SESSION_SETUP response that lacks the necessary SecurityBufferOffset and SecurityBufferLength fields required for NTLMSSP authentication negotiation.
The operational impact of this flaw is severe due to how the SMB client handles buffer reuse and memory allocation during protocol parsing. When the parser encounters these undersized responses, it proceeds to decode the NTLMSSP challenge without verifying that the referenced security buffers actually exist within the received data. The SecurityBufferOffset and SecurityBufferLength fields are read from stale bytes remaining in the receive buffer from a previous network frame. If an attacker can control or predict this residual data, they can craft a scenario where these values point to memory locations far beyond the current packet boundary. Specifically, a crafted long SecurityBufferLength value causes the decode_ntlmssp_challenge function to copy up to approximately 64 kilobytes of kernel memory from outside the receive object into an internal buffer. This copied data is subsequently included in the NTLMv2 AUTHENTICATE message sent back to the server as part of the target info field, effectively turning the vulnerability into a primitive for reading arbitrary kernel memory and potentially exfiltrating sensitive information such as cryptographic keys or other process data.
This issue aligns with CWE-134: Use of Externally-Controlled Format String and more accurately CWE-20: Improper Input Validation, as the core failure is the lack of rigorous boundary checking on protocol fields derived from untrusted network input. In terms of attack vectors, this vulnerability facilitates data exfiltration through a remote code execution context, mapping to MITRE ATT&CK techniques such as T1537: Transfer Data to Cloud Account or more specifically memory scraping via T1005: Data from Local System if the attacker can leverage the leaked information for further lateral movement. The flaw is not limited to specific kernel versions but affects all supported stable trees because both the early return logic and the unvalidated field read predate significant refactoring efforts in 2016 regarding raw NTLMSSP handling. This persistence indicates a deep-rooted architectural oversight in how error paths are handled relative to security buffer validation.
Mitigation requires immediate patching of the affected Linux kernel versions by vendors who maintain stable releases for enterprise distributions. The fix involves rejecting short SESSION_SETUP responses that indicate more processing is required before entering the short-error path, thereby ensuring that all subsequent parsing steps operate on valid data structures. Additionally, developers must implement strict validation checks to ensure that the sum of SecurityBufferOffset and SecurityBufferLength does not exceed the total length of the received response frame prior to decoding any challenge information. Until patches are applied, administrators should consider network-level controls such as firewalls or intrusion detection systems configured to drop SMB traffic from untrusted sources or detect anomalous packet sizes during authentication phases. Regular auditing of kernel updates and prioritizing security-focused patch deployments is essential to mitigate the risk of memory disclosure attacks exploiting this specific protocol parsing weakness.