CVE-2026-64389 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: validate NTLMv2 response before updating session key
ksmbd_auth_ntlmv2() derives the NTLMv2 session key into sess->sess_key before it verifies the NTLMv2 response. ksmbd_decode_ntlmssp_auth_blob() then continues into KEY_XCH even when ksmbd_auth_ntlmv2() failed.
With SMB3 multichannel binding, the failed authentication operates on an existing session and the session setup error path does not expire binding sessions. A client can send a binding session setup with a bad NT proof and KEY_XCH and still modify sess->sess_key before STATUS_LOGON_FAILURE is returned.
Relevant path:
smb2_sess_setup() -> conn->binding = true -> ntlm_authenticate() -> session_user() -> ksmbd_decode_ntlmssp_auth_blob() -> ksmbd_auth_ntlmv2() -> calc_ntlmv2_hash() -> hmac_md5_usingrawkey(..., sess->sess_key) -> crypto_memneq() returns mismatch -> KEY_XCH arc4_crypt(..., sess->sess_key, ...) -> out_err without expiring the binding session
Derive the base session key into a local buffer and copy it to sess->sess_key only after the proof matches. Return immediately on authentication failure so KEY_XCH is only processed after successful authentication.
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability resides in the ksmbd implementation within the Linux kernel where improper validation of NTLMv2 responses leads to potential session key manipulation during SMB3 multichannel operations. This flaw occurs when the authentication process attempts to derive the NTLMv2 session key before verifying the response, creating a window where malicious actors can exploit the system even after failed authentication attempts.
The technical implementation demonstrates a critical order of operations error in the authentication flow where ksmbd_auth_ntlmv2() function computes and stores the session key into sess->sess_key before performing any validation checks on the NTLMv2 response. This premature key derivation allows subsequent processing steps to operate with potentially compromised session state information, particularly when dealing with binding sessions that maintain their state across multiple connections.
The operational impact becomes significant in SMB3 multichannel environments where binding sessions can persist even after authentication failures. When a client submits a binding session setup with invalid NT proof data and proceeds through the KEY_XCH phase, the system continues processing with the compromised session key rather than properly rejecting the authentication attempt. This creates a persistent vulnerability that allows attackers to manipulate session state information without proper authorization.
According to CWE classification, this represents a weakness in the order of operations where authentication validation occurs after sensitive state changes have already been committed. The vulnerability aligns with ATT&CK technique T1566 which involves credential access through network protocols and authentication manipulation. The improper handling creates an opportunity for privilege escalation and session hijacking attacks.
The fix requires modifying the authentication flow to derive the base session key into a local buffer first, only copying this validated key to the permanent session storage after successful proof verification. This approach prevents any state modification during failed authentication attempts and ensures that cryptographic operations occur only after proper validation has been completed. The implementation should also include immediate return on authentication failure to prevent further processing steps like KEY_XCH from executing with compromised session data.
The solution addresses the fundamental security principle that sensitive operations should not proceed until all validation checks have been successfully completed, preventing the scenario where session state can be manipulated through failed authentication attempts. This remediation approach ensures proper separation of concerns between authentication validation and session state management, eliminating the window where malicious actors can exploit this timing vulnerability in SMB3 multichannel environments.