Linux EUVD-2026-15344

The ksmbd module is a Linux kernel-space SMB (Server Message Block) server implementation used for file sharing and network communication. MAC (Message Authentication Code) comparison is a critical cryptographic operation used to verify the authenticity and integrity of SMB protocol messages. The vulnerability stems from using memcmp(), which performs byte-by-byte comparison and exits early upon finding a mismatch, creating measurable timing differences based on the position of the first differing byte. This violates the constant-time requirement essential for cryptographic comparisons outlined in CWE-208 (Information Exposure Through Timing Discrepancy). The crypto_memneq() kernel function is specifically designed to compare cryptographic values in constant time, regardless of where differences occur, preventing attackers from deriving information through side-channel analysis. Linux kernel versions across multiple stable series (referenced by six distinct stable kernel commits) require patching to replace memcmp with crypto_memneq in the ksmbd authentication path.