Linux Kernel CVE-2026-31637

This vulnerability exists in the Linux kernel's RxRPC implementation, specifically in the rxkad (RX Kerberos-style Authentication and Data security) protocol handler used for securing AFS (Andrew File System) communications. The rxkad_decrypt_ticket() function uses the kernel's crypto_skcipher_decrypt() API to decrypt authentication response tickets but proceeds to parse the decryption buffer without checking the return code. In cryptographic implementations, failed decryption operations leave the buffer in an undefined state (typically containing unmodified ciphertext or garbage). By crafting a RESPONSE packet with a ticket length that is not aligned to the cipher's block size, an attacker can force decryption to fail while still controlling the bytes that the subsequent parser interprets as a 'decrypted' ticket structure. This violates the fundamental security principle that encrypted data integrity must be validated before use. The vulnerability has existed since commit 17926a79320afa9b (kernel 2.6.22 era) and affects all CPE-identified Linux kernel versions until the patches were applied. The fix adds explicit error checking and returns RXKADBADTICKET (bad ticket error code) to abort the connection when decryption fails.