Linux Kernel EUVD-2026-38698

AF_UNIX (Unix domain sockets) is a Linux kernel IPC subsystem supporting three socket types: SOCK_STREAM, SOCK_DGRAM, and SOCK_SEQPACKET. The SIOCATMARK ioctl queries whether a socket's receive queue is positioned at the out-of-band data mark, a concept tied exclusively to MSG_OOB semantics. In AF_UNIX, MSG_OOB is only meaningful for SOCK_STREAM; the other two types already return errors when MSG_OOB is specified in sendmsg() or recvmsg(). The vulnerability is a missing input validation gate in the af_unix SIOCATMARK handler: it did not check the socket type before proceeding to inspect the receive queue. The CWE is not formally assigned in the input data, but the root cause class is improper input validation or inconsistent state handling (analogous to CWE-754: Improper Check for Unusual or Exceptional Conditions). The affected code is identified via CPE cpe:2.3:a:linux:linux:*:*:*:*:*:*:*:* and traced to a specific introductory commit (314001f0bf927015e459c9d387d62a231fe93af3) across multiple stable branches.