sendmsg() is a Linux and Unix socket system call used to submit data, control messages, or multiple buffers to a socket for processing. Unlike simpler write() calls, it can carry structured metadata through ancillary data and is common in network code, IPC, and kernel-facing interfaces such as AF_ALG.
In security work, sendmsg() matters because it is often the entry point for attacker-controlled input into privileged networking or kernel subsystems. Bugs in how the kernel handles concurrent or malformed sendmsg() requests can lead to state corruption, race conditions, memory safety issues, or even privilege escalation. Defenders look for suspicious repeated calls, unusual socket families, and failures caused by concurrent access. When a vulnerability affects sendmsg()-driven paths, patching the kernel, limiting unnecessary socket features, and reducing exposure of legacy interfaces are practical defenses.