In FreeBSD 12.1-STABLE before r363918, 12.1-RELEASE before p8, 11.4-STABLE before r363919, 11.4-RELEASE before p2, and 11.3-RELEASE before p12, the sendmsg system call in the compat32 subsystem on 64-bit platforms has a time-of-check to time-of-use vulnerability allowing a mailcious userspace program to modify control message headers after they were validation.