Problem Description:
sigqueue(2) was marked as permitted in capability mode with the
introduction of Capsicum in 2011, but the implementation of
kern_sigqueue did not include a capability mode check restricting
signal delivery to the calling process's own PID.
Impact:
A process in capability mode can use sigqueue(2) to send signals
to any process it could signal following standard Unix permissions,
bypassing the Capsicum sandbox restriction. A compromised sandboxed
process could interfere with other processes, for example by sending
SIGKILL or SIGSTOP. This could be any process running as the same
user, or any process, for a superuser sandboxed process.