>We have found an issue in the XNU kernel of macOS High Sierra wherein an unprivileged user can terminate all running processes using the kill system call. In short, a completely unprivileged user can bring down the entire system with kill(-1, SIGKILL) (and, in a shell, kill SIGKILL -1), so long as there is at least one other process running owned by that user. In some cases we've seen it take a few tries in a loop to actually trigger the issue.
>We have reported the issue to Apple, who do not see it as a security concern. On its own the ability to easily bring down a multi-user system is concerning, but the fact that we found this accidentally and that the behavior is exactly what you'd expect if there were no permissions check for the kill call at all leads us to believe that there is likely more that can be done to exploit this issue. Some reports include log messages showing services being killed prior to the system breaking, though this has been difficult to reproduce.
https://archive.fo/g8s04 ( https://groups.google.com/forum/#!topic/nix-devel/KV9eomIdxWE )