An issue was discovered in GNU Hurd before 0.9 20210404-9. The use of an authentication protocol in the proc server is vulnerable to man-in-the-middle attacks, which can be exploited for local privilege escalation to get full root access.
https://www.mail-archive.com/bug-hurd%40gnu.org/msg32114.html
https://lists.gnu.org/archive/html/bug-hurd/2021-05/msg00079.html