A flaw was found in OpenSSH. A username entered on the ssh command line can still contain '$' or a non-trailing backslash. When a configuration expands %u inside ProxyCommand or Match exec, those characters are passed into the shell.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-101836
https://bugzilla.redhat.com/show_bug.cgi?id=2553034