ssh in OpenSSH before 10.1 allows control characters in usernames that originate from certain possibly untrusted sources, potentially leading to code execution when a ProxyCommand is used. The untrusted sources are the command line and %-sequence expansion of a configuration file. (A configuration file that provides a complete literal username is not categorized as an untrusted source.)
Exploitability
AV:LAC:HPR:LUI:NScope
S:UImpact
C:LI:LA:NCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N