Comment by munch117 | Hacker News Reader

Comment by munch117 | Hacker News Reader

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I'm slightly taken aback by the telnetd fix: The solution to the username "-f root" being interpreted as two arguments to /usr/bin/login is to add a "sanitize" function, really? I'm not seeing the sense in that. Surely in any case where the sanitize functions changes something, the login will fail. Better to error out early than to sanitize and try to hobble along.

What I'd like to know is how the arguments get interpreted like that in the first place. If I try giving that kind of argument /usr/bin/login directly, its argument parser chides me:

  $ login '-f root'
  login: illegal option --

What's telnetd doing differently? Is it invoking login via a shell?

You passed '-f root' to login (a single long string). telnetd is likely passing '-f' 'root' to login (two arguments instead of one, whether this is because it creates two, or it uses the shell (which then reparses) I don't know).

But '-f' is a valid option to login (man login):

login [-p] [-h host] [-H] [-f username|username]

...

-f Used to skip a login authentication. This option is usually used by the getty(8) autologin feature.

I was reading https://www.offsec.com/blog/cve-2026-24061/, which implies that precisely that single long string passes through getenv("USER") in the attack. The mystery is how that single long string in telnetd becomes two separate arguments for login. execv or posix_spawn certainly won't do that. So either there's a version of /usr/bin/login that parses arguments in some very sus way, or there's a shell involved somehow.

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