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procmail / Sendmail - five bugs
From: lcamtuf () IDS PL (Michal Zalewski)
Date: Thu, 23 Dec 1999 11:55:52 +0100

Hope you won't be angry for cross-post?:) This mail discusses five
interesting vulnerabilities in Berkeley Sendmail and 'procmail' utility, I
think it's good to let you know - but don't panic - at least for now, we
are too lazy and we have no idea if any of these holes can be exploited -
that's why I'm sending it to VULN-DEV as well, to give some ideas to
young, brilliant hackers around :)

1. procmail

Months ago, we found a problem with LINEBUF variable defined in
.procmailrc files. By setting arbitrary values of it - eg. negative or
extremely large ones - we're able to at least overwrite memory locations
with zero value (used to properly terminate string buffer, unfortunately
there's no range checking on signed int arithmetic in C ;). Actually, it
splits in two vulnerabilities:

a) On some glibc 2.0 machines (eg. RedHat), malloc(negative_integer) won't
result in EINVAL, but with valid pointer, for which malloc_usable_size()
returns size of 12 bytes. Heap overflows possible? Hmm, at least SEGVs in

b) With glibc 2.0/2.1, there's also some way to overwrite mem with '\0'
due to lack of range checking and signed<->unsigned integer conversion.
Nasty memory hacking required to exploit it.

2. Sendmail

Another time, quite old vulnerabilities, but, as we haven't reported it
till now, hopefully present in newest releases as well:

a) Sendmail (tested with 8.9.3 and previous) allows you to put mail
   addressed to eg. '|/bin/sh' (or any file) into mail queue. Fortunately,
   this queue file should contain also line like 'Croot' to be processed
   properly, while we have no idea how to put it there. But, anyway,
   seems to be dangerous - Sendmail should reject such crap immediately:

   /usr/sbin/sendmail -O 'DeliveryMode=d' '""|/bin/sh'

  (without these double-quotes, it _will_ immediately drop your message)

b) Into queue files (qf*), Sendmail stores internal delivery information,
   user-supplied headers (Hxxx) and server-added headers (H?x?xxx). These
   headers are handled in several different ways, depending on letter
   between '?' - but, in general, are used to store default values for
   other headers (like 'Date:' and 'Return-Path:'). But, headers like:

   ?D?My-Own-Header: junk

   are allowed in messages, and are put in unchanged form into queue
   files, allowing possible conflicts or MTA confusion :P For example,
   header '???This-Causes-Error: test' causes odd error messages during
   message processing. What does it mean? Probably nothing dangerous,
   but I haven't spend enough time investigating this issue to be
   sure, maybe it's some way to cause undesirable MTA behaviour.

c) Sendmail won't put X-Authentication-Warning header (eg. about
   protocol violations, suspected options etc) into message if there's
   one already present - no matter how stupid it is. Minor bug.

Michal Zalewski [lcamtuf () ids pl] [link / marchew] [dione.ids.pl SYSADM]
[Marchew Industries] ! [http://lcamtuf.na.export.pl] bash$ :(){ :|:&};:
[voice phone: +48 22 813 25 86] <=-=> [cellular phone: +48 501 4000 69]
Iterowac jest rzecza ludzka, wykonywac rekursywnie - boska [P. Deutsch]

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