Date: Thu, 23 Dec 1999 11:55:52 +0100
From: Michal Zalewski <[email protected]>
To: [email protected]Subject: procmail / Sendmail - five bugs
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
procmail :)
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 [[email protected]] [link / marchew] [dione.ids.pl SYSADM]
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