fetchmail-SA-2008-01: Crash on large log messages in verbose mode

Topics:    Crash in large log messages in verbose mode.

Author:    Matthias Andree
Version:  1.0
Announced:  2008-06-17
Type:    Dereferencing garbage pointer trigged by outside circumstances
Impact:    denial of service possible
Danger:    low
CVSS V2 vector: (AV:N/AC:M/Au:N/C:N/I:N/A:C/E:P/RL:O/RC:C)

Credits:  Petr Uzel (fix), Petr Cerny (analysis), Gunter Nau (bug report)
CVE Name:  CVE-2008-2711
URL:    http://www.fetchmail.info/fetchmail-SA-2008-01.txt
Project URL:  http://www.fetchmail.info/

Affects:  fetchmail release < 6.3.9 exclusively

Not affected:  fetchmail release 6.3.9 and newer
    systems without varargs (stdargs.h) support.

Corrected:  2008-06-13 fetchmail SVN (rev 5193)

References:  <https://bugzilla.novell.com/show_bug.cgi?id=354291>
    <http://developer.berlios.de/patch/?func=detailpatch&patch_id=2492&group_id=1824>


0. Release history
==================

2008-06-13 1.0  first draft for MITRE/CVE (visible in SVN,
    posted to oss-security)
2008-06-17 1.0  published on http://www.fetchmail.info/


1. Background
=============

fetchmail is a software package to retrieve mail from remote POP2, POP3,
IMAP, ETRN or ODMR servers and forward it to local SMTP, LMTP servers or
message delivery agents.

fetchmail ships with a graphical, Python/Tkinter based configuration
utility named "fetchmailconf" to help the user create configuration (run
control) files for fetchmail.


2. Problem description and Impact
=================================

Gunter Nau reported fetchmail crashing on some messages; further
debugging by Petr Uzel and Petr Cerny at Novell/SUSE Czech Republic
dug up that this happened when fetchmail was trying to print, in -v -v
verbose level, headers exceeding 2048 bytes. In this situation,
fetchmail would resize the buffer and fill in further parts of the
message, but forget to reinitialize its va_list typed source pointer,
thus reading data from a garbage address found on the stack at
addresses above the function arguments the caller passed in; usually
that would be the caller's stack frame.

It is unknown whether code can be injected remotely, but given that
the segmentation fault is caused by read accesses, the relevant data
is not under the remote attacker's control and no buffer overrun
situation is present that would allow altering program /flow/, it is
deemed rather unlikely that code can be injected.

Note that the required -vv configuration at hand is both non-default
and also not common in automated (cron job) setups, but usually used
in manual debugging, so not many systems would be affected by the
problem. Nonetheless, in vulnerable configurations, it is remotely
exploitable to effect a denial of service attack.



3. Solution
===========

There are two alternatives, either of them by itself is sufficient:

a. Apply the patch found in section B of this announcement to
   fetchmail 6.3.8, recompile and reinstall it.

b. Install fetchmail 6.3.9 or newer after it will have become available.
   The fetchmail source code is always available from
   <http://developer.berlios.de/project/showfiles.php?group_id=1824>.


4. Workaround
=============

Run fetchmail at low verbosity, avoid using two or three -v arguments;
internal messages are short and do not contain external message
sources so they do not cause buffer resizing. It is recommended to
replace the vulnerable code by a fixed version (see previous
section 3. Solution) as soon as reasonably possible.


A. Copyright, License and Warranty
==================================

(C) Copyright 2008 by Matthias Andree, <matthias.andree@gmx.de>.
Some rights reserved.

This work is licensed under the Creative Commons
Attribution-NonCommercial-NoDerivs German License. To view a copy of
this license, visit http://creativecommons.org/licenses/by-nc-nd/2.0/de/
or send a letter to Creative Commons; 559 Nathan Abbott Way;
Stanford, California 94305; USA.

THIS WORK IS PROVIDED FREE OF CHARGE AND WITHOUT ANY WARRANTIES.
Use the information herein at your own risk.



B. Patch to remedy the problem
==============================

diff --git a/report.c b/report.c
index 31d4e48..2a731ac 100644
--- a/report.c
+++ b/report.c
@@ -238,11 +238,17 @@ report_build (FILE *errfp, message, va_alist)
     rep_ensuresize();
 
 #if defined(VA_START)
-    VA_START (args, message);
     for ( ; ; )
     {
+  /*
+   * args has to be initialized before every call of vsnprintf(), 
+   * because vsnprintf() invokes va_arg macro and thus args is 
+   * undefined after the call.
+   */
+  VA_START(args, message);
   n = vsnprintf (partial_message + partial_message_size_used, partial_message_size - partial_message_size_used,
            message, args);
+  va_end (args);
 
   if (n >= 0
       && (unsigned)n < partial_message_size - partial_message_size_used)
@@ -254,7 +260,6 @@ report_build (FILE *errfp, message, va_alist)
   partial_message_size += 2048;
   partial_message = REALLOC (partial_message, partial_message_size);
     }
-    va_end (args);
 #else
     for ( ; ; )
     {

END OF fetchmail-SA-2008-01.txt