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bugtraq: fetchmail security announcement fetchmail-SA-2008-01 (C

fetchmail security announcement fetchmail-SA-2008-01 (CVE-2008-2711)

From: <ma+bt_at_nospam>
Date: Tue Jun 17 2008 - 12:58:02 GMT
To: bugtraq@securityfocus.com


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:

  1. Apply the patch found in section B of this announcement to fetchmail 6.3.8, recompile and reinstall it.
  2. 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>.
  3. 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.

  1. 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