fetchmail-SA-2010-01: Heap overrun in verbose SSL cert' info display.

Topics:		Heap overrun in verbose SSL certificate information display.

Author:		Matthias Andree
Version:	1.0
Announced:
Type:		malloc() Buffer overrun with printable characters
Impact:		Code injection (difficult).
Danger:		low
CVSSv2 vectors:

CVE Name:
URL:		http://www.fetchmail.info/fetchmail-SA-2010-01.txt
Project URL:	http://www.fetchmail.info/

Affects:	fetchmail releases 6.3.11, 6.3.12, and 6.3.13

Not affected:	fetchmail release 6.3.14 and newer

Corrected:	2010-02-04 fetchmail SVN (r5467)


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

2010-02-04 0.1	first draft (visible in SVN)


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. It supports SSL and TLS security layers through
the OpenSSL library, if enabled at compile time and if also enabled at
run time.


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

In verbose mode, fetchmail prints X.509 certificate subject and issuer
information to the user, and counts and allocates a malloc() buffer for
that purpose.

If the material to be displayed contains characters with high bit set
and the platform treats the "char" type as signed, this can cause a heap
buffer overrun because non-printing characters are escaped as
\xFF..FFnn, where nn is 80..FF in hex.

This might be exploitable to inject code if
- fetchmail is run in verbose mode
AND
- the host running fetchmail considers char unsigned
AND
- the server uses malicious certificates with non-printing characters
  that have the high bit set
AND
- these certificates manage to inject shell-code that consists purely of
  printable characters.

It is believed to be difficult to achieve all this.


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 6.3.13, recompile and reinstall it.

b. Install fetchmail 6.3.14 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 without and verbose options.


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

(C) Copyright 2010 by Matthias Andree, <matthias.andree@gmx.de>.
Some rights reserved.

This work is licensed under the Creative Commons
Attribution-Noncommercial-No Derivative Works 3.0 Germany License.
To view a copy of this license, visit
http://creativecommons.org/licenses/by-nc-nd/3.0/de/ or send a letter to

Creative Commons
171 Second Street
Suite 300
SAN FRANCISCO, CALIFORNIA 94105
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
==============================

Note that when taking this from a GnuPG clearsigned file, the lines
starting with a "-" character are prefixed by another "- " (dash +
blank) combination. Either feed this file through GnuPG to strip them,
or strip them manually. You may want to use the "-p1" flag to patch.

Whitespace differences can usually be ignored by invoking "patch -l",
so try this if the patch does not apply.

--- a/sdump.c
+++ b/sdump.c
@@ -36,7 +36,7 @@ char *sdump(const char *in, size_t len)
 	if (isprint((unsigned char)in[i])) {
 	    *(oi++) = in[i];
 	} else {
-	    oi += sprintf(oi, "\\x%02X", in[i]);
+	    oi += sprintf(oi, "\\x%02X", (unsigned char)in[i]);
 	}
     }
     *oi = '\0';

END OF fetchmail-SA-2010-01.txt