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fetchmail-SA-2009-01: Improper SSL certificate subject verification

Topics:    Improper SSL certificate subject verification

Author:    Matthias Andree
Version:  1.0
Announced:  2009-08-06
Type:    Allows undetected Man-in-the-middle attacks against SSL/TLS.
Impact:    Credential disclose to eavesdroppers.
Danger:    medium
CVSSv2 vectors:  (AV:N/AC:M/Au:N/C:P/I:N/A:N) (E:H/RL:OF/RC:C)

CVE Name:  CVE-2009-2666
URL:    http://www.fetchmail.info/fetchmail-SA-2009-01.txt
Project URL:  http://www.fetchmail.info/

Affects:  fetchmail releases up to and including 6.3.10

Not affected:  fetchmail release 6.3.11 and newer

Corrected:  2009-08-04 fetchmail SVN (rev 5389)

References:  "Null Prefix Attacks Against SSL/TLS Certificates",
    Moxie Marlinspike, 2009-07-29, Defcon 17, Blackhat 09.

    CVE-2009-2408, Mozilla Firefox <3.5 and NSS <3.12.3
    improper handling of '\0' characters in domain names in
    the Subject CN field of X.509 certificates.


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

2009-08-05 0.1  first draft (visible in SVN)
2009-08-06 1.0  first release


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
=================================

Moxie Marlinspike demonstrated in July 2009 that some CAs would sign
certificates that contain embedded NUL characters in the Common Name or
subjectAltName fields of ITU-T X.509 certificates.

Applications that would treat such X.509 strings as NUL-terminated C
strings (rather than strings that contain an explicit length field)
would only check the part up to and excluding the NUL character, so that
certificate names such as www.good.example\0www.bad.example.com would be
mistaken as a certificate name for www.good.example.  fetchmail also had
this design and implementation flaw.

Note that fetchmail should always be forced to use strict certificate
validation through either of these option combinations:

    --sslcertck --ssl --sslproto ssl3    (for service on SSL-wrapped ports)
or
    --sslcertck --sslproto tls1          (for STARTTLS-based services)

(These are for the command line, in the rcfile, you will need to omit
the respective leading --).

The default is relaxed checking for compatibility with historic versions.


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 6.3.10, recompile and reinstall it.

b. Install fetchmail 6.3.11 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
=============

Obtain the server fingerprints through a separate secure channel and
configure them with the sslfingerprint option, and enable the sslcertck
option.


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

(C) Copyright 2009 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.

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


Index: socket.c
===================================================================
- --- ./socket.c~
+++ ./socket.c
@@ -632,6 +632,12 @@
         report(stderr, GT_("Bad certificate: Subject CommonName too long!\n"));
         return (0);
       }
+      if ((size_t)i > strlen(buf)) {
+        /* Name contains embedded NUL characters, so we complain. This is likely
+         * a certificate spoofing attack. */
+        report(stderr, GT_("Bad certificate: Subject CommonName contains NUL, aborting!\n"));
+        return 0;
+      }
       if (_ssl_server_cname != NULL) {
         char *p1 = buf;
         char *p2 = _ssl_server_cname;
@@ -643,11 +649,18 @@
          * first find a match among alternative names */
         gens = (STACK_OF(GENERAL_NAME) *)X509_get_ext_d2i(x509_cert, NID_subject_alt_name, NULL, NULL);
         if (gens) {
- -          int i, r;
- -          for (i = 0, r = sk_GENERAL_NAME_num(gens); i < r; ++i) {
- -            const GENERAL_NAME *gn = sk_GENERAL_NAME_value(gens, i);
+          int j, r;
+          for (j = 0, r = sk_GENERAL_NAME_num(gens); j < r; ++j) {
+            const GENERAL_NAME *gn = sk_GENERAL_NAME_value(gens, j);
             if (gn->type == GEN_DNS) {
               char *p1 = (char *)gn->d.ia5->data;
               char *p2 = _ssl_server_cname;
+              /* Name contains embedded NUL characters, so we complain. This
+               * is likely a certificate spoofing attack. */
+              if ((size_t)gn->d.ia5->length != strlen(p1)) {
+                report(stderr, GT_("Bad certificate: Subject Alternative Name contains NUL, aborting!\n"));
+                sk_GENERAL_NAME_free(gens);
+                return 0;
+              }
               if (outlevel >= O_VERBOSE)
                 report(stderr, "Subject Alternative Name: %s\n", p1);

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