-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 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); END OF fetchmail-SA-2009-01.txt -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (GNU/Linux) iEYEARECAAYFAkp6GP8ACgkQvmGDOQUufZVuQwCgsD/kO/+KHC0/gopx/uiQr9V7 mXAAnjH6G5DfcxAjCzjmt9DKZHGsqoNv =6zGh -----END PGP SIGNATURE-----