Date: Fri, 27 Aug 2021 00:31:36 +0200 From: Matthias Andree <matthias.andree@....de> To: fetchmail-devel@...ts.sourceforge.net, fetchmail-users@...ts.sourceforge.net, fetchmail-announce@...ts.sourceforge.net, oss-security@...ts.openwall.com Subject: ANNOUNCE: fetchmail security announcement 2021-02 (CVE-2021-39272) - TLS bypass vulnerabilities ("NO STARTTLS") fetchmail-SA-2021-02: STARTTLS session encryption bypassing Topics: fetchmail fails to enforce an encrypted connection Author: Matthias Andree Version: 0.9 Announced: 2021-08-26 Type: failure to enforce configured security policy Impact: fetchmail continues an unencrypted connection, thus reading unauthenticated input and sending information unencrypted over its transport Danger: medium Acknowledgment: Andrew C. Aitchison for reporting this against fetchmail Damian Poddebniak, Fabian Ising, Hanno Böck, and Sebastian Schinzel for their Usenix Security 21 paper NO STARTTLS CVE Name: CVE-2021-39272 URL: https://www.fetchmail.info/fetchmail-SA-2021-02.txt Project URL: https://www.fetchmail.info/ Affects: - fetchmail releases up to and including 6.4.21 Not affected: - fetchmail releases 6.4.22 and newer Corrected in: 2021-08-26 fetchmail 6.4.22.rc1 release candidate TBD fetchmail 6.4.22 release tarball 0. History of this announcement =============================== 2021-08-10 Andrew C. Aitchison contacts fetchmail maintainer with pointer to Usenix Security 21 paper by Damian Poddebniak et al. 2021-08-16 a simplified recommendation to configure --ssl where possible (see section 3b. below) to mitigate impact was sent to the fetchmail mailing lists 2021-08-26 0.9 initial release along with fetchmail 6.4.22.rc1 1. Background ============= fetchmail is a software package to retrieve mail from remote POP3, IMAP, ETRN or ODMR servers and forward it to local SMTP, LMTP servers or message delivery agents. fetchmail supports SSL and TLS security layers through the OpenSSL library, if enabled at compile time and if also enabled at run time, in both SSL/TLS-wrapped mode on dedicated ports as well as in-band-negotiated "STARTTLS" and "STLS" modes through the regular protocol ports. 2. Problem description and Impact ================================= fetchmail permits requiring that an IMAP or POP3 protocol exchange uses a TLS-encrypted transport, in 6.4 by way of an --sslproto auto or similar configuration. This TLS encryption can be established either as Implicit TLS connection, which negotiates TLS first, or as a STARTTLS which starts as cleartext protocol exchange that gets upgraded in the same TCP stream to TLS. Without special configuration, fetchmail would opportunistically try to upgrade cleartext connections to TLS by STARTTLS, but allow cleartext protocol exchange, which is documented. IMAP also supports sessions that start in "authenticated state" (PREAUTH). In this latter case, IMAP (RFC-3501) does not permit sending STARTTLS negotiations, which are only permissible in not-authenticated state. In such a combination of circumstances (1. IMAP protocol in use, 2. the server greets with PREAUTH, announcing authenticated state, 3. the user configured TLS mandatory, 4. the user did not configure "ssl" mode that uses separate ports for Implicit SSL/TLS), fetchmail 6.4.21 and older would not encrypt the session. There was a similar situation for POP3: if the remote name contained @compuserve.com, and if the server supported a non-standard "AUTH" command without mechanism argument and if it responded with a list that contained "RPA" (also in mixed or lower case), then fetchmail would not attempt STARTTLS. While the password itself is then protected by the RPA scheme (which employs MD5 however), fetchmail 6.4.21 and older would not encrypt the session. Also, a configuration containing --auth ssh (meaning that fetchmail should not authenticate, on the assumption that the session will be pre-authenticated for instance through SSH running a mail server with --plugin, or TLS client certificates), would also defeat STARTTLS as result of an implementation defect. This affected both POP3 and IMAP. 3. Solutions ============ PREFACE: distributors backporting fixes to old versions are asked to diff the manual page and review the changes, and the NEWS file, because the manual page has been updated with newer recommendations. The same backport recommendations hold for the README.SSL file. 3a. Install fetchmail 6.4.22 or newer. The fetchmail source code is available from <https://sourceforge.net/projects/fetchmail/files/>. The Git-based source code repository is currently published via https://gitlab.com/fetchmail/fetchmail/-/tree/legacy_64 (primary) https://sourceforge.net/p/fetchmail/git/ci/legacy_64/tree/ (copy) 3b. Where the IMAP or POP3 server supports this form of access, fetchmail can be configured to use Implicit TLS, called "ssl" mode, meaning it will connect to a dedicated port (default: 993 for IMAP, 995 for POP3) and negotiate TLS without prior clear-text protocol exchange. Also, --ssl can be given on the command line, which switches all configured server statements to this Implicit TLS mode. A. Copyright, License and Non-Warranty ====================================== (C) Copyright 2021 by Matthias Andree, <matthias.andree@....de>. Some rights reserved. © Copyright 2021 by Matthias Andree. This file is licensed under CC BY-ND 4.0. To view a copy of this license, visit http://creativecommons.org/licenses/by-nd/4.0/ THIS WORK IS PROVIDED FREE OF CHARGE AND WITHOUT ANY WARRANTIES. Use the information herein at your own risk. END of fetchmail-SA-2021-02 Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)
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