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fetchmail-SA-2021-02: STARTTLS session encryption bypassing

Topics:		fetchmail fails to enforce an encrypted connection

Author:		Matthias Andree
Version:	1.0
Announced:	2021-08-26 (0.9), 2021-09-13 (1.0)
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:	2021-09-13	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
2021-08-27 0.9.1 update references to 6.4.22.rc2.
2021-08-29 0.9.2 update references to 6.4.22.rc3 and correct 0.9.1 date.
2021-09-13 1.0	remove reference to rc3 from "corrected in" and bump v. to 1.0.


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@gmx.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
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