fetchmail-SA-2021-02: failure to enforce STARTTLS session encryption

Topics:		fetchmail fails to enforce an encrypted connection

Author:		Matthias Andree
Version:	0.2
Announced:	TBC
Type:		failure to enforce configured security policy
Impact:		fetchmail continues an unencrypted connection,
		thus reading unauthenticated input and sending
		information unencrypted over its transport,
		including passwords
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:	TBC (if any)
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:	TBC	Git commit hash (both needed)
		TBC	fetchmail 6.4.21 release tarball

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

2021-08-10	initial report to maintainer
2021-08-10 0.1	first draft
2021-08-13 0.2	mention --auth ssh defeated STARTTLS


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 or fully-wrapped 
connections on dedicated TCP ports for the "secure" variants, or by initiating 
a clear-text protocol exchange and then requesting a TLS negotiation in-band.

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 continues with the 
unencrypted connection, rather than flagging the situation and aborting.

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 PREAUTH mail server with --plugin, or TLS client 
certificates), will also defeat STARTTLS as result of an implementation defect.

This can cause e-mail and in the first case, also potentially passwords, to be 
exposed to eavesdropping.


3. Solutions
============

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. Alternatively, where the IMAP server supports this form of access,
fetchmail can be configured to "ssl" mode, meaning it will connect to 
a dedicated port (default: 993 for IMAP) 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 ssl mode.


Distributors are encouraged to review the NEWS file and move forward to
6.4.22, rather than backport individual security fixes, because doing so
routinely misses other fixes crucial to fetchmail's proper operation,
for which no security announcements are issued, or documentation,
or translation updates.

Fetchmail 6.4.X releases have been made with a focus on unchanged user and
program interfaces so as to avoid disruptions when upgrading from 6.3.Z or
6.4.X to 6.4.Y with Y > X.  Care was taken to not change the interface
incompatibly.


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