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fetchmail-SA-2021-01: DoS or information disclosure logging long messages

Topics:		fetchmail denial of service or information disclosure when logging long messages

Author:		Matthias Andree
Version:	1.0
Announced:	2021-07-28
Type:		missing variable initialization can cause read from bad memory 
		locations
Impact:		fetchmail logs random information, or segfaults and aborts, 
		stalling inbound mail
Danger:		low
Acknowledgment:	Christian Herdtweck, Intra2net AG, Tübingen, Germany
		for analysis and report and a patch suggestion

CVE Name:	CVE-2021-36386
URL:		https://www.fetchmail.info/fetchmail-SA-2021-01.txt
Project URL:	https://www.fetchmail.info/

Affects:	- fetchmail releases up to and including 6.4.19

Not affected:	- fetchmail releases 6.4.20 and newer

Corrected in:	b381f5ea Git commit hash

		2021-07-28 fetchmail 6.4.20 release tarball


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

2021-07-07	initial report to maintainer
2021-07-28 1.0	release


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 has long had support to assemble log/error messages that are 
generated piecemeal, and takes care to reallocate the output buffer as needed.  
In the reallocation case, i. e. when long log messages are assembled that can 
stem from very long headers, and on systems that have a varargs.h/stdarg.h 
interface (all modern systems), fetchmail's code would fail to reinitialize 
the va_list argument to vsnprintf. 

The exact effects depend on the verbose mode (how many -v are given) of 
fetchmail, computer architecture, compiler, operating system and 
configuration.  On some systems, the code just works without ill effects, some 
systems log a garbage message (potentially disclosing sensitive information), 
some systems log literally "(null)", some systems trigger SIGSEGV (signal 
#11), which crashes fetchmail, causing a denial of service on fetchmail's end.


3. Solution
===========

Install fetchmail 6.4.20 or newer.

The fetchmail source code is available from
<https://sourceforge.net/projects/fetchmail/files/>.

Distributors are encouraged to review the NEWS file and move forward to 
6.4.20, 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.

fetchmail-SA-2021-01 © 2021 by Matthias Andree 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-2021-01
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