gnupg: [CVE-2006-6235] remotely controllable function pointer

Debian Bug report logs - #401894
gnupg: [CVE-2006-6235] remotely controllable function pointer

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Message #5 received at submit@bugs.debian.org (full text, mbox, reply):

From: Henrique de Moraes Holschuh <hmh@debian.org>

To: Debian Bug Tracking System <submit@bugs.debian.org>

Subject: gnupg: [CVE-2006-6235] remotely controllable function pointer

Date: Wed, 6 Dec 2006 15:21:50 -0200

Package: gnupg
Version: 1.4.1-1
Severity: grave
Tags: security
Justification: user security hole

http://lwn.net/Articles/212909/

From:      Werner Koch <wk-AT-g10code.com>
To:        bugtraq-AT-securityfocus.com
Subject:   GnuPG: remotely controllable function pointer [CVE-2006-6235]
Date:      Wed, 06 Dec 2006 16:58:16 +0100
Cc:        lwn-AT-lwn.net

     GnuPG: remotely controllable function pointer [CVE-2006-6235]
    ===============================================================
                              2006-12-04

Summary
=======

Tavis Ormandy of the Gentoo security team identified a severe and
exploitable bug in the processing of encrypted packets in GnuPG.

[ Please do not send private mail in response to this message.  The
  mailing list gnupg-devel is the best place to discuss this problem
  (please subscribe first so you don't need moderator approval [1]). ]


Impact
======

Using malformed OpenPGP packets an attacker is able to modify and
dereference a function pointer in GnuPG.  This is a remotely
exploitable bug and affects any use of GnuPG where an attacker can
control the data processed by GnuPG.  It is not necessary limited to
encrypted data, also signed data may be affected.

Affected versions: All versions of GnuPG   < 1.4.6
                   All versions of GnuPG-2 < 2.0.2
                   All beta versions of GnuPG-2 (1.9.0 .. 1.9.95)
Affected tools: gpg, gpgv, gpg2 and gpgv2.
Affected platforms: All.

gpg-agent, gpgsm as well as other tools are not affected.

A workaround is not known.


Solution
========

If you are using a vendor supplied version of GnuPG:

 * Wait for an update from your vendor.  Vendors have been informed on
   Saturday December 2, less than a day after this bug has been reported.

If you are using GnuPG 1.4:

 * Update as soon as possible to GnuPG 1.4.6. It has been uploaded to
   the usual location: ftp://ftp.gnupg.org/gcrypt/gnupg/.  This version
   was due to be released anyway this week.  See
   http://www.gnupg.org/download/ for details.

 * Or: As another and less intrusive option, apply the attached patch
   to GnuPG 1.4.5.  This is the smallest possible fix.

If you are using GnuPG 2.0:

 * Apply the attached patch against GnuPG 2.0.1.

 * Or: Stop using gpg2 and gpgv2, install GnuPG 1.4.6 and use gpg and gpgv
   instead.

If you are using a binary Windows version of GnuPG:

 * A binary version of GnuPG 1.4.6 for Windows is available as usual.

 * Gpg4win 1.0.8, including GnuPG 1.4.6, is available.  Please go to
   http://www.gpg4win.org .




Background
==========

GnuPG uses data structures called filters to process OpenPGP messages.
These filters ware used in a similar way as a pipelines in the shell.
For communication between these filters context structures are used.
These are usually allocated on the stack and passed to the filter
functions.  At most places the OpenPGP data stream fed into these
filters is closed before the context structure gets deallocated.
While decrypting encrypted packets, this may not happen in all cases
and the filter may use a void contest structure filled with garbage.
An attacker may control this garbage.  The filter context includes
another context used by the low-level decryption to access the
decryption algorithm.  This is done using a function pointer.  By
carefully crafting an OpenPGP message, an attacker may control this
function pointer and call an arbitrary function of the process.
Obviously an exploit needs to prepared for a specific version,
compiler, libc, etc to be successful - but it is definitely doable.

Fixing this is obvious: We need to allocate the context on the heap
and use a reference count to keep it valid as long as either the
controlling code or the filter code needs it.

We have checked all other usages of such a stack based filter contexts
but fortunately found no other vulnerable places.  This allows to
release a relatively small patch.  However, for reasons of code
cleanness and easier audits we will soon start to change all these
stack based filter contexts to heap based ones.


Support
=======

g10 Code GmbH, a Duesseldorf based company owned and headed by GnuPG's
principal author, is currently funding GnuPG development.  As evident
by the two vulnerabilities found within a week, a review of the entire
code base should be undertaken as soon as possible.  As maintainers we
try to do our best and are working slowly through the code.  The long
standing plan is to scrutinize the 2.0 code base, write more test
cases and to backport new fixes and cleanups to 1.4.  However, as a
small company our resources are limited and we need to prioritize
other projects which get us actual revenues.  Support contracts or
other financial backing would greatly help us to improve the quality
of GnuPG.


Thanks
======

Tavis Ormandy found this vulnerability.




[1] See http://lists.gnupg.org/mailman/listinfo/gnupg-devel .

-- System Information:
Debian Release: 4.0
  APT prefers unstable
  APT policy: (990, 'unstable')
Architecture: i386 (i686)
Shell:  /bin/sh linked to /bin/bash
Kernel: Linux 2.6.16.32-debian13+bluesmoke+lm85
Locale: LANG=pt_BR.ISO-8859-1, LC_CTYPE=pt_BR.ISO-8859-1 (charmap=ISO-8859-1)

Versions of packages gnupg depends on:
ii  gpgv                         1.4.5-3     GNU privacy guard - signature veri
ii  libbz2-1.0                   1.0.3-6     high-quality block-sorting file co
ii  libc6                        2.3.6.ds1-8 GNU C Library: Shared libraries
ii  libldap2                     2.1.30-13.2 OpenLDAP libraries
ii  libreadline5                 5.2-1       GNU readline and history libraries
ii  libusb-0.1-4                 2:0.1.12-2  userspace USB programming library
ii  makedev                      2.3.1-83    creates device files in /dev
ii  zlib1g                       1:1.2.3-13  compression library - runtime

gnupg recommends no packages.

-- no debconf information

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh

Message #16 received at 401894-done@bugs.debian.org (full text, mbox, reply):

From: Henrique de Moraes Holschuh <hmh@debian.org>

To: 401894-done@bugs.debian.org

Subject: fixed in 1.4.6-1 upload

Date: Fri, 8 Dec 2006 00:33:12 -0200

Version: 1.4.6-1

This bug has been fixed by the following upload to unstable; it remains open
in stable:

gnupg (1.4.6-1) unstable; urgency=high

  * New upstream release.
   * Fixes remotely controllable function pointer [CVE-2006-6235]

  * 27_filename_overflow.dpatch: merged upstream, dropped.
  * 24_gpgv_manpage_cleanup.dpatch: updated and a couple of additional
    trivial fixes.

  * debian/rules (binary-arch): info copy of manuals moved to
    /usr/share/info - remove them there instead.  Manuals are now built
    from texi source, so install them from build tree, not top level.

  * debian/copyright: update to add OpenSSL exemption for keyserver helper
    tools.

 -- James Troup <james@nocrew.org>  Thu,  7 Dec 2006 02:54:51 +0000


-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh

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