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GnuPG 1.4 and 2.0 buffer overflow
From: Werner Koch <wk () gnupg org>
Date: Mon, 27 Nov 2006 18:13:02 +0100

            GnuPG 1.4 and 2.0 buffer overflow
           ==================================

Summary
=======

While fixing a bug reported by Hugh Warrington, a buffer overflow has
been identified in all released GnuPG versions.  The current versions
1.4.5 and 2.0.0 are affected.  A small patch is provided.

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
======

When running GnuPG interactively, special crafted messages may be used
to crash gpg or gpg2.  Running gpg in batch mode, as done by all
software using gpg as a backend (e.g. mailers), is not affected by
this bug.

Exploiting this overflow seems to be possible.

gpg-agent, gpgsm, gpgv or other tools from the GnuPG suite are not
affected.



Solution
========

Apply the following patch to GnuPG.  It should apply cleanly to
current versions (1.4.5 as well as 2.0.0) but might also work for
older versions. 

2006-11-27  Werner Koch  <wk () g10code com>

        * openfile.c (ask_outfile_name): Fixed buffer overflow occurring
        if make_printable_string returns a longer string.  Fixes bug 728.

--- g10/openfile.c      (revision 4348)
+++ g10/openfile.c      (working copy)
@@ -144,8 +144,8 @@
 
     s = _("Enter new filename");
 
-    n = strlen(s) + namelen + 10;
     defname = name && namelen? make_printable_string( name, namelen, 0): NULL;
+    n = strlen(s) + (defname?strlen (defname):0) + 10;
     prompt = xmalloc(n);
     if( defname )
        sprintf(prompt, "%s [%s]: ", s, defname );



Background:
===========

The code in question has been introduced on July 1, 1999 and is a
pretty obvious bug.  make_printable_string is supposed to replace
possible dangerous characters from a prompt and returns a malloced
string.  Thus this string may be longer than the orginal one; the
buffer for the prompt has only be allocated at the size of the original
string - oops.  Note, that using snprintf would not have helped in
this case.  How I wish C-90 had introduced asprintf or at least it
would be available on more platforms.

The original bug report is at https://bugs.g10code.com/gnupg/issue728 .



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


-- 
Werner Koch                                      <wk () gnupg org>
The GnuPG Experts                                http://g10code.com
Join the Fellowship and protect your Freedom!    http://www.fsfe.org

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