             Realloc Bug with X.509 certificates in GnuPG
            ==============================================
                              2010-07-23


Summary
=======

While trying to import a server certificate for a CDN service, a segv
bug was found in GnuPG's GPGSM tool.  It is likely that this bug is
exploitable by sending a special crafted signed message and having a
user verify the signature.

[ 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
======

All applications using GnuPG's GPGSM tool to process S/MIME messages
or manage X.509 certificates are affected.  The bug exists in all
versions of GnuPG including the recently released GnuPG 2.0.16.

GPG (i.e. OpenPGP) is NOT affected.

GnuPG 1.x is NOT affected because it does not come with the GPGSM
tool.

An exploit is not yet known but it can't be ruled out for sure that
the problem has not already been identified by some dark forces.


Description
===========

Importing a certificate with more than 98 Subject Alternate Names [2]
via GPGSM's import command or implicitly while verifying a signature
causes GPGSM to reallocate an array with the names.  The bug is that
the reallocation code misses assigning the reallocated array to the
old array variable and thus the old and freed array will be used.
Usually this leads to a segv.

It might be possible to use one of the techniques to exploit
assignments to malloced and freed memory.  Such an exploit won't be
easy to write because the attack vector must fit into a valid ASN.1
DER encoded DN.  To further complicate the task, that DN is not used
directly but after a transformation to RFC-2253 format.



Solution
========

Apply the following patch.  The patch is required for all GnuPG
versions < 2.0.17.  It applies to 2.0.16 but should apply to many
older versions as well.


--- kbx/keybox-blob.c   (revision 5367)
+++ kbx/keybox-blob.c   (working copy)
@@ -898,6 +898,7 @@
               rc = gpg_error_from_syserror ();
               goto leave;
             }
+          names = tmp;
         }
       names[blob->nuids++] = p;
       if (!i && (p=x509_email_kludge (p)))




Support 
=======

g10 Code GmbH [3], a Duesseldorf based company owned and headed by
GnuPG's principal author, is currently funding GnuPG development.
Support contracts or other financial backing will greatly help us to
improve the quality of GnuPG.


Thanks
======

Peter Gutmann for his "A mighty fortress is our PKI" mail to the
cryptography ML which contained a pointer to a certificate to exhibit
the problem.  This bug was created, found and fixed by Werner Koch.


[1] See http://lists.gnupg.org/mailman/listinfo/gnupg-devel
[2] <http://cvs.gnupg.org/cgi-bin/viewcvs.cgi/*checkout*/trunk/tests/samplekeys/cert-with-117-akas.pem>
[3] See http://www.gnupg.org/service.html
