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         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

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