openssh unix dev - Oct 2015 - Segfault on invalid SSH keys.

Hi,

  I reported a bug against the Debian distribution,
 but it might be more useful to report it here.

  Via fuzzing I discovered a key which will cause 
 the ssh-keygen process to segfault when fingerprinting
 via:

        ssh-keygen -l -f bogus.key

  This segfault is a NULL pointer dereference, and is
 a denial of service attack if you run a service which
 allows SSH keys to be uploaded and display their fingerprints.
 (I run such a service.  Oops.)

  There is a simple patch which I've posted in the bug
 report which fixes the problem for me, but probably needs
 more eyes.  This is the patch:


--- sshkey.c.orig   2015-10-13 22:42:26.178252307 +0300
+++ sshkey.c    2015-10-13 22:42:58.781080815 +0300
@@ -1198,6 +1198,9 @@
            bits == 0 || bits > SSHBUF_MAX_BIGNUM * 8)
            return SSH_ERR_INVALID_FORMAT;  /* Bad bit count... */
 
+                if ( ret->rsa == NULL )
+                    return SSH_ERR_INVALID_FORMAT;
+
        /* Get public exponent, public modulus. */
        if ((r = read_decimal_bignum(&ep, ret->rsa->e)) < 0)
            return r;


  The crasher can be found in the bug-report (note there are
 two, the second is easier to deal with):

    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=801530:w

  I'm not a member of the list, but I'll keep an eye out for
 updates, via the archives, for the next few days in case
 there are questions.

Steve
-- 
http://www.steve.org.uk/

On Thu, 15 Oct 2015, Steve Kemp wrote:
> Hi,
> 
>   I reported a bug against the Debian distribution,
>  but it might be more useful to report it here.
> 
>   Via fuzzing I discovered a key which will cause 
>  the ssh-keygen process to segfault when fingerprinting
>  via:
> 
>         ssh-keygen -l -f bogus.key
Could you please share the key that causes this problem?

Thanks,
Damien

> >   Via fuzzing I discovered a key which will cause 
> >  the ssh-keygen process to segfault when fingerprinting
> >  via:
> > 
> >         ssh-keygen -l -f bogus.key
> 
> Could you please share the key that causes this problem?
  The key was attached to the referenced bug report, but
 please find attached a copy to this mail.

  Usage is:

    $ gunzip crash.min.pub.gz
    $ ssh-keygen -l -f ./crash.min.pub 
    Segmentation fault

  (It also crashes when running "ssh -i x.pub user at host", but
that's
 less interesting.)

Steve
-- 
http://www.steve.org.uk/
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