Heap-based buffer underflow in the modmul function in sshbn.c in PuTTY before 0.63 allows remote SSH servers to cause a denial of service (crash) and possibly trigger memory corruption or code execution via a crafted DSA signature, which is not properly handled when performing certain bit-shifting operations during modular multiplication.
http://www.openwall.com/lists/oss-security/2013/08/06/11
http://www.debian.org/security/2013/dsa-2736
http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-modmul.html
http://svn.tartarus.org/sgt/putty/sshbn.c?sortby=date&r1=9977&r2=9976&pathrev=9977
http://secunia.com/advisories/54533
http://secunia.com/advisories/54379
http://lists.opensuse.org/opensuse-updates/2013-08/msg00035.html