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://lists.opensuse.org/opensuse-updates/2013-08/msg00035.html
http://secunia.com/advisories/54379
http://secunia.com/advisories/54533
http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-modmul.html