Multiple heap-based buffer overflows in the modpow function in PuTTY before 0.55 allow (1) remote attackers to execute arbitrary code via an SSH2 packet with a base argument that is larger than the mod argument, which causes the modpow function to write memory before the beginning of its buffer, and (2) remote malicious servers to cause a denial of service (client crash) and possibly execute arbitrary code via a large bignum during authentication.
https://exchange.xforce.ibmcloud.com/vulnerabilities/16885
http://www.securityfocus.com/bid/10850
http://www.gentoo.org/security/en/glsa/glsa-200408-04.xml
http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-modpow.html
http://www.chiark.greenend.org.uk/~sgtatham/putty/changes.html