Integer overflow in the terminal emulator in PuTTY before 0.66 allows remote attackers to cause a denial of service (memory corruption) or possibly execute arbitrary code via an ECH (erase characters) escape sequence with a large parameter value, which triggers a buffer underflow.
https://security.gentoo.org/glsa/201606-01
http://www.securitytracker.com/id/1034308
http://www.debian.org/security/2015/dsa-3409
http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-ech-overflow.html
http://lists.opensuse.org/opensuse-updates/2015-11/msg00099.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-December/173094.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-December/173021.html