PuTTY 0.53b and earlier does not clear logon credentials from memory, including plaintext passwords, which could allow attackers with access to memory to steal the SSH credentials.

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.

Integer signedness error in the ssh2rdpkt function in PuTTY before 0.56 allows remote attackers to execute arbitrary code via a SSH2MSGDEBUG packet with a modified stringlen parameter, which leads to a buffer overflow.