Calls to EVP_CipherUpdate, EVP_EncryptUpdate and EVP_DecryptUpdate may overflow the output length argument in some cases where the input length is close to the maximum permissible length for an integer on the platform. In such cases the return value from the function call will be 1 , but the output length value will be negative. This could cause applications to behave incorrectly or crash. The OpenSSL public API function X509_issuer_and_serial_hash attempts to create a unique hash value based on the issuer and serial number data contained within an X509 certificate. However it fails to correctly handle any errors that may occur while parsing the issuer field . This may subsequently result in a NULL pointer deref and a crash leading to a potential denial of service attack