When a malformed ASN1 structure is received, it's contents are freed up and zeroed and an error condition returned. On a small number of platforms where sizeof(long) < sizeof(void *) (for example WIN64) this can cause an invalid memory access later resulting in a crash when some invalid structures are read, for example RSA public keys ). Any OpenSSL application which uses the public key of an untrusted certificate could be crashed by a malformed structure. Including SSL servers, clients, CA and S/MIME software.