According to Mandiant 83% of all backdoors used by APT attackers are outgoing sessions to TCP port 80 or 443. The reason for why APT, as well as other attackers, are using these two ports is primarily because most organizations allow outgoing connections on TCP 80 as well as 443. Many organizations try to counter this by using web-proxies, which can inspect the HTTP traffic and block any malicious behavior. But TCP 443 cannot be inspected in this way since SSL relies on end-to-end encryption.
Well, something that many people aren’t aware of is that the initial part of an SSL session isn’t encrypted. In fact, there are some pieces of relevant information being transmitted in clear text, especially the X.509 certificate that is sent from the SSL server.