The security of Internet-based applications fundamentally relies on the trustworthiness of Certificate Authorities (CAs). We practically demonstrate for the first time that even a weak off-path attacker can effectively subvert the trustworthiness of popular commercially used CAs. Our attack targets CAs which use Domain Validation (DV) for authenticating domain ownership; collectively these CAs control 99% of the certificates market. The attack utilises DNS Cache poisoning and tricks the CA into issuing fraudulent certificates for domains the attacker does not legitimately own -- namely certificates binding the attacker's public key to a victim domain. We discuss short and long term defences, but argue that they fall short of securing DV. To mitigate the threats we propose Domain Validation ++ (DV++). DV++ replaces the need in cryptography through assumptions in distributed systems. While retaining the benefits of DV (automation, efficiency and low costs) DV++ is secure even against Man-in-the-Middle (MitM) attackers. Deployment of DV++ is simple and does not require changing the existing infrastructure nor systems of the CAs. We demonstrate security of DV++ under realistic assumptions and provide open source access to DV++ implementation.
