Domain Impersonation is Feasible: A Study of CA Domain Validation Vulnerabilities

Web security relies on the assumption that certificate authorities (CAs) issue certificates to rightful domain owners only. However, we show that CAs expose vulnerabilities which allow an attacker to obtain certificates from major CAs for domains he does not own. We present a measurement method that allows us to check CAs for a list of technical weaknesses during their domain validation procedures. Our results show that all tested CAs are vulnerable in one or even multiple ways, because they rely on a combination of insecure protocols like DNS and HTTP and do not implement existing secure alternatives like DNSSEC and TLS. We have validated our methodology experimentally and disclosed these vulnerabilities to CAs. Based upon our findings we provide recommendations to domain owners and CAs to close this fundamental weakness in web security.