Domain Validation
The commercial CAs that issue the bulk of certificates that clients trust for email servers and public HTTPS servers typically use a technique called "domain validation" to authenticate the recipient of the certificate. Domain validation involves sending an email containing an authentication token or link, to an email address that is known to be administratively responsible for the domain. This could be the technical contact email address listed in the domain's WHOIS entry, or an administrative email like postmaster@ or root@ the domain. The theory behind domain validation is that only the legitimate owner of a domain would be able to read emails sent to these administrative addresses.
Domain validation suffers from certain structural security limitations. In particular, it is always vulnerable to attacks that allow an adversary to observe the domain validation emails that CAs send. These can include attacks against the DNS, TCP, or BGP protocols (which lack the cryptographic protections of TLS/SSL), or the compromise of routers. Such attacks are possible either on the network near a CA, or near the victim domain itself.
Some Certificate Authorities offer Extended Validation (EV) certificates as a more rigorous alternative to domain validated certificates. One limitation of EV as a solution to the weaknesses of domain validation is that attackers could still obtain a domain validated certificate for the victim domain, and deploy it during an attack; if that occurred, the only difference observable to the victim user would be a blue HTTPS address bar rather than a green one. Few users would be likely to recognise this difference as indicative of an attack being in progress.
Domain validation implementations have also sometimes been a source of security vulnerabilities. In one instance, security researchers showed that attackers could obtain certificates for webmail sites because a CA was willing to use an email address like SSLCertificates@domain.com for domain.com, but not all webmail systems had reserved the "SSLCertificates" username to prevent attackers from registering it.
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