Before issuing a certificate, a Certification Authority (CA) must verify that the requestor legitimately controls the domain. Most modern validation relies on “challenge-response” mechanisms, for example, a CA might provide a random value for the requestor to place in a specific location, like a DNS TXT record, which the CA then verifies.
Historically, other methods validated control through indirect means, such as looking up contact information in WHOIS records or sending an email to a domain contact. These methods have been proven vulnerable (example) and the recent efforts retire these weaker checks in favor of robust, automated alternatives.
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