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===== Email Challenge-Response: =====
Many CAs use an email challenge-response mechanism to verify that the SSL certificate subscriber owns/controls the domain to be included in the certificate. Some CAs allow applicants to select an address from a predetermined list to be used for this verification. See [[CA:Problematic_Practices#Email_Address_Prefixes_for_DV_Certs|Mozilla's restrictions on the set of verification addresses that may be used.]]
CAs using this method of domain validation must ensure that they are using the full email, and that none of their systems misinterpret the localpart of an email. If your system fails to process the exact email, it will misinterpret the '+' sign as a delimiter. Consider, for example, the WHOIS contact information for a domain of "user+word@example.com" or "user=word@example.com". Both of these email addresses conform to the rules set out in RFC 822 with respect to the 'localpart' syntax. If your system emails to "word@example.com", rather than to the full "user+word@example.com" / "user=word@example.com", it will allow an attacker (who obtains "word@example.com") to cause and obtain certificates for example.com. Please work with your engineering department to ensure your systems properly handle such cases, as well as handling productions such as quoted-string, as also detailed in RFC 822 and subsequent RFCs. This is especially important as EAI addresses (described in RFC 6530 and related) come into play.
=== Verifying Email Address Control ===
