WHOWAS —
WHOIS data is unreliable. So why is it used in TLS certificate applications?
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Certificate authorities and browser makers are planning to end the use of WHOIS data verifying domain ownership following a report that demonstrated how threat actors could abuse the process to obtain fraudulently issued TLS certificates.
TLS certificates are the cryptographic credentials that underpin HTTPS connections, a critical component of online communications verifying that a server belongs to a trusted entity and encrypts all traffic passing between it and an end user. These credentials are issued by any one of hundreds of CAs (certificate authorities) to domain owners. The rules for how certificates are issued and the process for verifying the rightful owner of a domain are left to the CA/Browser Forum. One "base requirement rule" allows CAs to send an email to an address listed in the WHOIS record for the domain being applied for. When the receiver clicks an enclosed link, the certificate is automatically approved.
Non-trivial dependencies
Researchers from security firm watchTowr recently demonstrated how threat actors could abuse the rule to obtain fraudulently issued certificates for domains they didn’t own. The security failure resulted from a lack of uniform rules for determining the validity of sites claiming to provide official WHOIS records.
Specifically, watchTowr researchers were able to receive a verification link for any domain ending in .mobi, including ones they didn’t own. The researchers did this by deploying a fake WHOIS server and populating it with fake records. Creation of the fake server was possible because dotmobiregistry.net—the previous domain hosting the WHOIS server for .mobi domains—was allowed to expire after the server was relocated to a new domain. watchTowr researchers registered the domain, set up the imposter WHOIS server, and found that CAs continued to rely on it to verify ownership of .mobi domains.
The research didn’t escape the notice of the CA/Browser Forum (CAB Forum). On Monday, a member representing Google proposed ending the reliance on WHOIS data for domain ownership verification “in light of recent events where research from watchTowr Labs demonstrated how threat actors could exploit WHOIS to obtain fraudulently issued TLS certificates."
The formal proposal calls for reliance on WHOIS data to “sunset” in early November. It establishes specifically that “CAs MUST NOT rely on WHOIS to identify Domain Contacts” and that “Effective November 1, 2024, validations using this [email verification] method MUST NOT rely on WHOIS to identify Domain Contact information.”
Since Monday’s submission, more than 50 follow-up comments have been posted. Many of the responses expressed support for the proposed change. Others have questioned the need for a change as proposed, given that the security failure watchTowr uncovered is known to affect only a single top-level domain.
An Amazon representative, meanwhile, noted that the company previously implemented a unilateral change in which the AWS Certificate Manager will fully transition away from reliance on WHOIS records. The representative told CAB Forum members that Google’s proposed deadline of November 1 may be too stringent.
“We got feedback from customers that for some this is a non-trivial dependency to remove,” the Amazon representative wrote. “It’s not uncommon for companies to have built automation on top of email validation. Based on the information we got I recommend a date of April 30, 2025.”
CA Digicert endorsed Amazon’s proposal to extend the deadline. Digicert went on to propose that instead of using WHOIS records, CAs instead use the WHOIS successor known as the Registration Data Access Protocol.
The proposed changes are formally in the discussion phase of deliberations. It’s unclear when formal voting on the change will begin.