Get in the bin: Let's Encrypt gives admins until February 13 to switch off TLS-SNI-01

End-of-life followed 2018 fake Website certificate drama

If you're still using TLS-SNI-01, stop: a year after a slip-up allowed miscreants to claim Let's Encrypt certificates for domains they didn't own, the free certificate authority has announced the final sunset of the protocol involved.

In January 2018, Let's Encrypt discovered that validation based on TLS-SNI-01 and its planned successor TLS-SNI-02 could be abused. As we explained at the time: "A company might have investors.techcorp.com set up and pointed at a cloud-based web host to serve content, but not investor.techcorp.com. An attacker could potentially create an account on said cloud provider, and add a HTTPS server for investor.techcorp.com to that account, allowing the miscreant to masquerade as that business – and with a Let's Encrypt HTTPS cert, too, via TLS-SNI-01, to make it look totally legit.”

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The SNI extension to the TLS protocol is supposed to validate the name presented by the server, something particularly important when a single IP address is serving a large number of websites. As we noted last year, the opportunity for abuse arises if the hosting provider doesn't verify ownership of a domain.

Let's Encrypt's response at the time was to block TLS-SNI-01 for new accounts. However, it decided to continue support for certificates already issued.

That's going to end on February 13, 2019, the organisation has now confirmed.

In the blog post, Internet Security Research Group executive director Josh Aas explained that anyone still using TLS-SNI need to switch to DNS-01 and HTTP-01 as their validation mechanism.

"We apologize for any inconvenience but we believe this is the right thing to do for the integrity of the Web PKI," Aas concluded. ®




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