Let’s Encrypt – a SSL/TLS certificate authority run by the non-profit Internet Security Research Group (ISRG) to programmatically provide websites with free certs for their HTTPS websites – on Thursday said it is discontinuing TLS-SNI validation because it’s insecure in the context of many shared hosting providers.
TLS-SNI is one of three ways Let’s Encrypt’s Automatic Certificate Management Environment (ACME) protocol validates requests for TLS certificates, which enable secure connections when browsing the web, along with the confidence-inspiring display of a lock icon. The other two validation methods, HTTP-01 and DNS-01, are not implicated in this issue.
The problem is that TLS-SNI-01 and its planned successor TLS-SNI-02 can be abused under specific circumstances to allow an attacker to obtain HTTPS certificates for websites that he or she does not own.
Such a person could, for example, find an orphaned domain name pointed at a hosting service, and use the domain – with an unauthorized certificate to make fake pages appear more credible – without actually owning the domain.