Bulletproof TLS Newsletter #37
Cloud provider vulnerability causes Let's Encrypt to disable SNI domain validation
31 January 2018
Author: Hanno Böck

This issue was distributed to 42,763 email subscribers.

Bulletproof TLS Newsletter is a free periodic newsletter bringing you commentary and news surrounding SSL/TLS and Internet PKI, designed to keep you informed about the latest developments in this space.

In this issue:

  1. Cloud provider vulnerability causes Let's Encrypt to disable SNI domain validation
  2. Short news

Cloud provider vulnerability causes Let's Encrypt to disable SNI domain validation

A major issue with some cloud providers allowed the unauthorized issuance of Let’s Encrypt certificates. Although the issue clearly lies with the cloud providers, Let’s Encrypt nevertheless has decided to disable the corresponding validation method.

Frans Rosén discovered that he could use the SNI validation method from the ACME protocol to issue certificates for domains hosted on certain cloud providers. He explicitly mentions Heroku and Amazon CloudFront.

The core of the issue is that these providers allow users to upload certificates that the system will serve automatically to TLS requests with the corresponding server name. The ACME SNI validation method uses temporary certificates that end with .acme.invalid.

After the issue was reported, Let’s Encrypt almost immediately disabled the TLS-SNI-01 validation method. Let’s Encrypt subsequently decided that, with a few exceptions, it will stay disabled. The newer TLS-SNI-02 method is vulnerable as well. A new TLS-SNI-03 method that considers this problem is being developed, but for the time being users should switch to either the HTTP or the DNS validation method.

Short news