Bulletproof TLS Newsletter #67
Intermediate certificates with OCSP capability cause trouble
30 Jul 2020
Author: Hanno Böck

This issue was distributed to 55,298 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. Intermediate certificates with OCSP capability cause trouble
  2. Short news

Intermediate certificates with OCSP capability cause trouble

A large number of certificates are affected by an issue in which intermediate certificates can also act as OCSP responder certificates.

This issue was highlighted by Google developer Ryan Sleevi in early July. Sleevi found over 200 certificates that were issued by publicly trusted certificate authorities with an extended key usage flag that allows them to act as OCSP responder certificates. However, these certificates were missing an extension (id-pkix-ocsp-nocheck) that is required for such certificates, so they are in violation of the Baseline Requirements.

Many of these certificates were not intended as OCSP responder certificates; they were really intermediate certificates used to issue normal web page certificates. The reason these certificates have been created seems to be due to some behavior of Microsoft’s CA software, which was discussed in an email thread in 2019.

Usually certificates that violate the Baseline Requirements should be revoked within seven days. But Ryan Sleevi argues that revoking may not be sufficient in this case. Because these certificates can sign OCSP responses, they could be used to sign valid OCSP responses for themselves. Sleevi therefore argues that CAs should destroy the private keys corresponding to these certificates and provide a report documenting the key destruction.

Another problem in this case is that a large number of web page certificates rely on these intermediates and it is unclear when they will all be revoked. Nevertheless, web page operators of affected sites should ask their certificate authorities for replacement certificates and replace them as soon as possible.

Various tools are available to help website operators check if they are affected. The Hardenize service shows a warning for affected certificates. The TLS Certificate Health Checker from Oh Dear can also be used to check for this issue. And a simple shell script to check hosts is provided by the author of this newsletter.

Short news