30 Jun 2020
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. Maintained by Hanno Böck.
The expiration of an old root certificate by Sectigo was the cause of some trouble on May 30. The AddTrust certificate was used to cross-sign the newer USERTrust certificate, and in some situations the expired cross-signature certificate caused verification failures. Andrew Ayer has written a blog post with a detailed technical explanation.
A client that implements certificate verification and path building correctly should not have any issues with such an expired root certificate. It should detect that it could also validate the certificate against the newer USERTrust certificate and ignore the expired AddTrust intermediate and root. Browsers were largely unaffected by this issue. Most issues happened with tools either using GnuTLS or older versions of OpenSSL.
The issue could be mitigated in various ways. Although the fault lies with the client library, administrators could mitigate this issue by no longer serving the expired AddTrust intermediate certificate in the chain. Removing the old AddTrust certificate from the chain is also recommended for performance reasons. Shortly after the incident, GnuTLS released an updated version that fixed certificate validation in such instances, and the patch was also backported into most Linux distributions.
In the aftermath of the AddTrust incident, Google developer Ryan Sleevi wrote two blog posts detailing certificate validation issues in TLS libraries. In the first post, Sleevi describes how certificate path validation should ideally be done, and in the second post the validation code of several popular TLS libraries is analyzed. To better analyze certificate paths, Rob Stradling has implemented the possibility to display certificate path graphs in the crt.sh certificate search engine.
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