In cases where we expect OpenSSL to be validating the chain, we expect that ISRG Root X1 is also in the trust store (unlike older versions of Android, where we know that it hasn't been added). As such, there will be two certificates in the chain which are also in the local trust store: ISRG Root X1 and the expired DST Root CA X3.
It is my understanding that OpenSSL 1.1.0+, with the `trusted_first` method as the default chain-building method, will go through the following steps: 1) Receive the chain "EE <-- R3 <-- ISRG Root X1 (cross-signed by DST Root CA X3)" from the server 2) Look to see if it can complete this chain using certificates from `-CAfile`, `-CApath`, or `-trusted` 3) See that ISRG Root X1 is already trusted 4) Return this chain, which successfully verifies. The evidence that this works on OpenSSL 1.1.0+ comes from the very similar situation this past May. In that case, many servers were serving the chain "EE <-- Sectigo RSA Domain Validation Secure Server CA <-- USERTrust RSA Certification Authority <-- AddTrust External CA Root". In that situation, both the USERTrust RSA Certification Authority and the AddTrust External CA Root were in various trust stores, and then the AddTrust External CA Root expired. Clients which were using OpenSSL 1.1.0+ did not begin to fail at that time, because they were still able to trust the USERTrust RSA Certification Authority. Clients using OpenSSL 1.0.x were failing, because they couldn't recognize that one of the intermediates in the chain was in their own trust store. If this understanding is incorrect or missing something, we'd love to be informed. Thanks again, Aaron On Thu, Jan 7, 2021 at 1:10 AM Kurt Roeckx via dev-security-policy < dev-security-policy@lists.mozilla.org> wrote: > On 2021-01-07 01:48, Aaron Gable wrote: > > As mentioned in the blog post, and as we'll elaborate on further in an > > upcoming post, one of the drawbacks of this arrangement is that there > > actually is a class of clients for which chaining to an expired root > > doesn't work: versions of OpenSSL prior to 1.1. This is the same failure > > mode as various clients ran into on May 30th of 2020, when the AddTrust > > External CA root expired. > > I'm not sure why you mention OpenSSL prior to 1.1. There was a bug in > 1.1.1h that no longer checked for expired roots, but it was fixed in > 1.1.1i. OpenSSL has no plan to allow expired roots by default. > > > Kurt > _______________________________________________ > dev-security-policy mailing list > dev-security-policy@lists.mozilla.org > https://lists.mozilla.org/listinfo/dev-security-policy > _______________________________________________ dev-security-policy mailing list dev-security-policy@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-security-policy
