Combining wildcard and subdomain-specific SSL certificates

Question

Not quite sure which Google terms to use to find the answer, so I thought I'd ask it here, presented as a scenario:

Say you have a corporate entity (Example Co) with a wildcard SSL cert issued for *.example.com. They contract with a SaaS provider and requests the SaaS provider use a custom domain: saas.example.com.

The SaaS provider creates their own private key (not shared with Example Co), generates a CSR and passes it on to Example Co., who then has it signed by their CA and returns it to the SaaS provider for use in production.

Later that year, the private key for Example Co's wildcard cert is compromised, but they don't know (so they don't revoke it). If the thieves man-in-the-middle connections to saas.example.com and present the wildcard cert, would the customer getting man-in-the-middled get any sort of SSL-related warning from their browser?

Answer 1

If the thieves man-in-the-middle connections to saas.example.com and present the wildcard cert, would the customer getting man-in-the-middled get any sort of SSL-related warning from their browser?

Usually not because the attacker presents a non-revoked certificate which is valid for the accessed host. The client would only reject the valid certificate if some kind of certificate pinning was used and the certificate did not match the expected public key.