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Dai
Asked: 2016-05-05 19:10:55 +0800 CST

Trusting an untrustworthy CA - Can I restrict how system trusts it?

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(Posted to ServerFault instead of StackOverflow because I feel it concerns OS configuration more than programming code).

I'm currently responsible for maintaining a system which connects to a third-party webservice. This webservice requires client authentication certificates, which is fair enough, but the webservice itself is secured with a self-signed certificate created by a self-created root certification authority certificate - the same root that creates the client auth certificates.

It would be enough to merely add the current service certificate to the known-trusted list and to ignore the self-created authority certificate, unfortunately the service certificate changes regularly so the authority certificate must be trusted to ensure the application doesn't break when the service cert is renewed.

However I don't (personally) trust the CA cert based on my experience with the company running the webservice - it would not surprise me if it would be leaked to the web - and worryingly the CA cert has no key-usage restrictions placed on it (while external MITM attacks are a possibility, though remote, I'm more concerned about a leaked certificate used for code-signing, for example).

Is it possible for me to tell my computer (currently a server box, but in future ordinary desktop client boxes) to trust a CA but only for a given set of key-usages and a small set of possible subject-names (domain-names)?

The server is currently Windows Server 2012 R2, but it could be running on a Linux box - though the desktop machines are all Windows boxes.

certificate-authority ssl-certificate x509

1 Answers

  1. Best Answer

    Yes, it is possible. In the case of Windows, there is a feature called Cross-Certification or Qualified Subordination.

    The idea is that you sign third party's issuing CA certificate in your environment. As the result remote SSL certificate is chained to your own root CA certificate. In order to protect yourself from possible rogue certificates, you implement a Name Constraints certificate extension where you specify a list of acceptable names. If third party CA issue certificate for any other name (not explicitly specified in the Name Constraints extension), it will be automatically rejected by your CryptoAPI provider.

    In addition to name constraints, you can describe Enhanced Key Usages constraint by defining the Application Policies certificate extension in the cross-certificate. So, your trust provider will successfully validate only usages specified in the Application Policies extension.

    More information: Planning and Implementing Cross-Certification and Qualified Subordination Using Windows Server 2003

    p.s. although, the article is written against Windows Server 2003, the article still applies to the most recent Windows Server version.

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