Revocation doesn't work (18 Mar 2011)

When an HTTPS certificate is issued, it's typically valid for a year or two. But what if something bad happens? What if the site loses control of its key?

In that case you would really need a way to invalidate a certificate before it expires and that's what revocation does. Certificates contain instructions for how to find out whether they are revoked and clients should check this before accepting a certificate.

There are basically two methods for checking if a certificate is revoked: certificate revocation lists (CRLs) and OCSP. CRLs are long lists of serial numbers that have been revoked while OCSP only deals with a single certificate. But the details are unimportant, they are both methods of getting signed and timestamped statements about the status of a certificate.

But both methods rely on the CA being available to answer CRL or OCSP queries. If a CA went down then it could take out huge sections of the web. Because of this, clients (and I'm thinking mainly of browsers) have historically been forgiving of an unavailable CA.

But an event this week gave me cause to wonder how well revocation actually works. So I wrote the the world's dumbest HTTP proxy. It's just a convenient way to intercept network traffic from a browser. HTTPS requests involve the CONNECT method, which is implemented. All other requests (including all revocation checks) simply return a 500 error. This isn't even as advanced as Moxie's trick of returning 3.

To be clear, the proxy is just for testing. An actual attack would intercept TCP connections. The results:

Firstly, IE 8 on Windows 7:

No indication of a problem at all even though the revocation checks returned 500s. It's even EV.

(Aside: I used Wireshark to confirm that revocation checks weren't bypassing the proxy. It's also the case that SChannel can cache revocation information. I don't know how to clear this cache, so I simply used a site that I hadn't visited before. Also confirming that a cache wasn't in effect is the fact that Chrome uses SChannel to verify certificates and Chrome knew that there was an issue..)

Firefox 3.6 on Windows 7, no indication:

Read more: ImperialViolet

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