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January 21, 2018

Google's Certificate Revocation Server Is Down - What Does It Mean?

Earlier today, someone reported to the mozilla.dev.security.policy mailing list that they were unable to access any Google websites over HTTPS because Google's OCSP responder was down. David E. Ross says the problem started two days ago, and several Tweets confirm this. Google has since acknowledged the report. As of publication time, the responder is still down for me, though Ross reports it's back up. (Update: a fix is being rolled out.)

What's an OCSP Responder?

OCSP, which stands for Online Certificate Status Protocol, is the system used by SSL/TLS clients (such as web browsers) to determine if an SSL/TLS certificate is revoked or not. When an OCSP-using TLS client connects to a TLS server such as https://www.google.com, it sends a query to the OCSP responder URL listed in the TLS server's certificate to see if the certificate is revoked. If the OCSP responder replies that it is, the TLS client aborts the connection. OCSP responders are operated by the certificate authority which issued the certificate. Google has its own publicly-trusted certificate authority (Google Internet Authority G2) which issues certificates for Google websites.

I thought Chrome didn't support OCSP?

You're correct. Chrome famously does not use OCSP. Chrome users connecting to Google websites are blissfully unaware that Google's OCSP responder is down.

But other TLS clients do use OCSP, and as a publicly-trusted certificate authority, Google is required by the Baseline Requirements to operate an OCSP responder. However, certificate authorities have historically done a bad job operating reliable OCSP responders, and firewalls often get in the way of OCSP queries. Consequentially, although web browsers like Edge, Safari, and Firefox do contact OCSP responders, they use "soft fail" and allow a connection if they don't get a well-formed response from the responder. Otherwise, they'd constantly reject connections that they shouldn't. (This renders OCSP almost entirely pointless from a security perspective, since an attacker with a revoked certificate can usually just block the OCSP response and the browser will accept the revoked certificate.) Therefore, the practical impact from Google's OCSP responder outage is probably very small. Nearly all clients are going to completely ignore the fact that Google's OCSP responder is down.

That said, Google operates some of the most heavily trafficked sites on the Internet. A wide variety of devices connect to Google servers (Google's FAQ for Certificate Changes mention set-top boxes, gaming consoles, printers, and even cameras). Inevitably, at least some of these devices are going to use "hard fail" and reject connections when an OCSP responder is down. There are also people, like the mozilla.dev.security.policy poster, who configure their web browser to use hard fail. Without a doubt, there are people who are noticing problems right now.

I thought Google had Site Reliability Engineers?

Indeed they do, which is why this incident is noteworthy. As I mentioned, certificate authorities tend to do a poor job operating OCSP responders. But most certificate authorities run off-the-shelf software and employ no software engineers. Some regional European certificate authorities even complain when you report security incidents to them during their months-long summer vacations. So no one is surprised when those certificate authorities have OCSP responder outages. Google, on the other hand, sets higher expectations.

Posted on 2018-01-21 at 19:35:36 UTC

← How will Certificate Transparency Logs be Audited in Practice?

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Hi, I'm Andrew. I'm the founder of SSLMate, which makes SSL certificates easy through automation, great software, and friendly support.

I blog about security, PKI, Linux, and more. If you liked this post, check out my other posts or subscribe to my Atom feed.

My email address is andrew@agwa.name. I'm AGWA at GitHub and @__agwa on Twitter.

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