Hostile HTTPS interception on the modern web is now increasingly costly and risky

January 24, 2016

One of the things that HTTPS is vulnerable to is a state level actor armed with enough money that is willing (and able) to compromise a CA and get certificates issued for sites that they want to run a MITM attack against. This is nothing new; it is the core security problem of SSL on the web, namely that any of the hundreds of CAs that are trusted by your browser can generate a certificate for anyone.

Technically, that is still the case. All of the trusted CAs in the world can still issue certificates for, say,, and quite a lot of browsers will trust those certificates. But not Chrome. If you are an attacker and you try this against a Chrome user, Chrome will try hard to scream bloody murder about this back to Google (and refuse to go ahead). Then pretty soon Google's security people will get to write another blog post and your nice compromised CA will be lost to you (one way or another).

Chrome has been doing this for a while (this is part of how Google has gotten to write a number of blog posts about this sort of thing), but it is not alone. On the modern web, there are a steadily increasing number of things that are more or less automatically looking for and reporting bogus certificates and an increasing number of ways to block many of them from being useful to attack your site. On the one hand, many of the better things are not included in web browsers by default; on the other hand, many of the people that a state level actor is likely to be most interested in MITM'ing are exactly the sort of people who may install things like HTTPS Everywhere and enable its reporting features.

Based on what I've read from the security circles that I follow, the net effect of all of these changes is that mounting anything but an extremely carefully targeted MITM attack is almost certain to cost you the compromised CA you were able to exploit. Each compromised CA you have is good for exactly one attack, if that.

(See for example the Twitter conversation linked to here.)

This doesn't make HTTPS interception impossible, of course. CAs can still be compromised. But it means that no one is going to do this for anything except very high priority needs, which in practice makes us all safer by reducing how often it happens.

An important contributing factor to the increased chanciness of HTTPS interception is that browsers are increasingly willing to say no. There was a time when you could MITM a significant number of people with a plain old bogus certificate (no CA compromise required, just generate it on the fly in your MITM box). Those days are mostly over, especially for some popular sites, and increasingly even a real certificate from a compromised CA may not work due to things like HPKP.

Written on 24 January 2016.
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Last modified: Sun Jan 24 01:54:02 2016
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