Microsoft recently announced that they were enhancing their “SmartScreen” system to send back to Microsoft every SSL certificate that every IE user encounters. They will use this information to try and detect SSL misissuances on their back end servers.

They may or may not be successful in doing that, but this implementation raises significant questions of privacy.

SmartScreen is a service to submit the full URLs you visited in IE (including query strings) to Microsoft for reputation testing and possible blocking. While Microsoft tries to reassure users by saying that this information passes to them over SSL, that doesn’t help much. It means an attacker with control of the network can’t see where you are browsing from this information – but if they have control of your network, they can see a lot about where you are browsing anyway. And Microsoft has full access to the data. The link to “our privacy statement” in the original SmartScreen announcement is, rather worryingly, broken. This is the current one, and it also tells us Each SmartScreen request comes with a unique identifier. That doesn’t contain any personal information, but it does allow Microsoft, or someone else with a subpoena, to reconstruct an IE user’s browsing history. The privacy policy also says nothing about whether Microsoft might use this information to e.g. find out what’s currently trending on the web. It seems they don’t need to provide a popular analytics service to get that sort of insight.

You might say that if you are already using SmartScreen, then sending the certificates as well doesn’t reveal much more information to Microsoft about your browsing than they already have. I’d say that’s not much comfort – but it’s also not quite true. SmartScreen does have a local whitelist for high traffic sites and so they don’t find out when you visit those sites. However (I assume), every certificate you encounter is sent to Microsoft, including high-traffic sites – as they are the most likely to be victims of misissuance. So Microsoft now know every site your browser visits, not just the less common ones.

By contrast, Firefox’s (and Chrome’s) implementation of the original function of SmartScreen, SafeBrowsing, uses a downloaded list of attack sites, so that the URLs you visit are not sent to Google or anyone else. And Certificate Transparency, the Google approach to detecting certificate misissuance after the fact which is now being standardized at the IETF, also does not violate the privacy of web users, because it does not require the browser to provide information to a third-party site. (Mozilla is currently evaluating CT.)

If I were someone who wanted to keep my privacy, I know which solution I’d prefer.