Risk Assessment / Security & Hacktivism

New attack on HTTPS crypto might reveal if you’re pregnant or have cancer

Scientist-devised technique determines precise address of SSL-protected websites.




"The message motivating the technical points and techniques presented in the paper is that, in the case of many HTTPS deployments, maintaining the privacy of which pages a user views within a site matters," Brad Miller, a PhD candidate at the University of California at Berkeley and the lead author of the paper, wrote in an e-mail. "To see why, consider (1) the range of intermediary parties which can access user traffic and (2) what can be inferred from the pages you view within a website."

The researchers targeted 10 websites, including those belonging to the American Civil Liberties Union, Bank of America, Kaiser Permanente, Legal Zoom, the Mayo Clinic, Netflix, Planned Parenthood, Vanguard, Wells Fargo, and YouTube. To create a body of training data, they carefully measured the characteristics of specific URLs of each by visiting them ahead of time. They then monitored other encrypted Web sessions on those sites and used the characteristics to determine when those pages were visited. With an average accuracy rate of 89 percent, they were able to determine the individual pages within those sites. A small sampling on the subject matters, along with the corresponding URLs included:

Gauss what?

The researchers relied on Gaussian distribution to find similarities between the traffic observed during the training period and the HTTPS-protected communications traveling between test machines and the websites they visited. To return to the earlier metaphor, the technique is similar to studying the weight, shape, and size of the contents inside a wrapped present. Something small, square, and light is likely to be a Blu-ray disk. Something long, heavy, and showing the outline of a bicycle is probably a bike.

The researchers' job was made much harder by the common use of Web caching and browser cookies, which often remove or modify the contents contained in encrypted traffic from what was observed in the training data. Miller explained:

In the context of the bicycle analogy, imagine trying to figure out if somebody is building a unicycle, bicycle or a tricycle based on individually wrapped parts they receive. Without caching, you can just count the number of wheel shaped objects and you know what is being built. With caching, it could be the case that somebody already has a wheel or two in the garage, so even if you just see one wheel it could be a unicycle, bicycle or tricycle. The upshot is that you have to pay closer attention to the rest of the objects and be able to pick up on smaller details.

The technique should be of concern to privacy advocates, since it could be used by employers to determine if a worker is planning to get pregnant, or by repressive governments to know when people are viewing videos or other content deemed subversive. The paper is another reminder that the mere fact that data is encrypted doesn't mean it doesn't reveal highly personal information. The researchers also provide several countermeasures website operators can employ to prevent leakage, including a technique they dubbed Burst. The paper and its recommendations should be required for any site that provides privacy assurances to its visitors.

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