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Admins warned: Drill SSL knowledge into your Chrome users

Google research finds whopping SSL click-through rates

Admins of Chrome shops unite – your users are dabbling with dodgy SSL, and you must teach them how to be safer online until Google updates its browser.

That's the gist of a new report from Google researcher Adrienne Porter Felt and University of California, Berkeley graduate student Devdatta Akhawe, who trawled some 25 million data points in a quest to figure out how effective phishing, malware, and SSL warnings are for users of Chrome and Firefox.

The paper in which the flaws are discussed – Alice in Warningland: A Large-Scale Field Study of Browser Security Warning Effectiveness – will be presented next week at the USENIX Security Symposium 2013 in Washington, DC.

It finds that Chrome could borrow a number of useful traits from Firefox to reduce the rate at which users click through SSL warnings, potentially opening their computers to being compromised.

"Google Chrome users are 2.1 times more likely to click through an SSL warning than Mozilla Firefox users," the researchers write. They believe this high click-through rate comes from a combination of aesthetics, the storage of user-set SSL exemptions, and different demographics from users of different operating systems.

The report found that Firefox's use of a stylized policeman combined with the use of the word "untrusted" in the title likely had an effect on stopping users from bypassing the warning.

It also noted that Firefox forces users to make three clicks versus one in Chrome to bypass the warning, and this is likely to have had an effect as well.

However, both browsers have specific technologies that skew their own hit rates up (Google), and down (Firefox).

Chrome, for instance, ships with a technology called "certificate pinning" that skews Google's click-through rate upward. Pinning adds a list of certificated preloaded HTTP Strict Transport Security sites, such as Google, PayPal, and Twitter, where users cannot click past SSL warnings.

This means that some 20 per cent of all Google Chrome SSL warning impressions are non-bypassable, compared with Firefox's 1 per cent. Therefore, Firefox users see warnings for sites that Google users do not see, and by not clicking through on these critical warnings, Firefox's SSL click-through rate is skewed down as compared to Chrome's.

Further contributing to this is the fact Firefox lets users permanently make exceptions for specific sites also lowered that browser's SSL click-through rate:

We suspect that people do repeatedly visit sites with warnings (e.g., a favorite site with a self-signed certificate). If future work were to confirm this, there could be two implications. First, if users are repeatedly visiting the same websites with errors, the errors are likely false positives; this would mean that the lack of an exception-storing mechanism noticeably raises the false positive rate in Google Chrome.

Second, warning fatigue could be a factor. If Google Chrome users are exposed to more SSL warnings because they cannot save exceptions, they might pay less attention to each warning that they encounter.

Though these two specific technologies are likely shifting the click-through rates among the surveyed population, that does not account for the yawning gulf in click-throughs between Firefox and Chrome, the researchers write.

In light of the study, Google plans to test an exception-remembering feature in Chrome to halt "warning fatigue" among users and make them more careful when confronted with warnings. It has also begun a series of A/B tests to test the effectiveness of "a number of improvements".

For the time being, however, it seems the greatest advice an admin can dispense to their users is as familiar as ever: RTFW – Read The Flipping Warning. ®

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