At our company, we use a variety of research technologies to identify and study maliciousness on the Web. One of these tools is an automated system that forces a Web browser inside a Windows virtual machine to visit a URL to see what happens to the browser, its plugins, and the operating system. The resulting network-level actions of the virtual machine help us determine, without prior knowledge of specific exploits served to the browser or its extensions, whether a URL serves malicious content.
A few months ago, we began using the above-described system to examine the Alexa's 25,000 most popular domains. (Alexa is a Web information company that provides data about websites including top sites, Internet traffic statistics and metrics.) As these sites are popular and long-lived, many people assume that it is safe to visit them. However, automated examination of the Alexa top 25,000 each day for the month of February 2012 -- which found 58 sites serving drive-by download exploits -- shows that this assumption does not always hold.
While Alexa does not publish the total number of page views it uses to determine site rankings, there exists sufficient information to determine that number. As an example, Wikipedia, which represented about 0.54 percent of total Alexa views in February 2012, reported about 15.75 billion views for the previous month. Working backwards, we can thus calculate that Alexa used an average of (15,756 * 1,000,000)/(29 * (0.5416/100)) = roughly 100.31 billion views each day to rank the popularity of websites.
Using the above number, we can calculate the affected views for a given site in a 24-hour period. As an example, free-tv-video-online[.]me, which via an ad network served visitors malicious content on 13 February, represented about 0.0053 percent of the total Alexa views, which yields 5,366,895 affected views for that day. However, to estimate how many users were served exploit content, this number must be adjusted to account for the average number of views per user. Fortunately, Alexa makes this information available. Continuing with the example, free-tv-video-online[.]me has an average of 7.2 views per user. Thus, for this site, 5,366,895 views equates to 745,402 users served malicious content on 13 February. Across all 58 sites that (directly or indirectly) served malicious content, there were 44,160,016 affected views from 10,541,379 users.
Of course, not every user served malicious content was compromised. To estimate the number of successfully exploited users, we used several different sources, including Wikipedia's browser statistics. To begin, if we examine platform and browser popularity, only about half (or 50.81 percent) of users (who run Windows and IE or Firefox) possess properties conducive to exploitation.
To convert the number of possibly compromised users into those probably compromised, we conservatively adjusted according to the most popular mechanism of exploitation: the Java plugin. According to Adobe, 73 percent of PC users have the Java plugin installed. According to Qualys, 42 percent of users with the Java plugin installed have versions vulnerable to exploitation. Thus, of 10,541,379 users served malicious content, 42 percent (insecure Java) of 73 percent (Java installed) of 50.81 percent (Windows and Firefox/IE), or 1,642,172, were likely compromised.
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