Rogue ads overtake porn as top mobile malware attack method

But smut remains proportionately more risky

Rogue adverts that use social engineering to persuade users to install malware have displaced porn as the leading method of attack on mobile devices, according to a report from security firm Blue Coat.

Using numbers pulled out from the firm's WebPulse filtering system, in 2013 web ads accounted for nearly 20 percent of encounters with mobile malware, up from under 6 percent a year earlier. This surpassed even porn, which recorded a rate of 16.5 percent, down from 22.1 percent.

Surprisingly, these ads are often served from legitimate but unregulated ad networks that have proved easy to fool into serving rogue 'malvertising', the firm said.

Blue Coat offers no hard numbers so it is important to put this into context. Ads now serve the greatest amount of malware but this is not surprising given the huge growth in ad traffic. Porn, by contrast, is a far smaller volume of traffic - barely 1 percent - but remains far more likely to take users to malware as a proportion of visits.

"While users don't access pornography that frequently, when they do, they are very vulnerable to malware," said Blue Coat's researchers in a summary of its findings.

Once they have lured the user to a malware source, the criminals still have to persuade them to install it, something that requires more social engineering than would be the case on a PC.

One technique for doing this is to throw up a bogus security alert that asks the user to run a security scan that recommends an app install. All the criminals have to do is to persuade the user to remove the block on installing apps from a third-party site and the malware has found a new home inside a mobile (read Android) host.

It is exactly the same approach tried by fake antivirus scams on Windows and there is no doubt it will work often enough to represent a threat.

"While mobile users are not yet subject to the same drive-by downloads that PC users face, mobile ads are increasingly being used as part of many socially engineering attacks. The increased frequency of mobile ads conditions users to see them as normal, which makes users more vulnerable to the attacks that are launched through ads," said Blue Coat.

On the other hand, a separate report from security firm F-Secure offers some explanation of why malware writers are having to develop more elaborate strategies to get malware on to users' Android devices - Android has simply proved less prone to fundamental operating system vulnerabilities than Windows.

If that's true, the real threat to Android users and mobile users in general remains rogue apps that get into official markets, even if only for short periods of time. Users simply assume apps on markets are secure and trustworthy even though this is not always the case.

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