By Andrew Brandt

A new version of Trojan-Pushu is doing some interesting stuff to bypass captchas used by Microsoft’s Hotmail/Live.com/MSN webmail services in order to spam people with links to malicious Yahoo Groups pages.

The three-year-old spy (known by a variety of other aliases, including Cutwail, Pushdo, Diehard, and Rabbit) has always been, primarily, a spam bot. In this case, however, the spy is not sending spam by connecting to open mail relays or more traditional means; It’s spamming through the Hotmail/Live.com Web mail interface. Most interestingly, during the course of the spam sessions, the spy apparently pulls down “audio captchas” and successfully sends back the correct response, which permits it to continue spamming.

Audio captchas are just what they sound like they are: A voice, often female, reads a sequence of 10 numbers in an artificially noisy background. The purpose is simple: to ensure that a human being, and not some automated process, is entering data into a form. Just as you would type in the scrambled-up letters from a captcha image to proceed, with an audio captcha you have to type the correct numbers from the recording, or the site won’t let you continue.

That doesn’t seem to be a problem for this Pushu variant. We’ve seen Trojans attempt to crack visual captchas a number of ways, including using optical character recognition; employing a mechanical turk service (where humans are paid fractions of a penny for each correctly entered captcha); or by prompting the victim him- or herself to enter captcha text, disguising the captcha form as some sort of Windows prompt. This is the first time I’ve heard of a Trojan attempt to crack the audio captcha, let alone succeed.

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