Spammers are using a virtual stripper as bait to dupe people into helping criminals crack codes they need to send more spam or boost the rankings of parasitic Web sites, security researchers said Tuesday.
A series of photographs shows "Melissa," no relation to the 1999 worm by the same name, with progressively fewer clothes and
more skin each time the user correctly enters the characters in an accompanying CAPTCHA (Completely Automatic Public Turing
Test to Tell Computers and Humans Apart), the distorted, scrambled codes that most Web mail services use to block bots from
registering hundreds or thousands of accounts. Spammers rely on Web e-mail accounts because they're disposable; by the time
filters have blocked the address, the spammers throw it away and move on to another.
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The CAPTCHAs that Melissa feeds to users are, in fact, legitimate codes snatched from Yahoo Mail's sign-up screens, said analysts
at Trend Micro. The hackers, frustrated at their inability to come up with a way to automate account registration, are getting
users to do their dirty work.
"They're using human beings in semi-real time to translate CAPTCHAs by proxy," said Paul Ferguson, a network architect at
Trend Micro. "You have to give them this, it's clever."
Each time the user correctly decodes the CAPTCHA, a new Melissa photo is revealed, pulled from a hacker-controlled server
in Israel, according to Symantec. The plain-text decodes are sent to that same server, where they are presumably banked for
future use in generating large numbers of Yahoo Mail accounts.
Fumble-fingered typists are even encouraged by Melissa to try their luck again: "Hmmm, nope, the word you entered is incorrect
honey! Lets [sic] try again?" the virtual stripper replies.
Trend Micro said the striptease was part of a Trojan horse called CAPTCHA.a; rival Symantec dubbed it Captchar.a instead.
The Trojan horse may be part of a multistage attack, downloaded to a PC that's been compromised by other, more malicious code,
or can be encountered as a drive-by Web-based exploit.
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