Spammers employ stripper to crack CAPTCHAs

[Aram Perez]

'Melissa' disrobes in ploy that relies on people, not CPUs, to crack  
squiggly codes

October 30, 2007 (Computerworld) -- 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 today.

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.

The CAPTCHAs that Melissa feeds to users are, in fact, legitimate  
codes snatched from Yahoo Mail's signup screens, said analysts at  
Trend Micro Inc. 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 Corp. 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.

"This isn't the first time that they've tried to bust CAPTCHAs," said  
Ferguson, noting past attempts by bot-driven malware to apply optical  
character-recognition technology to deciphering the squiggles and  
obscured letters. Nor is it the first time human beings have been put  
to work decoding CAPTCHAs. "Work-at-home money mule schemes run by  
criminals have hired people to do this same thing," Ferguson said.  
"They're told to log on to this Web page and type the CAPTCHA. They  
have a quota."

In some cases, those CAPTCHAs have been used to sidestep bot  
protection for blog commenting rights; hackers will flood a blog  
they've created with fraudulent comments to drive up its search-engine  
ranking, expecting that the higher placement will translate into more  
traffic and thus more clicks on the ads displayed on the blog page.  
"Sometimes they use [CAPTCHAs] just to bump up their page [ranking],"  
Ferguson said.

The Trojan horse can strike PCs running Windows 98, Me, NT, 2000, XP  
and Server 2003.

---------------------------------------------------------------------
The Cryptography Mailing List
Unsubscribe by sending "unsubscribe cryptography" to majordomo at metzdowd.com