Posted by Virus Bulletin on Feb 11, 2008
Spammers use botnet to register accounts on popular free webmail service.
Spammers have written a program that cracks the CAPTCHAS used by the Windows Live Mail registration system. The program, thought to have been installed on a large botnet of compromised systems, enables the automated creation of email accounts which can then be used to send spam.
Windows Live Mail, the successor of Hotmail, is a popular free webmail service owned by Microsoft. Like most free webmail services, it uses a CAPTCHA system to prevent automatic account registration. CAPTCHA images contain a simple code that needs to be entered correctly into a form on the web page to provide proof that the page is being viewed by a genuine person. The images are distorted in a way that supposedly prevents computer recognition, while allowing human viewers to decipher them and enter the codes they display.
Compromised systems making up a large botnet are thought to be running trojans which run through the registration process for Windows Live Mail accounts. When presented with the CAPTCHA, the image is sent off to a central server which attempts to decode it and return the result. Although the decipher technique is only successful around 35 per cent of the time, the fact that large numbers of infected systems are running repeated attempts means that the botnet's controllers are yielding a high number of new accounts to use for spamming.
Windows Live Mail is a particularly valuable resource for spammers - it provides free access to powerful mailing resources, and with its broad popularity and large legitimate user base, it provides a domain address that is unlikely to be blocked by spam filters. After reports in January of Yahoo!'s CAPTCHA system being cracked, this is another blow for the anti-spam world.