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.
More detail can be found at
The Register
here
and at
Websense
here
.
Posted on 11 February 2008 by
Virus Bulletin
Leave a Reply