A strain of malware capable of setting up bogus Hotmail and Yahoo! accounts in order to send spam has been adapted to also target Gmail accounts.

The HotLan Trojan creates automatically-generated webmail accounts, implying that spammers have discovered a means to defeat Captcha challenge-response systems. Captcha systems, which typically prevent accounts being created until a user correctly identifies letters depicted in an image, are designed to ensure requests are made by a human rather than an automated program.

Since the arrival of the first variant of the Trojan last month, more than 500,000 spam email accounts have been created, according to Romanian anti-virus firm BitDefender. A joint effort between the security teams of BitDefender and Yahoo! appears to have stymied attempts to generate and use Yahoo! accounts to send spam.

However, this has pushed the problem onto Hotmail and Gmail (a new target of a latter variant of the Trojan) rather than having the desired effect of bringing the creation of bogus accounts under control.

The use of compromised PCs to send spam has been going on for years. The HotLan Trojan follows a more complex routine. Each active copy of the Trojan attempts to set up a webmail account, sending off the captcha image in an encrypted form to a spammer-controlled website. Servers behind this site process the image and extract the solution to the captcha challenge, which is then posted in the appropriate field.

Once a webmail account is established, encrypted spam emails are sent from a website onto infected machines. The HotLan Trojan then decrypts these junk emails and sends them to (presumably valid) addresses taken from yet another website.