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The two webmail providers have fallen victim to a new trojan called Trojan.Spammer.HotLan.A.

The malware not only generates new webmail accounts automatically but has also found a way around the anti-spam CAPTCHA system, which requires people to enter the letters depicted in an image.

The trojan reportedly accesses the webmail account, pulls encrypted spam messages from another website, decrypts them and then sends them out to legitimate email addresses. The messages send users to a site selling pharmacy products.

"There are only about 500 or so new accounts being created every hour," claims, Viorel Canja, head of BitDefender's Antivirus Lab. "But still, we've seen 15,000-plus Hotmail accounts being used so far. It's hard to estimate how many spam e-mails have already been sent."

Microsoft couldn't be reached for comment, while a Yahoo spokesperson said he would investigate the claims before commenting.