The widespread availability of CAPTCHA-breaking tools has led to an increase in spam on blogging sites, says MessageLabs.

The security company's October Intelligence report highlights the increasing levels of spam pollution on free sites such as Google's Blogspot.

The underlying mechanism is the ongoing collapse of traditional defence systems such as Completely Automated Public Turing tests to tell Computers and Humans Apart (CAPTCHA), a variety of which have been broken over recent months.

Indeed, CAPTCHA's vulnerabilities have turned into one of the security themes of 2008, which began with the cracking open of Yahoo's much-vaunted system in January, continuing with a bot assault on Microsoft's equivalent, culminating in the use of Gmail to create fake user accounts in March.

Holes in CAPTCHA have allowed criminals set up large numbers of fake blogs and content, which are then used to feed bogus profiles to social networking systems. Messages and requests from these domains are a simple way around reputation-based anti-spam technology because they emanate from trusted sites not as aggressively filtered by such software.

"With the exploitation of Google Blogspot we are again seeing two common spamming practices converge - CAPTCHA breaking techniques and exploitation of free hosted services," said MessageLabs' Mark Sunner.

"The spammers are now taking it one step further and experimenting with the capabilities of social networking sites, like Bebo. As a result, users of social networking sites are receiving more friend requests from fake profiles wishing to connect."

Earlier this month, MessageLabs agreed to a $695 million (£421 million) buy-out by Symantec.