Security researchers have identified an "insidious" threat affecting Yahoo Messenger.

A self-propagating worm, named yhoo32.explr, installs a piece of software called 'Safety Browser' and then hijacks the Internet Explorer homepage, leading users to a site that puts spyware on their PCs.

Because Safety Browser uses the Internet Explorer icon to identify itself, users can easily mistake it for the legitimate Microsoft browser.

This is the first recorded incidence of malware installing its own web browser on a PC without the user's permission, according to security firm FaceTime. 

The self-propagating worm spreads the infection to all contacts in Yahoo Messenger by sending a website link that loads a command file onto the user's PC and installs Safety Browser.

"This is one of oddest and more insidious pieces of malware we have encountered in years, and the first instance of a complete web browser hijack without the user's awareness," said Tyler Wells, senior director of research at FaceTime Security Labs.

"Similar 'rogue' browsers, such as 'Yapbrowser', have demonstrated the potential for serious damage by directing end-users to potentially illegal or illicit material. 'Rogue' browsers seem to be the hot new thing among hackers."