Oh dear, not good news, especially for google and its advertisers.

A London-based team of security researchers has exposed a scheme which inserted unauthorised adverts into Google's YouTube.

Spider.io discovered two programmes which placed ads on YouTube's website when viewed by affected PCs. It said the plug-in had been promoted as tools to download videos from the service. It said some directed users to malware.

This was in breach of YouTube's rules.

Plug In Adds Unauthorised Ads To YouTube [bbc.co.uk]