The companies used special computer code that tricks Apple's Safari Web-browsing software into letting them monitor many users. Safari, the most widely used browser on mobile devices, is designed to block such tracking by default.
Google disabled its code after being contacted by The Wall Street Journal.
The Google code was spotted by Stanford researcher Jonathan Mayer and independently confirmed by a technical adviser to the Journal, Ashkan Soltani, who found that ads on 22 of the top 100 websites installed the Google tracking code on a test computer, and ads on 23 sites installed it on an iPhone browser.
The technique reaches far beyond those websites, however, because once the coding was activated, it could enable Google tracking across the vast majority of websites. Three other online-ad companies were found using similar techniques: Vibrant Media Inc., WPP PLC's Media Innovation Group LLC and Gannett Co.'s PointRoll Inc.
See the rest of this article at WSJ online. It's definitely worth a read. Just so you know.