Here's what happened: Apple's Safari browser is set to block third-party cookies by default, accepting cookies only from sites that a user visits or interacts with. However, there is an exception to this rule that allows cookies if you interact with a form or advertisement in certain ways. The Journal reports that Google and other ad networks took advantage of this exception by using an invisible form and its +1 Google+ recommendation system. Essentially, Google allowed Safari users who had signed into Google+ to interact with DoubleClick ads using an embedded '+1' button. This would then send off an invisible form that would have Safari think the user had provided permission for cookies to be stored.
For its part, Google says that it used this workaround to enable signed-in users to give +1 votes to content, but was unaware that it inadvertently enabled the advertising cookies. The search giant has since disabled the feature. It said in a statement to Electronista that users who had opted out of its interest-based ad program (via Google's Ad Preferences Manager) were not affected by the work around. Check out the full statement below:
Google wasn't the only one using this loophole. The code, which was discovered by Stanford researcher Jonathan Mayer, was also used by Media Innovation Group, PointRoll, and Vibrant Media. However, Google is the most high-profile of the listed offenders, and with recent discussions over the search giant's attitude to user privacy, it's hardly surprising that is receiving more attention than others over this.
[UPDATE] Well, it's all fun and games until someone phones the FTC, isn't it? Ars reports that Consumer Watchdog has asked the Federal Trade Commission to investigate this issue. Specifically, they want to know if Google has violated a previous agreement with the FTC by tracking cookies in this way.