Apple has announced that in rare cases the Apple 5W European USB power adapter may overheat, which could work itself into a safety risk. This particular adapter came with the iPhone 3GS, iPhone 4 and iPhone 4S models shipped between October 2009 to September 2012 in select countries. The company is offering to replace faulty chargers with replacements free of charge.
First thing's first. You need to determine whether or not you have an affected charger. Check the image below and see if the color of the letters "CE" are in solid gray and you have the model number A1300. If so, you'll need to exchange this charger for a new one, which will have "CE" outlined in gray and the model number A1400. We recommend using another USB charger (or a PC) while the charger in question is replaced by Apple.
To kick start the replacement program, head to an Apple retail store or Apple authorized service provider with your iPhone (for serial number verification). If you're unable to reach a retailer, contact Apple technical support for more information. Be sure to check out the official announcement on the Apple website for more details.