Internet payment system Worldpay is under hack attack from unknown assailants, hitting thousands of online retailers around the world, the BBC reports. The company’s payment and administration networks have been flooded with computer-generated requests, clogging the system and slowing transactions – also known as a “denial-of-service” attack. Worldpay is owned by the Royal Bank of Scotland Group. It has 27,000 clients around the world, ranging from heavyweights like Vodafone and Sony Music Entertainment to numerous small online retailers.