A cyber attack which crippled more than 200,000 computers in 150 countries could strike again today, coming back to life in systems that have already been fixed. The anonymous British IT expert who discovered a “kill switch” which slowed the spread of the ransomware fears that it has a back door installed – meaning hackers could easily inject a new strain of the virus. Concerns have been raised that today could see a resurgence of the chaos which struck at the end of last week as office workers power up their machines. The attack, which began on Friday, struck banks, hospitals and government agencies, exploiting known vulnerabilities in old Microsoft computer operating systems. Experts fear that an email containing the virus could be laying dormant in hundreds of thousands of inboxes across the globe. But now MalwareTech, the cyber expert who halted the first strain of the ransomware, has revealed that a new wave of chaos could begin across the globe if hackers have installed a back door.