A newly discovered security bug nicknamed Heartbleed has exposed millions of usernames, passwords and credit card numbers — a major problem that hackers could have exploited during the more than two years it went undetected. Unlike most security breaches, which affect a single website or organization, this flaw affects code used across the web to keep servers secure. The bug could affect thousands of websites including online banking portals, marketplaces and social media sites. A fix has been circulated, but it is unclear how quickly and widely it was being implemented. Use this page to test whether a website is at risk. Technical information can be found here.