A newly discovered bug in widely used Web encryption technology has made data on many of the world's major websites vulnerable to theft by hackers in what experts say is one of the most serious security flaws uncovered in recent years.

The finding of the so-called "Heartbleed" vulnerability, by researchers with Google Inc. and a small security firm Codenomicon, prompted the U.S. government's Department of Homeland Security to advise businesses on Tuesday to review their servers to see if they were using vulnerable versions a type of software known as OpenSSL.

It said updates are already available to address the vulnerability in OpenSSL, which could enable remote attackers to access sensitive data including passwords and secret keys that can decode traffic as it travels across the Internet.