Researchers at Google and Codenomicon have discovered a "Heartbleed" vulnerability in widely used web encryption technology that has reportedly made data on a number of the world's biggest websites susceptible to theft by hackers, according to Reuters.

A number of experts are calling it one of the most serious security flaws discovered in recent years.

The finding caused the U.S. government's Department of Homeland Security to advise companies on April 8 to analyze their servers to see if they were using a vulnerable version of the software known as OpenSSL.

Updates have been made available to address the vulnerability in OpenSSL, which could allow remote hackers to access "sensitive data" like passwords or secret keys used to decode traffic as it makes its way across the internet, according to Reuters.

"We have tested some of our own services from attacker's perspective. We attacked ourselves from outside, without leaving a trace," Codenomicon said on Heartbleed.com, which it made to provide information on possible attacks.

Victims won't be able to tell if their data has been access because the bug has existed for at least two years, according to computer security experts.

"If a website is vulnerable I could see things like your password, banking information and healthcare data, which you were under the impression you were sending securely to your website," said Michael Coates, director of product security for Shape Security, according to Reuters.

Chris Eng, vice president of research with software security firm Veracode, said this week that he believes hundreds of thousands of web and email servers around the world need to be patched ASAP to protect them from a possible attack.

Hackers will be rushing to exploit the vulnerability now that it is public information.

Technology website Ars Technica released a report this week saying that that security researcher Mark Loman was able to extract data from Yahoo Mail servers just by using a free tool.

"We are working to implement the fix across the rest of our sites right now," said a Yahoo spokesperson, according to Reuters.

Yahoo has confirmed that its Yahoo Mail service was vulnerable to attack, but said it has been patched, along with other Yahoo sites like Yahoo Search, Sports, Finance, Tumblr, and Flickr.

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