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.

"We have tested some of our own services from attacker's perspective. We attacked ourselves from outside, without leaving a trace," Codenomicon said on a website it built to provide information about the threat, heartbleed.com.

Computer security experts warned that means victims cannot tell if their data has been accessed which is troubling because the bug has existed for about two years.

"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.

Chris Eng, vice president of research with software security firm Veracode, said he estimates that hundreds of thousands of web and email servers around the globe need to be patched as soon as possible to protect them from attack by hackers who will rush to exploit the vulnerability now that it is publicly known.

The technology website Ars Technica reported that security researcher Mark Loman was able to extract data from Yahoo Mail servers by using a free tool.

A spokesperson for Yahoo Inc confirmed that Yahoo Mail was vulnerable to attack, but said it had been patched along with other main Yahoo sites such as Yahoo Search, Finance, Sports, Flickr and Tumblr.

"We are working to implement the fix across the rest of our sites right now," she said on Tuesday evening.