Internet security researchers say people should not rush to change their passwords after the discovery of a widespread “catastrophic” software flaw that could expose website user details to hackers.

The flaw, dubbed “Heartbleed”, could reveal anything which is currently being processed by a web server – including usernames, passwords and cryptographic keys being used inside the site. Those at risk include Deutsche Bank, Yahoo and its subsidiary sites Flickr and Tumblr, photo-sharing site Imgur, and the FBI.

About half a million sites worldwide are reckoned to be insecure. “Catastrophic is the right word,” commented Bruce Schneier, an independent security expert. “On the scale of 1 to 10, this is an 11.”