Hundreds of thousands of web and email servers worldwide have a software flaw that lets attackers steal the cryptographic keys used to secure online commerce and web connections, experts say.
They could also leak personal information to hackers when people carry out searches or log into email.
The bug, called "Heartbleed", affects web servers running a package called OpenSSL.
Among the systems confirmed to be affected are Imgur, OKCupid, Eventbrite, and the FBI's website, all of which run affected versions of OpenSSL. Attacks using the vulnerability are already in the wild: one lets a hacker look at the cookies of the last person to visit an affected server, revealing personal information. Connections to Google are not vulnerable, researchers say.
SSL is the most common technology used to secure websites. Web servers that use it securely send an encryption key to the visitor; that is then used to protect all other information coming to and from the server.
It is crucial in protecting services like online shopping or banking from eavesdropping, as it renders users immune to so-called man in the middle attacks, where a third party intercepts both streams of traffic and uses them to discover confidential information.
The Heartbleed bug – so called because it exploits a failure in an extension called heartbeat – not only lets attackers read the confidential encrypted data; it also allows them to take the encryption keys used to secure the data. That means that even servers which fix the bug, using a patch supplied by OpenSSL, must also update all their keys or risk remaining vulnerable.
More worryingly still, the bug can cause servers to leak other information stored on the server which wouldn't normally be available at all. For instance, one developer reports the ability to see searches made by other users on privacy-focused search engine DuckDuckGo, while another reports similar data leakage from Yahoo. Worse still, Yahoo has been found to be leaking user credentials due to the bug. Yahoo did not return requests for comment.
That data leakage means that servers vulnerable to Heartbleed are less secure than they would be if they simply had no encryption at all. "This allows attackers to eavesdrop communications, steal data directly from the services and users, and to impersonate services and users," explained security group Codenomicon, which discovered the flaw.