THE Heartbleed bug sounds like a nasty coronary condition. But it is in fact a software flaw that has left up to two-thirds of the world’s websites vulnerable to attack by hackers. “This is potentially the most dangerous bug that we have seen for a long, long time,” says James Beeson, the chief information security officer of GE Capital Americas, an arm of GE. Since its existence was revealed on April 7th by researchers at Codenomicon, a security outfit, and Google, countless companies around the world that rely on the internet for part or all of their business have been scrambling to fix the flaw.

Ironically, the bug was discovered in OpenSSL, encryption software that was designed to make the internet more secure. Available free, this open-source code is popular with businesses and governments, which use it to help secure everything from online credit-card transactions to public services. On April 9th, for instance, Canada’s tax authority shut off public access to its online services while it checked the security of its systems in the light of news about the bug.

The flaw makes it possible for hackers to trick a server into spewing out data held in its memory. OpenSSL has a feature known as a “heartbeat” that allows a computer at one end of an encrypted link to send occasional signals to the computer at the other end of it, to check that it is still online. The researchers discovered that a hacker with knowledge of the bug could replicate this signal and use it to steal all manner of data from a remote computer.

Those data could include encryption keys that let hackers decipher traffic. To make matters worse, the researchers found that the bug, which is present in some versions of OpenSSL that have been available since March 2012, allows attacks to be mounted without leaving a trace in targeted computers’ “server logs”, so victims are unaware their systems have been compromised. That means it is impossible to tell for sure what damage has been done.

The bug has forced companies to find out fast how many of their systems employ the vulnerable versions of OpenSSL. “Everyone knows they have to patch their customer-facing internet websites, but that is only the tip of the iceberg,” says Jonathan Sander of STEALTHbits Technologies, a security firm that is helping one of America’s biggest banks work out where it has deployed the buggy software. Web-connected systems that handle things such as accounting and personnel data will also need to be checked for the bug.

Mr Sander likens the discovery of the Heartbleed bug to finding a faulty part in nearly every make and model of car. The problem is that the internet cannot be recalled. Big web companies such as Google and Yahoo have moved fast to deal with the bug. But millions of smaller e-commerce sites and other businesses face the worrying prospect of being attacked by hackers alerted to the bug’s existence as the firms race to fix the problem.

The cure includes applying a software “patch” and then choosing new encryption keys to replace those that may have been compromised. Once this has been done, customers will often need to change their passwords too. Tumblr, a blogging service owned by Yahoo, has urged its users to change the passwords they use for all of the secure online services that hold sensitive data about them. Some companies even chose to suspend services while they were working on a fix. Bitstamp, a Bitcoin e-currency exchange, temporarily suspended new account registrations and logins to its existing accounts.

Another Y2 K?

Perhaps the risk posed by the Heartbleed bug will turn out to be overblown. But if it emerges that companies’ systems have indeed been hacked because of it, this could open a legal can of worms. Firms could argue that they ought not to be punished for using widely trusted security software. But aggrieved customers—and their lawyers—may see things differently.

Quite how the bug got into the OpenSSL software in the first place is a mystery. Bruce Schneier, an internet-security expert, argues in a blog post that “the probability is close to one” that intelligence agencies have exploited the glitch to nab the encryption keys needed to decipher information about their targets. His guess is that the glitch is the result of a coding error rather than the handiwork of spies, though he says he cannot be sure.

No matter who is to blame, this episode is another reminder of the security challenges companies face as ever more economic activity shifts online. According to eMarketer, a research outfit, worldwide business-to-consumer e-commerce sales are likely to grow by just over a fifth this year, to $1.5 trillion. That is a huge commercial opportunity, but it will also encourage cyber-crooks to target businesses even more vigorously. Expect more computer-security heartburn in boardrooms.