A study of the public-facing web servers run by some of the world's largest firms has suggested only three per cent of the machines have been fully protected against the OpenSSL vulnerability known as Heartbleed.

The research, carried out by security specialists at Venafi Labs, examined 550,000 servers belonging to 1,639 companies on the Forbes top Global 2000 list, and showed that 99 per cent of the companies checked had patched the data-leaking Heartbleed flaw.

But, Venafi tells us, only 15,000 of the patched servers have changed their private keys, and as well as being issued with new SSL certificates and having the old ones revoked. Given that Heartbleed can be exploited to snaffle private keys out of a vulnerable computer's memory, it should be assumed the server keys and certs have been compromised, we're told.

"Mopping up after an incident isn't as simple as it used to be," Kevin Bocek, VP of security strategy and threat intelligence at Venafi, told The Register. "You can't just stick a patch on and call it done."

He pointed out that the OpenSSL flaw had been exploitable for two years before it was finally spotted in April. During that time passwords were retrievable by those capable of exploiting the flaw, but so were encryption keys and certification data that could be used to masquerade as corporate servers.

Bocek said the situation was even worse in servers that aren't public facing, and in many cases servers residing inside corporate firewalls hadn't even been properly patched against the Heartbleed flaw.

In terms of the types of business that had fully patched against Heartbleed, the computer services sector unsurprisingly performed best, ahead of broadcasting firms, banks, and the semiconductor industry. ®