The widespread publicity about Heartbleed
The widespread publicity about Heartbleed had led operators of many websites to update vulnerable software and urge users to change passwords.
Paul Mutton, a security researcher at net monitoring firm Netcraft, explained that while that meant there was no "significant risk of further direct exploitation of the bug", it did not mean all danger had passed.
He said the problem had been compounded by the fact that a large number of sites had not cleaned up all their security credentials put at risk by Heartbleed.
In particular, he said, many sites had yet to invalidate or revoke the security certificates used as a guarantee of their identity.
"If a compromised certificate has not been revoked, an attacker can still use it to impersonate that website," said Mr Mutton.
The dangers posed by Heartbleed will persist for years, warn security experts
In addition, he said, web browsers did a poor job of checking whether security certificates had been revoked.
"Consequently, the dangers posed by the Heartbleed bug could persist for a few more years."
His comments were echoed by James Lyne, global head of security research at security software developer Sophos.
"There is a very long tail of sites that are going to be vulnerable for a very long time," said Mr Lyne, who pointed out that the list of devices that Heartbleed put at risk was growing.
Many so-called smart devices, such as home routers, CCTV cameras, baby monitors and home-management gadgets that control heating and power, were now known to be vulnerable to Heartbleed-based attacks, he said.
A survey by tech news site Wired found that smart thermostats, cloud-based data services, printers, firewalls and video-conferencing systems were all vulnerable.
Other reports suggest the makers of some industrial control systems are also now producing patches for their software to limit the potential for attack.
How tempting this was for malicious attackers was difficult to gauge, said Mr Lyne.
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