Akamai Technologies, whose network handles up to 30 per cent of all Internet traffic, said a researcher found a fault in custom code that the company thought shielded most of its customers from the Heartbleed bug.
As a result, Akamai is now reissuing all Secure Sockets Layer (SSL) certificates and security keys used to create encrypted connections between its customer's websites and visitors to those sites.
"In short, we had a bug," wrote Andy Ellis, Akamai's CTO, in a blog post.
Akamai's customers include some of the world's largest banks, media and e-commerce retailers. The company, which runs 147,000 servers in 92 countries, is one of thousands of organizations and companies that use the open-source OpenSSL cryptographic library.
Two years ago, a German programmer modified OpenSSL and made a mistake that could cause a Web server to divulge the private key used to create an SSL connection, indicated by a padlock, or other recent data sent to a server, such as usernames and passwords. It is one of the most serious bugs to affect the Internet in recent memory.
Akamai's servers would have been vulnerable to Heartbleed between August 2012 through April 4, Ellis wrote last Friday. During that period, it would have been possible for attackers to intercept passwords or steal other data such as session cookies.
But Ellis also wrote Akamai customers would have been less vulnerable to an attack using Heartbleed to obtain a private SSL key.
The reason is that Akamai had added customized code to its OpenSSL deployment about a decade ago that modified how the secret keys used to create an SSL connection were stored.
Soon after the OpenSSL fault was made public, Ellis wrote that Akamai was confident its code provided "better protection" that the stock OpenSSL code.
On Friday, a principal engineer at Akamai, Rich Salz, wrote on a forum that the company decided to publicly release a variation of that code, which had been in use at Akamai.
Salz did warn that the code should not be seen as a full patch or put into "production," or used on a live system, without further review.
On Sunday, Ellis wrote that an independent security researcher, Willem Pinckaers, had found defects in it. Pinckaers wrote on his website he found the flaws within 15 minutes.
Akamai "should not be sending out non-functional, bug ridden patches to the OpenSSL community, while claiming they protected Akamai against the Heartbleed attack," Pinckaers wrote.
Ellis wrote that Akamai's code did not protect three of six critical values of an RSA key, which is a long number generated by an algorithm and is used to create an encrypted connection. Those values could be exposed by exploiting the Heartbleed bug, which could allow an attacker to calculate the private key, he wrote. The company is evaluating other claims made by Pinckaers.
With a private SSL key, it's possible for a hacker to set up a fake website that passes a cryptographic security verification. It would also allow for a man-in-the middle attack, where encrypted traffic is intercepted and read.
Ellis wrote that issuing new SSL keys and certificates may be fast in some cases, but those that require extra validation with certificate authorities may take longer.
Akamai could not be immediately reached for further comment on Sunday.
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