A firm that yesterday claimed it has discovered a hack that opens up Transmission Control Protocol (TCP) communications has been slammed by a leading computer-security expert

Tim Newsham, a senior research scientist with Guardent found the flaw, the company said. It claims his research revealed a weakness in the generation of TCP's ISNs (Initial Sequence Numbers), which are used to maintain session information between network devices. ISNs are used as a "handshake" between two machines to identify legitimate data-packet traffic.

But Steve Gibson, of California-based Gibson Research, stresses that the flaw was recognised by the architects of the early Internet – who knew lack of randomness in the way that ISNs are chosen would be a problem as far back as the mid-1980s.

"This hack is extremely difficult to do. It's a theoretical attack. It's weird they're talking about something like this. It's as old as the hills," he told ZDNet.

Guardent says a hacker could launch DoS (denial of service) attacks to cut off individual Web server connections, commence an information poisoning attack to taint legitimate data, and hijack a user's session on a computer system.

However, Guardent vice president of research and development Jerry Brady admits the company has no hard evidence that the vulnerability has been exploited by hackers: "This is not the first time that ISN weaknesses have been discovered. Improvements to TCP were made in 1989 and again in 1995 to ensure more secure TCP sessions."

Guardent has shared its information with the Computer Emergency Response Team (CERT) Coordination Center at Carnegie Mellon University in Pittsburgh, as well as with network equipment vendors, operating system vendors and government agencies.