The government's National Infrastructure Security Co-ordination Centre highlighted the vulnerability, posting a global alert.
Phil Cracknell, chief technology officer at security consultancy netSurity, said the problem with TCP was a known flaw but had been considered low-risk.
To exploit the vulnerability an attacker would have to guess a sequence of numbers, a scenario that experts deemed highly unlikely. But Cracknell said, "It seems that a researcher has stumbled upon scenarios where any number of values can be used to great effect."
Network hardware supplier Cisco said all its products that use TCP were susceptible to this vulnerability and advised users to update their IOS network operating system software. The vulnerability could cause a TCP connection to break, which, Cisco said, in most cases would cause little harm to the network.
But it warned that router-to-router connections using Border Gateway Protocol (BGP) could be severely affected.
Richard Brain, technical director at security consultancy Procheckup, said, "BGP relies on a permanent TCP connection between two routers. A connection could last for minutes or even hours, so it is more likely to be affected [by the flaw]. If exploited, a hacker could prevent one part of a corporate network from communicating with another part."
Applying patches might not be a straightforward process. Chris Anley, joint founder of security testing company NGS Software, said, "Companies attempting to close the gaps may find they have to prevent some users getting access to their networks while they upgrade the operating software on their routers and switches. This may lead to network outages for some as it is a major task."