Software in Cisco switches and routers has a vulnerability which could allow hackers to intercept and modify traffic originating from them.
In a warning published last week, Cisco said the flaw in its Internetworking Operating Systems versions 11 and 12 allows the successful prediction of packet sequence information. The problem affects nearly all of Cisco's switches and routers, which are among the most widely used in the world.
Graham Titterington, an analyst with Ovum, said the vulnerability was "serious" and recommended that network managers move quickly to fix it.
"This could be a vehicle for denial of service attacks at least, though how effective an attack is depends on the authentification method the end-user employs. Once the [sequence data] is discovered it is easy to predict," he said.
"Network managers need to determine whether they have these vulnerabilities. Then they need to get the patch and install it. In general it is OK to publicise these vulnerabilities but in this case it is debatable because once this hole is known it can be quickly exploited," he added.