A flaw in the most popular communications protocol for sending data on the Net could let attackers shut down connections between servers and routers, according to an advisory released Tuesday by Britain's national emergency response team.

TCP--the Transmission Control Protocol--contains a flaw that "varies by vendor and application, but in some deployment scenarios...is rated critical," said the advisory, published by the United Kingdom's National Infrastructure Security Co-ordination Centre. Networking-hardware maker Juniper Networks has determined that its products are vulnerable. Cisco Systems, Hitachi, NEC, and others are studying the issue, according to the advisory.

The vulnerability allows for what's known as a reset attack. Many network appliances and software programs rely on a continuous stream of data from a single source--called a session--and prematurely ending the session can cause a wide variety of problems for devices. Security researcher Paul Watson discovered a method that makes disrupting the data flow far easier than previously thought.