A security researcher has developed a new attack for a well-known flaw in the TCP protocol that allows an attacker to effectively shut down targeted routers and terminate existing TCP sessions at will. The scenario has many security experts worried, given the ubiquity of TCP and the fact that there's an attack tool already circulating on the Internet. . . .
A security researcher has developed a new attack for a well-known flaw in the TCP protocol that allows an attacker to effectively shut down targeted routers and terminate existing TCP sessions at will. The scenario has many security experts worried, given the ubiquity of TCP and the fact that there's an attack tool already circulating on the Internet.

The basic problem lies in the fact that existing TCP sessions can be reset by sending specially crafted RST (reset) or Syn (synchronization) packets to either of the machines involved in the session. This is in fact an intended feature of the protocol.

However, the source IP addresses on these packets can be forged, which makes it possible for attackers not involved in the TCP session to terminate the connection, causing a de facto denial of service.

Security experts have known for some time that such an attack was possible in theory, but had thought it to be impractical to implement in the real world because of the difficulty of guessing the random numbers used to establish new TCP sessions.

Machines on the receiving end of TCP packets look for this number as a way of determining the authenticity of incoming requests. The numbers are randomly generated and come from a pool of about 4 billion possible 32-bit sequences.

But a researcher named Paul Watson has discovered that machines receiving TCP packets will accept packets containing numbers that are within a certain range of the actual sequence number. This makes it far easier to create authentic-looking packets capable of shutting down TCP sessions, according to an analysis of the attack posted Tuesday by the National Infrastructure Security Coordination Center, England's national clearinghouse for security data.

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