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 theres 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, Englands national clearinghouse for security data.
Known as a "window," this range of acceptable sequence numbers is established during the initial TCP handshake and varies depending on the devices and applications involved. A larger window size makes it easier for this attack to succeed. And with an automated attack tool already out there, experts expect to see quite a bit of activity in the coming days.
"It takes about 15 seconds for the attack tool to resize the window and guess the number and crash the device," said Chris Rouland, vice president of the X-Force research team at Internet Security Systems Inc. in Atlanta. "This certainly will become another tool in the arsenal [of attackers]."
Experts say BGP (Border Gateway Protocol) is likely to be most vulnerable to this issue because it relies on a persistent TCP connection between peers. ISPs use the protocol to exchange routing information, and resetting BGP connections often creates the need to rebuild routing tables altogether.
Many of the backbone service providers have updated their devices to guard against the new attack, Rouland said, as they were given advance notice of the public release of the information.
The likelihood of actual attacks using this technique is lessened somewhat by the fact that attackers need to know both the source and destination IP addresses as well as the source and destination ports for whatever connection they want to go after.
Also, using IP Sec wherever possible to encrypt TCP sessions prevents attackers from being able to see TCP data for those sessions.
Watson plans to discuss the new technique in more detail at the CanSecWest security conference this week in Vancouver, British Columbia.
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