[ISN] Flaw Leaves Internet Open to Attacks

[InfoSec News]

http://www.eweek.com/article2/0,1759,1571185,00.asp

By Dennis Fisher 
April 20, 2004   
 
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. 

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.