Date Discovered

April 20, 2004


A vulnerability in TCP implementations may permit unauthorized remote users to reset TCP sessions. This issue affects products released by multiple vendors. Exploiting this issue may permit remote attackers to more easily approximate TCP sequence numbers. The problem is that affected implementations will accept TCP sequence numbers within a certain range of the expected sequence number for a packet in the session. This will permit a remote attacker to inject a SYN or RST packet into the session, causing it to be reset and effectively allowing denial-of-service attacks. An attacker would exploit this issue by sending a packet to a receiving implementation with an approximated sequence number and a forged source IP and TCP port. Few factors may present viable target implementations, such as imlementations that: – depend on long-lived TCP connections – have known or easily guessed IP address endpoints – have known or easily guessed TCP source ports. Note that Border Gateway Protocol (BGP) is reported to be particularly vulnerable to this type of attack. As a result, this issue is likely to affect a number of routing platforms. Note also that while a number of vendors have confirmed this issue in various products, investigations are ongoing and it is likely that many other vendors and products will turn out to be vulnerable as the issue is investigated further. Other consequences may also result from this issue, such as injecting specific data in TCP sessions, but this has not been confirmed. **Update: Microsoft platforms are also reported prone to this vulnerability. Vendor reports indicate that an attacker will require knowledge of the IP address and port numbers of the source and destination of an existent legitimate TCP connection in order to exploit this vulnerability on Microsoft platforms. Connections that involve persistent sessions, for example Border Gateway Protocol sessions, may be more exposed to this vulnerability than other TCP/IP sessions.

Technologies Affected


Block external access at the network boundary, unless external parties require service.
Employ ingress and egress filtering to verify source IP addresses at the network gateway.

Communicate sensitive information over encrypted channels.
Implementing IP Security (IPSEC) to encrypt TCP traffic can help against an attack. Authenticated BGP may also be deployed to mitigate attacks versus BGP implementations.

Communicate sensitive information over encrypted channels.
Using AUTHENTICATED BGP with strong passwords may help mitigate an attack via BGP.

Review and adjust according to policy any default configuration settings.
Limiting TCP source port information may conceal sensitive information from an attacker that may be useful in carrying out an attack.

Please see the referenced advisories for more information on obtaining and applying fixes.



Discovery is credited to Paul A. Watson.

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