Waraxe IT Security Portal

0xbadfaceX · Regular user

TCP Session Hijacking.

By 0xbadfaceX. Razz

TCP Session Hijacking allows you to redirect a TCP flow. A hacker can then override protection with a password (like telnet or FTP). The need for listening (sniffing) restricts the scope of this attack to the physical network of the target. Before detailing the attack, we explain some basic principles of TCP.

We will not disclose here the mysteries of the TCP protocol, but only the main points to understand the attack. The header TCP is made up of several fields:

* Port source and destination port, to identify the connection between two machines;
* The sequence number that identifies each byte sent;
* Acknowledgement number that corresponds to the number of acquittal last byte received;
* Flags, along with those that will interest us are:
o SYN which synchronizes the sequence numbers when establishing a connection;
o ACK, the flag of a TCP segment;
o HSP, which tells the receiver to trace the data to the application.

The sequence numbers will change depending on the number of bytes of data sent. The number sequence is represented by Seq, the acknowledgement number is found after the HSP and ACK flags.

The attack creates a state of desynchronization on each side of the TCP connection, making possible the flight session. A connection is out of sync when the sequence number of the next byte sent by the A machine is different number sequence next byte to receive a B. And conversely, there is desynchronization when the sequence number of the next byte sent by the machine B is different from the sequence number of the next byte to be received by A.

When B receives its package, A waits for a package with a number of acquittal x +60. If the next package by B does not have this number of acquittal then A and B are as desynchronized.

Specifically, a pirate with a machine C wants to steal a Telnet session between machines A and B. In a first step, the C machine sniffs the Telnet traffic (TCP port 23) between A and B. Once the attacker thinks A had time to authenticate to the Telnet service on the B machine, it desynchronizes the machine A against B. For this, he forges a package with the source IP address of the A machine and the TCP acknowledgement number expected by B. The B machine accepts the package. In addition desynchronizing the TCP connection, this package allows the attacker to inject a command via Telnet session previously established by A. Indeed, this package can carry data (HSP flag equal to 1).

These techniques are implemented by the hunt program. Wink

lenny · Valuable expert

Interesting, thanks for that! Yet another weapon to add to my armoury =]