Transmission Control Protocol (TCP) Hijack
Wall Script
Wall Script
Saturday, December 20, 2008

Transmission Control Protocol (TCP) Hijack

This guide is meant for ethical hacking or audit with authorization purposes only. The author is not responsible for any consequences otherwise. The material is copyrighted.

Requirements: Linux OS, Connect to the same LAN or wireless network as the victim,

Once a malicious user gains access to the FTP session traffic he can now begin to monitor the session and wait for an opportunity to hijack the session. A hijack occurs when the attacker is able to intercept the communication between the client and server after the session has been authenticated. The simplest method to hijack the session would be to send a reset to the user forcing the client application to close the FTP session but he also have to prevent the client from resetting the port on the server end. If he does not prevent this packet from reaching the server then the connection will be terminated and he will have to wait for another opportunity to hijack a session. Once he has successfully closed the client, he now has the opportunity to send queries to the server requesting files or upload his own malicious files to the server. Since he was monitoring the entire session between the server and client he has the right sequence number and acknowledgment number so that the server thinks its still communicating with the original client.

If the attacker chooses to keep both the client and server running then he will have to keep track of the sequence number and acknowledgment number being sent between the client and server. Any command that the attacker sends to the server will change the sequence number and acknowledgment numbers and will cause the client and server to be out of synchronization and they will not be able to communicate thus causing the connection to close. This method is more difficult because the attacker now has to continually change the client and server sequence/acknowledgment numbers to reflect the commands that he injected towards the server and the data he received from the server.

You have to be on the same wireless or LAN network to accomplish this.

The process of FTP Hijack:

ARP Spoof

Arp spoof the victim to the gateway (Victim:; Gateway: using arpspoof from the attacking machine ( to redirect all traffic through the attacker.
# echo 1 >; /proc/sys/net/ipv4/ip_forward
# arpspoof -t
# arpspoof -t


Hunt is a program for intruding into a connection, watching it and resetting it. Hunt operates on Ethernet and is best used for connections which can be watched through it. However, it is possible to do something even for hosts on another segments or hosts that are on switched ports. Hunt doesn't distinguish between local network connections and connections going to/from Internet. It can handle all connections it sees. Connection hijacking is aimed primarily at the telnet or rlogin traffic but it can be used for another traffic too. Features: connection management (watching, spoofing, detecting, hijacking, resetting), daemons (resetting, arp spoof/relayer daemon, MAC discovery daemon for collecting MAC addresses, sniff daemon for logging TCP traffic), host resolving, packet engine (TCP, UDP, ICMP and ARP traffic; collecting TCP connections with sequence numbers and the ACK storm detection), switched environment (hosts on switched ports can be spoofed, sniffed and hijacked too). This latest release includes lots of debugging and fixes in order to get the hunt running against hosts on switched ports, timejobs, dropping IP fragments, verbose status bar, options, new connection indicator, various fixes.

By default, Hunt only monitors telnet (port 23) and rlogin (port 513) sessions, but the code is written in such a way that it would be very easy to add other types. In the file hunt.c, in the initialization code for the entry function, is this line:


This function is located in the addpolicy.c file and here's the function in question:

api->;dst_ports[2] = htons(21); was added to incorporate FTP sessions.

void add_telnet_rlogin_policy(void)
struct add_policy_info *api;

api = malloc(sizeof(struct add_policy_info));
memset(api, 0, sizeof(sizeof(struct add_policy_info)));
api->;src_addr = 0;
api->;src_mask = 0;
api->;dst_addr = 0;
api->;dst_mask = 0;
api->;src_ports[0] = 0;
api->;dst_ports[0] = htons(23);
api->;dst_ports[1] = htons(513);
api->;dst_ports[2] = htons(21); //This port was added for FTP
api->;dst_ports[3] = 0;
list_push(&;l_add_policy, api);

The source files were compiled and hunt.c executed.

* hunt 1.5
* multipurpose connection intruder / sniffer for Linux
* (c) 1998-2000 by kra
starting hunt
--- Main Menu --- rcvpkt 0, free/alloc 64/64 ------
l/w/r) list/watch/reset connections
u) host up tests
a) arp/simple hijack (avoids ack storm if arp used)
s) simple hijack
d) daemons rst/arp/sniff/mac
o) options
x) exit

HUNT Preparations

Customize Options and Start Daemons

o is typed to customize options. The MAC base is changed to attacker's NIC 00:ab:cd:ef:gh:mn. Host resolving, arp spoof with MAC base and learn IP from MAC discovery are all enabled.

From the main menu, d daemons -- a arp spoof daemon is started. Hunt can also arp spoof the hosts and targets if specified.

FTP Hijack

From the main menu, l gives a list of connections.

0) [32777] -- 95.623.58.102 [21]

w - Watches the above connection.
a - Performs a simple hijack.

Once you hijack, you have access to the files being sent. You can manipulate them using a tool like frag route to craft evil packets. If the connection is telnet on port 23, you will have the shell on both the machines.


? Access to Data
? Access to the command shell
? DOS Attack

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