TCP/IP Hijacking

TCP/IP hijackingTCP/IP hijacking is a technique that uses spoofed packets to take over a connection between a victim and a host machine. It is similar to a man-in-the-middle attack, except that the rogue agent sends a reset request to the client so that the client loses contact with the server while the rogue system assumes the role of the legitimate client, continuing the session. This technique is especially useful when the victim uses a one-time password to connect to the host machine. A one-time password can, as its name implies, be used to authenticate once and only once; thus, sniffing the authentication is useless for the attacker.

To carry out a TCP/IP hijacking attack, the attacker must be on the same network as the victim. This gives the attacker the ability to sniff the local network segment and, as a result, all the details of open TCP connections can be pulled from the headers. Each TCP packet contains a sequence number in its header. This sequence number is incremented with each packet sent to ensure that packets are received in the correct order. While sniffing packets, the attacker has access to the sequence numbers for a connection between a victim and a host machine. Then the attacker sends a spoofed packet from the victim’s IP address to the host machine, using the sniffed sequence number to provide the proper acknowledgment number. The host machine will receive the spoofed packet with the correct acknowledgment number and will have no reason to believe the packet did not come from the victim’s machine; thus the TCP/IP hijacking attempt will be successful.


Forms of TCP/IP Hijacking

One form of TCP/IP hijacking is to inject an authentic-looking reset (RST) packet. If the source is spoofed and the acknowledgment number is correct, the receiving side will believe that the source actually sent the reset packet, and the connection will be reset. The attacker could perform such an attack using a program that uses the libpcap and libnet libraries. libpcap would sniff the packets, and libnet would inject RST packets. The program does not need to look at every packet, but only established TCP connections to a target IP, so the libcpap function calls would be structured accordingly. It is relatively easy to come up with a filter rule for packets that have a certain destination IP. It is somewhat more difficult to filter for established connections, but since all established connections will have the ACK flag in the TCP header TCP flags, the program can look for that.

Another type of TCP/IP hijacking is continued hijacking. The spoofed packet does not need to be an RST packet; the spoof packet can contain data. When the host receives the spoofed packet, it will increment the sequence number and responds to the victim’s IP. Since the victim’s machine does not know about the spoofed packet, the host machine’s response has an incorrect sequence number, so the victim ignores that response packet. And since the victim’s machine ignored the host machine’s response packet, the victim’s sequence number count is off. Therefore, any packet the victim tries to send to the host machine will have an incorrect sequence number as well, causing the host machine to ignore it. In this instance, both legitimate sides of the connection have incorrect sequence numbers, resulting in a desynchronized state. And since the attacker sent out the first spoofed packet that caused all this chaos, it can keep track of sequence numbers and continue spoofing packets from the victim’s IP address to the host machine. This lets the attacker continue communicating with the host machine while the victim’s connection hangs.


External Links:

TCP Hijacking at TechRepublic

Replay Attacks and Possible Countermeasures

replay attackReplay attacks are a variation on the man-in-the-middle theme. In a replay attack an agent is once again placed within the client/server line of communication. In the case of a replay attack, however, the transaction data is recorded for the express purpose of allowing the data to be modified and replayed to the server at a later time for nefarious purposes.

An example of a replay attack is an instance where one party wants to prove their identity to a another party. If a third party eavesdrops on the conversation, they can intercept the password. Once the exchange is over, the eavesdropper can send the password and impersonate the party to whom the password belongs to gain unauthorized access to the other party.

Defenses Against Replay Attacks

As with other man-in-the-middle attacks, replay attacks can be countered using encryption, timestamps, serial numbers and packet sequences so that the server can detect that the data is being replayed from a previous session. One effective method of avoiding replay attacks which uses encryption is to use session tokens. Let us assume that A is required to send a password to B. If session tokens are used, B will send a one-time token to A, which A will then use to transform the password and send the result to B. On the other side, B performs the same transformation, and if both values match, the login will be successful. If C eavesdrops on A and B and captures the transformed password, C can try to use it to authenticate with B. But B will send a session token, and if C sends the transformed password captured earlier, the transformations will not match, and authentication will fail.


If C knows that B is using session tokens, C might be able to pose as B, presenting some predicted future token, and convince A to use that token in A’s transformation. C can then replay A’s reply at a later time, when the previously predicted token is presented by B, and B will accept the authentication. For that reason, session tokens should be chosen by a pseudo-random process.

One-time passwords are similar to session tokens in that the password expires after it has been used or after a very short period of time. They can be used to authenticate individual transactions in addition to sessions.

Replay attacks can also be thwarted by the use of message authentication codes (MACs), short pieces of information used to authenticate a message and to provide integrity and authenticity assurances on the message. MAC algorithms accept as input a secret key and an arbitrary-length message to be authentication, and outputs a MAC. This value protects both a message’s data integrity and its authenticity by virtue of the fact that the verifiers possessing the secret key to detect any changes to the message content.

Timestamping is another means of preventing a replay attack. Synchronization is achieved using a secure protocol. As an example, B can broadcast the time on their clock along with a message authentication code (MAC). If A wants to send B a message, they can include an estimate of the time on B’s clock in their message (which also sends a MAC). B only accepts messages for which the timestamp is within a reasonable tolerance.


External Links:

Replay attack on Wikipedia

Man-in-the-Middle Attacks

man-in-the-middle attackMan-in-the-middle attacks are perhaps one of the more complex and sophisticated forms of security breaching approaches. As the name implies, such an attack involves the surreptitious placement of a software agent between the client and server ends of a communication. In this scenario, neither end of the communication is aware that the malicious agent is in the line of communication. For the most part, the man in the middle simply relays the data transmissions between client and server as though nothing is happening. What is generally happening in parallel with this process is that the agent is also recording the data as it is passed through. A man-in-the-middle attack can succeed only when the attacker can impersonate each endpoint to the satisfaction of the other. Such an attack results in a third party gaining access to a variety of different types of data, from login and password credentials to proprietary and confidential information. In addition, it is possible for the man-in-the-middle agent to modify data, causing unsold problems for the victim.

Man-in-the-middle attacks have increased considerably since the introduction of wireless networking. As a result, there is no need for the hacker to connect to a wire. Instead, the data can simply be intercepted from anywhere within range of the wireless signal.


Preventing Man-in-the-Middle Attacks

In order to prevent MITM attacks, some form of endpoint authentication is helpful. Just using public key encryption is not enough to prevent such an attack. As an example, suppose A and B are trying to communicate, and C is trying to intercept said communications. If B sends A his public key and C intercepts it, he can replace B’s public key with his own and send it to A. If A then encrypts a message with C’s public key (believing it to be B’s public key), then when it is sent, C can intercept and read it, decrypting it with his private key. He can also re-encrypt the message using C’s public key and send it to C.

Thus, any private-public key system requires some means of ensuring that a MITM attack does not compromise its integrity. One possible method is public key infrastructures (PKI). The main defense in a mutual authentication. In this case, as well as the application validating the user, the user’s devices validate the application – hence distinguishing rogue applications from genuine applications. Another possibility is a recorded media attestment, which can be either a verbal communication of a shared value for each session, or an audio/visual communication of the public key hash. In addition, stronger mutual authentication, such as secret keys and passwords often helps thwart man-in-the-middle attacks.

Latency examination may be a useful means of detecting man-in-the-middle attacks. For example, if each party performs a long cryptographic hash function calculation that takes 20 seconds normally, and the calculation takes 60 seconds to reach each party, this can indicate a third party.

The integrity of public keys must generally be assured in some manner, but need not be secret. Passwords and shared secret keys have the additional secrecy requirement. Public keys can be verified by a certificate authority whose public key is distributed through a secure channel. Public keys can also be verified by a web of trust that distributes public keys through a secure channel.

Quantum cryptography protocols, which use quantum communication and quantum communication to perform cryptographic tasks, can be used to thwart man-in-the-middle attacks. One method quantum cryptography employs is quantum key distribution (QKD), which establishes a shared key between two parties. If a third party tries to eavesdrop and learn these bits, the messages will be disturbed and the original two parties will notice. The key is then typically used for encrypted communication.


External Links:

Man-in-the-middle attack on Wikipedia

IP Spoofing and Defenses

IP spoofingIP address spoofing is the creation of IP packets with a source IP address with the purpose of concealing the identity of the sender or impersonating another computer system. The basis of spoofing involves masquerading as a trusted system in order to gain unauthorized access to a secure environment. IP spoofing involves modifying data to make it appear to originate from the IP address of a system that is trusted by a server or firewall. Using this approach, a host is able to pass through the IP filtering that would otherwise serve to prevent access.

The objective of IP spoofing in most, but not all cases, is to gain unauthorized access to a server or service. DNS spoofing differs from IP spoofing in that the objective is to send users to a different location than the one to which they thought they were going. For example, assume a user wants to login to Facebook. He enters the URL of Facebook into his browser. The browser contacts a Domain Name Server (DNS) which looks up the IP address which matches the URL. The user is then taken to the site located at that IP address, where he enters his login name and password. DNS spoofing involves the DNS server being compromised such that the Facebook URL is set to point to the IP address of a malicious party where a web site that looks like Facebook has been set up. Now when the user enters the URL in a browser, he is taken to the fake web site where his login name and password are captured and stored. The web site might then report that Facebook is offline for maintenance. The user decides to try again later. In the meantime, the attacker uses the victim’s credentials to log into his Facebook account and gain a foothold in committing identity theft. Even more nefarious would be if the attacker used DNS spoofing to point to a fake bank web site or another site where the attacker may be able to gain access to sensitive data.


IP spoofing is not, however, always carried out with malicious intent. In performance testing of websites, hundreds or even thousands of virtual users may be created, each executing a test script against the web site under test, in order to simulate what will happen when the system goes live and a large number of users log on at once. Commercial testing products can use IP spoofing, allowing each user its own IP address.

IP Spoofing: Defenses

There are several possible defenses against IP spoofing. Packet filtering is one defense against IP spoofing attacks. The gateway to a network usually performs ingress filtering, which is blocking of packets from outside the network with a source address inside the network. This prevents an outside attacker spoofing the address of an internal machine. Ideally, the gateway would also perform egress filtering on outgoing packets, which is blocking of packets from inside the network with a source address that is not inside. This prevents an attacker within the network performing filtering from launching IP spoofing attacks against external machines. In addition, many firewalls (pfSense included) practice bogon filtering, which means that IP packets from the Internet that claim to be from an area of the IP address space reserved, but not yet allocated or delegated by the Internet Assigned Numbers Authority (IANA) or a delegated Regional Internet Registry (RIR), are blocked.
Some upper layer protocols provide their own defense against IP spoofing attacks. For example, Transport Control Protocol (TCP) uses sequence numbers negotiated with the remote machine to ensure that arriving packets are part of an established connection. Since the attacker normally cannot see any reply packets, the sequence number must be guessed in order to hijack the connection.

Implementing encryption and authentication will also reduce spoofing threats. Both of these features are included in Ipv6, which will eliminate current spoofing threats. Additionally, a system administrator should eliminate all host-based authentication measures, which are sometimes common for machines on the same subnet. You should ensure that the proper authentication measures are in place and carries out over a secure, encrypted channel.

IP spoofing is a common problem without a simple solution, since it is inherent in the design of the TCP/IP protocol suite. Understanding how and why spoofing attacks are used, along with a few simple prevention methods, can help protect your network from these nefarious techniques.


External Links:

IP spoofing on Wikipedia

Phishing: Common Variations

phishingPhishing is the attempt to acquire sensitive information such as usernames, passwords, and credit card details be masquerading as a trustworthy entity in electronic communications. Communications purporting to be from popular social networking sites, auction sites, banks, online payment processors or IT administrators are commonly used to lure unsuspecting people. A phishing attack is most often initiated with a special type of spam containing a link to a misleading domain name, which appears to be a legitimate site. The e-mail tricks the recipient into visiting the spoofed web site, which mimics a site where the person would feel comfortable entering a username and password or other personal information.

Phishing has also been explained as leveraging or exploiting the design of web pages in a social engineering attack that tricks the user into thinking that they are in a legitimate and secure web session with a trusted site. In reality, the phishing site is designed to install malicious software or acquire personal information. The information is then used by the phisher for identity theft, to steal money, or to commit other fraudulent schemes.


Variations on Phishing

There are several variations on phishing. For example, “spear phishing” is targeted communication toward employees or members of a certain organization or online group. E-mails or other forms of communication are customized with information publicly available on web sites like Facebook or MySpace. In cases where e-mails are utilized, the e-mails will often direct people to a fake login page. One such early example was the early phishing attempts on AOL. A phisher would pose as an AOL staff member and send an instant message to a potential victim, asking them to reveal their password. In order to lure the victim into giving up sensitive information, the message might include imperatives such as “verify your account” or “confirm billing information”. Once the victim had revealed the password, the attacker could access and use the victim’s account for fraudulent purposes or spamming. Phishing became so prevalent on AOL that they added a line on all instant messages stating: “no one working at AOL will ask you for your password or billing information”, though even this did not prevent some people from giving away their passwords and personal information.

“Whaling” is phishing that is targeted at corporate executives, affluent people, and other “big phish”. Like spear phishing, whaling e-mails are often customized with information directed to the resident and sent to a relatively small number of people. One example of whaling was when thousands of bogus subpoenas appearing to be from the U.S. District Court in San Diego were “served” by e-mail on corporate executives. The e-mail contained an image of the official seal from the court and contained a link which purportedly linked to a copy of the entire subpoena. However, the link actually linked to a software installer that installed key-logging software on the user’s computer.

“Clone phishing” is a type of phishing attack whereby a legitimate, and previously delivered, e-mail containing an attachment or link has its content and recipient address (or addresses) taken and used to create an almost identical e-mail. The attachment or link within the e-mail is replaced with a malicious version and then sent from an e-mail address spoofed to appear to come from the original sender. It may claim to be a re-send of the original or possibly an updated version of the original. This technique could be used by the attacker to pivot from a previously infected machine and gain a foothold on another machine.


External Links:

Phishing on Wikipedia

Back Door Attacks

Back door attacks

Back Orifice in action.

Back door attacks utilize programs that provide a mechanism for entering a system without going through the usual authentication process. This can either take the form of hidden access points intentionally put into an application by the original developers to aid in maintaining and debugging the software which were then left in when the software was installed by customers, or a malicious program that is placed on a system via a virus or other method which opens up the system to unauthorized access.

Back Door Attacks: Back Orifice, NetBus and Sub7

A number of back door programs have been discovered over the years which can be used in back door attacks. Here are some of them:

  • Back Orifice: The brainchild of Sir Dystic of Cult of the Dead Cow, its initial purpose was to show the lack of security in Windows 98 (it was released in 1998), Back Orifice has legitimate purposes, such as remote administration. But it also has attributes that make it suited for less benign uses such as back door attacks. The server can hide itself from cursory looks by users of the system. As the server can be installed without user interaction, it can be distributed as the payload of a Trojan horse. As a result, the antivirus industry immediately categorized the tool as malware and appended Back Orifice to their quarantine lists. Two sequel applications followed: Back Orifice 2000 (released in 1999), and Deep Back Orifice by French Canadian hacking group QHA.


  • NetBus: Another program that can be used in back door attacks, this is a software program for remotely controlling a Microsoft Windows computer system over a network. It was released in 1998, a few months before Back Orifice. There are two components to the client-server architecture. The server must be installed and run on the computer that should be remotely controlled. The server was an .EXE file with a size of about 500 KB. When started for the first time, the server would install itself on the host computer, including modifying the Windows registry so that i starts automatically on each system startup. The server is a faceless process listening for connections on port 12345, with the port number adjustable on later versions. the client was a separate program presenting a graphical user interface that allowed the user to perform a number of activities on the remote computer, such as keystroke logging, screen captures, file browsing, opening and closing the CD-tray, and using tunneling protocols. The NetBus client was designed work under Windows 95/98/ME/NT 4.0, as well as Windows 2000/XP. Major parts of the protocol are textual, and as a result, the server can be controlled by typing commands over a raw TCP connection from a non-Windows computer.
  • Sub7: Originally designed by someone with the handle “mobman”, the name “Sub7” was derived by spelling “NetBus” backwards (“SubTen”) and swapping “ten” with “seven”. Because its typical use is to allow undetected and unauthorized access, Sub7 is usually described as a trojan horse by security experts. Like Back Orifice and NetBus, Sub7 is distributed with a server and a client. Sub7 has more features than NetBus, such as webcam capture, multiple port redirect, a user-friendly registry editor and chat, as well as penetration testing features, including a port scanner and a port redirector. Customizations possible with the Sub7 server editor included changing the port addresses, displaying a customized message upon installation. If the intent of the person deploying Sub7 is to launch a back door attack on a system, then the customized message can be used to deceive the victim and mask the true intent of the program. Nearly all antivirus programs can detect Sub7 and prevent it from being installed unless steps are taken to hide it.

Although the installation of any of the above mentioned back door programs will compromise network security, all of these threats can be prevented effectively by implementing a comprehensive virus scanning strategy.


External Links:

Back Orifice on Wikipedia

NetBus on Wikipedia

Sub7 on Wikipedia

Distributed Denial of Service (DDoS) Attacks

distributed denial of serviceIn the previous article, we discussed denial of service (DoS) attacks. These attacks involve the use of a single client to launch an attack on a system or service. Distributed denial of service (DDoS) attacks use the same basic attack methodologies as outlined in the previous article, with the exception that the attacks are initiated from multiple client systems.

The way this typically works is that malicious parties will use viruses to subtly gain control over large numbers of computers (typically poorly-defended home computers connected to broadband Internet connections). Unbeknownst to the owner of the computer (which generally continues to function as normal) the system is essentially a zombie waiting to be given instructions. Once the malicious party has gathered an army of zombie computers they are instructed to participate in massive distributed denial of service attacks on unsuspecting victims. A large enough volume of zombie systems can, and indeed have been known to bring down even the largest and most scalable enterprise infrastructure, and even bring parts of the Internet itself to a grinding halt. Merely purchasing more incoming bandwidth than the current volume of attack might not help, because the attacker might be able to simply add more attack machines.

Distributed Denial of Service Attacks: Advantages and Types

There are several advantages to launching a distributed denial of service attack:

  1. Multiple machines can generate more attack traffic than one machine.
  2. Multiple machines are harder to turn off than one attack machine.
  3. The behavior of each attack machine can be stealthier, making it harder to track and shut down.

Distributed denial of service can take several forms. Malware can carry distributed denial of service attack mechanisms. One of the better-known examples of this was MyDoom. Its DoS mechanism was triggered on a specific date and time. this type of distributed denial of service involved hardcoding the target IP address prior to the release of the malware. No further interaction was necessary to launch the attack.


A system may also be compromised with a trojan, allowing the attacker to download a zombie agent, or the trojan may contain one. Attackers can also break into systems using automated tools that exploit flaws in programs that listen for connections from remote hosts. A compromised system becomes known as a bot, and they are controlled by handlers run by the attacker, known as botnets. Many of these tools use classic DoS attack methods centered on IP spoofing and amplification like smurf and fraggle attacks, as well as SYN floods.

A distributed denial of service attack may involve sending forged requests to a very large number of computers that will reply to the requests. Using Internet Protocol address spoofing, the source address is set to that of the targeted victim, which means that all the replies will go to and flood the target.

The primary line of defense for blocking distributed denial of service attacks, as with DoS attacks, is the firewall. Firewalls can be set up to have simple rules to allow or deny protocols, ports or IP addresses. In the case of a simple attack coming from a small number of unusual IP addresses for instance, one could put up a simple rule to drop all incoming traffic from those attackers. But most complex attacks will be hard to block with simple rules. Additionally, firewalls may be too deep in the network hierarchy, although they can prevent users from launching simple flooding type attacks from machines behind the firewall.

Some stateful firewalls, like OpenBSD’s pf (and pfSense, since it’s based on pf), can act as a proxy for connections. Normally when a client initiates a TCP connection to a server, PF will pass the handshake packets between the two endpoints as they arrive. pf can proxy the handshake: pf itself will complete the handshake with the client, initiate a handshake with the server, and then pass packets between the two. In the case of a TCP SYN flood attack, the attacker never completes the three-way handshake, so the attacker’s packets never reach the protected server, but legitimate clients will complete the handshake and get passed. this minimizes te impact of spoofed TCP SYN floods on the protected service, handling it in pf instead.

Most switched also have some automatic and system-wide rate limiting, traffic shaping, delayed binding, deep packet inspection and Bogon (bogus IP) filtering to detect and block denial of service attacks. This will work as long as the distributed denial of service attack is something that can be prevented by using them. SYN floods can be prevented using delayed binding. Content-based DoS or DDoS may be prevented using deep packet inspection. And attacks originating from dark addresses can be prevented using Bogon filtering.


External Links:

Denial of service attack on Wikipedia

PF: Packet Filtering at www.openbsd.org

Denial of Service (DoS) Attacks

denial of serviceDenial of Service (DoS) attacks are undertaken with the express purpose of preventing users from accessing and using a service they should otherwise be able to access. such attacks make malicious use of a variety of different standard protocols and tools. There is no single denial of service attack method, and the term has come to encompass a variety of different forms of attack. Some of the different types of denial of service attacks will be outlined here.

Types of Denial of Service (DoS) Attacks

  • Ping flood: This attack uses the Internet Message Protocol (ICMP) ping request to a server as a denial of service method. The strategy either involves sending ping requests in such vast quantities that the receiving system is unable to respond to valid user requests, or sending ping messages which are so large (known as the ping of death) that the system is unable to handle the request.
  • Smurfing: As with ping flood attacks, smurfing makes use of the TCP Internet Message Protocol (ICMP) ping request to mount DoS attacks. In a typical smurfing attack, the attacker sends a ping request to the broadcast address of the network containing the IP address of the victim, rather than to a specific machine. The network then acts as a smurf amplifier. The ping request is sent to all computers on the broadcast network, which in turn all reply to the IP address of the victim system, thereby overloading the victim with ping responses. The primary method for preventing smurf attacks is to block ICMP traffic through routers so that the ping responses are blocked from reaching internal servers. In addition, services like the Smurf Amplifier registry have given network service providers the ability to identify misconfigured networks and to take appropriate action.
  • TCP SYN Flood: We have already discussed SYN flood attacks as a means of achieving denial of service on this website, but I’ll mention it here again. This attack begins with a client attempting to establish a TCP connection with the victim server. The client sends a request to the server, which in turn returns an ACK package to acknowledge the connection. At this point in the communication, the client should respond with a message accepting the connection. Instead, the client sends another ACK which is respnded to by the server with yet another ACK. The client continues to send ACKs to the server with the effect of causing the server to hold sessions open in anticipation of the client sending the final packet required to complete the connection. As a result the server uses up all available sessions serving the malicious client, thereby prevneting access to other users. One possible countermeasure is to limit the number of connections from any one client (which can easily be done in pfSense), but if the SYN flood is coming from several different clients, it is hardly the ideal solution. Moreover, if the attacker may be using a spoofed IP address, so limiting the number of connections from that IP address may not help at all. Another possibility is to set up a SYN proxy, so that clients do not connect to a server until the SYN/SYN-ACK/ACK handshake is complete.


  • Fraggle: A fraggle attack is similar to a smurfing attack with the exception that the User Datagram Protocol (UDP) is used instead of using ICMP.
  • Land: Under a land attack, the attacker creates a fake SYN packet containing the same source and destination IP addresses and ports and sends it to the victim, causing the system to become confused when trying to respond to the packet.
  • Teardrop: A teardrop type of denial of service attack exploits a weakness in the TCP/IP implementation on some operating systems. The attack works by sending messages fragmented into multiple UDP packages. Ordinarily the operating system is able to reassemble the packets into a complete message by referencing data in each UDB packet. The teardrop attack works by corrupting the offset data in the UDP packets, making it impossible for the system to rebuild the original packets. On systems that are unable to handle this corruption, a crash is the most likely outcome of a teardrop attack.
  • Bonk: An effective attack on some Windows systems involving the transmission corrupted UDP packets to the DNS port (port 53) resulting in a system crash.
  • Boink: This is similar to the Bonk attack except that the corrupted UDP packets are sent to multiple ports, not just port 53.

These are the most common forms of denial of service attacks. In the next article, we will look at distributed denial of service (DDoS) attacks.


External Links:

Denial-of-service attack on Wikipedia

© 2013 David Zientara. All rights reserved. Privacy Policy