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Denial of Service and Session Hijacking

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In this sample chapter from Certified Ethical Hacker (CEH) Exam Cram, you will explore types of protocol attacks, common tools used for denial of service (DoS) attacks, and countermeasures.

This chapter is from the book

Denial of Service

Denial of service (DoS) attacks, as the name suggests, are not about breaking into a system but rather about denying legitimate users the opportunity to use the system. In most cases, a DoS attack is easy to execute. This makes DoS attacks a very serious problem. Every technology has limits; if you can exceed those limits, then you can make a system unusable.

Protocol Attacks

A protocol attack tries to exploit some vulnerability in the protocol being used. Exploiting such vulnerabilities can cause a system to become unresponsive. The magnitude of a protocol attack is measured in packets per second (pps).

TCP SYN Flood Attacks

A TCP SYN flood attack is an older type of DoS attack, but it illustrates the concepts of denial of service quite well. This particular type of attack depends on the hacker’s knowledge of how connections to a server are made. When a session is initiated between a client and a server in a network using TCP, a packet is sent to the server with a 1-bit flag called a SYN flag set. (SYN is short for synchronize.) This packet is asking the target server to synchronize communications. The server allocates appropriate resources and then sends to the client a packet with both the SYN (synchronize) and ACK (acknowledge) flags set. The client machine is then supposed to respond with an ACK flag set. This process, called a three-way handshake, is summarized as follows:

  1. The client sends a packet with the SYN flag set.

  2. The server allocates resources for the client and then responds with the SYN and ACK flags set.

  3. The client responds with the ACK flag set.

There have been a number of well-known SYN flood attacks on web servers. This attack type is popular because any machine that engages in TCP communication is vulnerable to it—and all machines connected to the Internet engage in TCP communications. Such communication is obviously the entire reason for web servers. The easiest way to block DoS attacks is via firewall rules.

Teardrop Attacks

Fragmentation attacks in general try to prevent targets from being able to reassemble packet fragments. They usually involve sending a large number of fragmented packets to the target. A teardrop attack is a specific type of fragmentation attack. In a teardrop attack, the attacker sends a fragmented message, where the two fragments overlap in ways that make it impossible to reassemble them properly without destroying the individual packet headers. Therefore, when the victim attempts to reconstruct the message, the message is destroyed. This causes the target system to halt or crash. There are a number of variations on the basic teardrop attack, such as TearDrop2, Boink, targa, Nestea Boink, NewTear, and SYNdrop.

ACK Flood Attacks

As the name suggests, an ACK flood attack involves sending a flood of TCP ACK packets. Normally an ACK packet is an acknowledgment of something being received, be it data or a synchronization request. Some devices or services are stateful, which means they process each packet. When a target receives a flood of ACK packets, it tries to process it but, because it is not actually an acknowledgment of anything, it can overwhelm the target.

TCP State Exhaustion Attacks

There are a variety of state exhaustion attacks, and the idea behind them all is essentially the same. They attack weaknesses in Layers 3 and 4 of the protocol stack and overconsume resources. Invalid name queries to a DNS server are a type of state exhaustion attack. TCP state exhaustion attacks operate on some aspect of the TCP handshake. For example, a SYN flood attack is a type of TCP state exhaustion.

Application Layer Attacks

Application layer DoS attacks work to consume a given application’s resources. The magnitude is usually measured in requests per second (rps). Basically, overwhelming a target server with too many requests is the basis for most application layer attacks.

HTTP Post DoS Attacks

An HTTP post DoS attack involves sending a legitimate HTTP post message. Part of the post message is the content length, which indicates the size of the message to follow. In this type of attack, the attacker sends the actual message body at an extremely slow rate. The web server is then hung as it waits for the message to complete. For more robust servers, the attacker needs to use multiple HTTP post attacks simultaneously.

Slowloris Attacks

A Slowloris attack is another attack against web servers. The attacker sends partial HTTP requests. When the target receives these requests, it opens a connection and waits for the requests to complete. But rather than complete a request, the attacker continues to send multiple partial requests. Eventually, the server has opened so many connections that it exhausts its maximum connection pool limit and can no longer respond to legitimate requests.

Volumetric Attacks

All volumetric attacks seek to overwhelm the target with an overwhelming number of packets. These attacks are not particularly sophisticated or difficult. They simply overwhelm the target. The magnitude of a volumetric attack is usually measured in bits per second (bps).

Smurf IP Attacks

A UDP attack is a type of volumetric attack, and a Smurf attack is a very popular version of a DoS attack. An ICMP (Internet Control Message Protocol) packet is sent out to the broadcast address of the network. The network responds by echoing the packet out to the network hosts, which then send it to the spoofed source address. Also, the spoofed source address can be anywhere on the Internet, not just on the local subnet. A hacker who can continually send such packets can cause the network itself to perform a DoS attack on one or more of its member servers. This attack is clever and rather simple. The only problem for the hacker is getting the packets started on the target network. This task can be accomplished via some software, such as a virus or Trojan horse, that begins sending the packets.

In a Smurf attack, there are three people/systems involved: the attacker, the intermediary (who can also be a victim), and the victim. The attacker first sends an ICMP echo request packet to the intermediary’s IP broadcast addresses. Since this is sent to the IP broadcast address, many of the machines on the intermediary’s network receive this request packet and send back an ICMP echo reply packet. If all the machines on a network are responding to this request, the network becomes congested, and there may be outages.

The attacker impacts the third party—the intended victim—by creating forged packets that contain the spoofed source address of the victim. Therefore, when all the machines on the intermediary’s network start replying to the echo request, those replies flood the victim’s network. Thus, another network becomes congested and could become unusable. This type of attack is illustrated in Figure 4.4 in Chapter 4, “Malware.”

UDP Flood Attacks

The UDP flood attack is another example of a volumetric attack. Keep in mind that UDP (User Datagram Protocol) is a protocol that does not verify each packet’s delivery. In a UDP flood attack, the attacker sends a UDP packet to a random port on a target system. When the target system receives a UDP packet, the attacker determines what application is listening on the destination port. Then, if the attacker wants to attack that application, he or she just starts a flood of UDP packets to the IP address and port. If enough UDP packets are delivered to ports on the target, the system becomes overloaded trying to determine awaiting applications (which do not exist) and then generating and sending packets back.

ICMP Flood Attacks

The ICMP flood attack is another volumetric attack. ICMP flood attacks are usually accomplished by broadcasting a large number of either pings or UDP packets. Like other flood attacks, the idea is to send so much data to the target system that the system slows down. If it can be forced to slow down enough, the target will time out (i.e., not send replies fast enough) and be disconnected from the Internet. This type of attack is far less effective against modern computers than it was against older ones. Even a low-end desktop PC now has 4 GB (or more) of RAM and a dual-core processor, making it difficult to generate enough pings to knock the machine offline. However, at one time, this was a very common form of DoS attack.

Ping of Death Attacks

A ping of death attack, often simply called a PoD attack, is accomplished by sending malformed ICMP packets (e.g., sending a packet that is 65,536 bytes in size). RFC 791 specifies a maximum packet size of 65,535 bytes. A PoD attack can cause a vulnerable system to crash.

Other DoS Attacks

Some DoS attack types don’t fit neatly into one of the previously discussed categories. These attacks can nonetheless be quite effective against target systems.

Multi-Vector Attacks

As the name suggests, a multi-vector attack is a combination of two or more of the other attacks (e.g., launching a SYN flood attack and a teardrop attack at the same time). Another method is to launch one type of attack and then, after a time, to shift to a different attack vector. This method can overcome DoS countermeasures the target may have implemented.

DHCP Starvation Attacks

DHCP (Dynamic Host Configuration Protocol) is used to dynamically assign IP addresses to systems on a network. If an attacker floods a target network with DHCP requests for dynamic IP addresses, the attacker can completely exhaust the address space allocated by the DHCP server. Then legitimate users cannot get an IP address assigned and thus cannot connect to the network. There are tools such as gobblers that can do this for an attacker.

PDoS Attacks

Though not terribly common, it is possible to have a DoS attack that leaves the system either inoperable or needing the operating system completely reinstalled. These are referred to as permanent denial of service (PDoS) attacks, or phlashing. Such attacks usually involve DoS attacks on a device’s firmware.

Registration DoS Attacks

A registration DoS attack is a very simplistic attack used against websites. The attacker creates a script or program that just keeps registering fake users on a website. This is one reason many registration websites use CAPTCHA.

Login DoS Attacks

Login DoS attacks are similar to registration DoS attacks and also frequently use scripts or programs. The attacker tries to overload the login process by continually sending login information. This can overwhelm the target system or at least slow it down. Many websites use CAPTCHA to prevent automated login attempts.

DDoS Attacks

Perhaps the most common form of DoS attack today is the DDoS attack. This type of attack is accomplished by getting various machines to attack the target. This is commonly done by sending out a Trojan horse that causes infected computers to attack a specified target at a particular date and time—which is a very effective way to execute a DDoS attack on any target. In this form of DDoS attack, the attacker does not have direct control of the various machines used in the attack. These machines are simply infected by some malware that causes them to participate in the attack on a particular date and at a particular time.

Another method is to use a botnet to orchestrate a DDoS attack. A botnet is a network of computers that have been compromised by an attacker so that the attacker has control of the computers. This is often accomplished via delivery of a Trojan horse. However, unlike in the previous DDoS example, the attacker has direct control over the attacking machines in the botnet.

A botnet usually has a command and control (C&C) that controls the various compromised machines. Then the botnet can be used for whatever the attacker wishes. DDoS is only one application of a botnet. Password cracking and sending phishing emails are other uses. The compromised systems can be attacked in any of the ways that malware is usually distributed: via phishing emails, compromised websites, vulnerable target systems, etc.

Peer-to-Peer Attacks

While peer-to-peer (P2P) apps have become quite popular, so have P2P DoS attacks. One method is to force the client to disconnect from the legitimate P2P hub and get the client to connect to the attacker’s fake hub. There have also been massive DDoS attacks on peer-to-peer networks. In addition, attackers attempt to exploit flaws in the protocols used, such as the Direct Connect (DC++) protocol that is used to share files between peer-to-peer clients.

Distributed Reflection DoS Attacks

As previously stated, DDoS attacks are becoming more common. Most such attacks rely on getting various machines (i.e., servers or workstations) to attack the target. A distributed reflection DoS attack is a special type of DoS attack. As with all such attacks, it is accomplished by the hacker getting a number of machines to attack the selected target. However, this attack works a bit differently than other DoS attacks. Rather than getting computers to attack the target, this method tricks Internet routers into attacking a target.

Many of the routers on the Internet backbone communicate on port 179, particularly using BGP (Border Gateway Protocol) to exchange routing information. A distributed reflection DoS attack exploits that communication line and gets routers to attack a target system. What makes this attack particularly wicked is that it does not require the routers in question to be compromised in any way. The attacker does not need to get any sort of software on a router to get it to participate in the attack. Instead, the hacker sends a stream of packets to the various routers, requesting a connection. The packets have been altered so that they appear to come from the target system’s IP address. The routers respond by initiating connections with the target system. What occurs is a flood of connections from multiple routers, all hitting the same target system. This has the effect of rendering the target system unreachable.

Common Tools Used for DoS Attacks

As with any of the other security issues discussed in this book, you will find that hackers have at their disposal a vast array of tools in the DoS arena. While it is certainly well beyond the scope of this book to begin to categorize or discuss all of these tools, a brief introduction to just a few of them will prove useful.


LOIC (Low Orbit Ion Cannon) is one of the most widely known DoS tools available. It has a very easy-to-use graphical user interface, shown in Figure 6.1.



This tool is very easy to use. As you can see in Figure 6.1, it simply requires the user to enter the target URL or IP address and then begin the attack. Fortunately, this tool also does nothing to hide the attacker’s address and thus makes it relatively easy to trace the attack back to its source. It is an older tool but still widely used today. There is a tool similar to this named HOIC, which we discuss later in this section.


DoSHTTP is another tool that is simple to use. You select the target, the agent (i.e., the browser type to simulate), the number of sockets, and the requests and then start the flood. You can see this in Figure 6.2.




XOIC, which is similar to LOIC, has three modes: send a message, execute a brief test, or start a DoS attack. You can see these options in Figure 6.3.



Like LOIC, XOIC is very easy to use. It is just a point-and-click graphical user interface. Even attackers with minimal skill can launch a DoS attack using XOIC.


HOIC (High Orbit Ion Cannon) was developed by the Anonymous collective as an improvement on LOIC. It is available at https://sourceforge.net/projects/highorbitioncannon/. Although HOIC was meant to be more powerful than LOIC, it still has a very simple user interface, which can be seen in Figure 6.4.



Other Tools for DoS and DDoS Attacks

There are many other tools for DoS and DDoS. A few are listed here:

Countermeasures to DoS and DDoS Attacks

The CEH exam will ask you about countermeasures to DoS and DDoS attacks. A few of them have already been discussed. For example, CAPTCHA can mitigate web DoS attacks. In general, three categories can be used in the case of overwhelming attacks:

  • arrow.jpg Simply shut down the targeted service. This is usually not a good choice, as it essentially means capitulating to the attack.

  • arrow.jpg Keep the critical services functioning by stopping noncritical services and use those resources for the critical services.

  • arrow.jpg Absorb the attack. This method is popular with internet service providers (ISPs; for an added charge). When the ISP detects a DoS or DDoS attack in progress, it allocates additional bandwidth to absorb that attack.

A good antivirus approach coupled with regular system updates can prevent one of your systems from becoming compromised and becoming part of a botnet. Filtering incoming and outgoing traffic to your network can also mitigate DoS attacks. Rate limiting any service or IP address so that it can consume only a finite percentage of resources also helps mitigate DoS attacks.

Honeypots are gaining popularity in deflecting all sorts of attacks, including DoS attacks. A honeypot is a fake system set up for the sole purpose of attracting hackers. Essentially, if a honeypot looks realistic enough, the attacker may go after it rather than after a real system.

Robust network configuration can also help mitigate DoS attacks. Load balancing critical services is a very good first step in helping mitigate DoS attacks. Throttling or limiting traffic for a given service can also help. Being able to drop incoming requests when a certain threshold is reached is also helpful.

There is actually a standard for filtering. RFC 3704, “Ingress Filtering for Multihomed Networks,” is a standard to help limit the impact of DDoS attacks by blocking any traffic with spoofed IP addresses.

Black hole filtering is another common technique. A black hole is a network location where traffic is simply discarded/dropped, typically by sending traffic to an IP address that is not in use. When a DoS attack is detected, suspected DoS traffic can be forwarded to the network black hole.

As mentioned earlier in this book, the CEH exam has a strong emphasis on Cisco. You therefore need to be familiar with a couple Cisco commands that can help mitigate DoS attacks:

  • arrow.jpg access-list access-list-number {deny | permit} tcp any destination destination-wildcard: Defines an IP extended access list

  • arrow.jpg ip tcp Intercept list access-list-number: Enables TCP intercept

There are also a number of devices that can be added to a network to help mitigate DoS attacks, including:

There are also software solutions that can help mitigate DoS attacks:

SPI (stateful packet inspection) is an excellent way to mitigate DoS attacks. Many modern firewalls use SPI. These types of firewalls not only apply rules to each packet but maintain the state of communication between the client and the server. As an example of how this mitigates attacks, the firewall realizes that multiple SYN packets are coming from the same IP address and then blocks those packets. This is one major reason SYN floods are not seen much today. In addition, next-generation firewalls (NGFWs) combine traditional firewall capabilities and other functions, such as those of an application firewall or an intrusion detection system/prevention system (IDS/IPS). Using a modern advanced firewall is an excellent way to mitigate DoS and DDoS attacks.

DoS in the Real World

According to the security consulting firm Calyptix Security, the first quarter of 2018 set records for DoS and DDoS attacks. This included a massive DDoS attack against the GitHub site on February 28, 2018, peaking at 1.3 Tbps. This illustrates how effective and damaging these attacks can be, for the amount of data sent in DoS attacks is growing all the time.

One creative example comes from 2017. In February 2017, a new DDoS attack vector emerged. Attackers used memcache, a database caching system, to amplify traffic volume. A request could be amplified by a factor of several thousand by using this method. The aforementioned GitHub attack involved memcaching. This illustrates that new methods of DoS are being developed, and you should expect to see them out in the real world (though not on the CEH exam).

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