SY0-501 Section 3.2- Summarize various types of attacks.

A computer connected to a computing network is potentially vulnerable to an attack. An “attack” is the exploitation of a flaw in a computing system (operating system, software program or user system) for purposes that are not known by the system operator and that are generally harmful.

Attacks are always taking place on the Internet, at a rate of several attacks per minute on each connected machine. These attacks are mostly launched automatically from infected machines (by viruses, Trojan horses, worms, etc.) without their owner’s knowledge. In rarer cases, they are launched by computer hackers.

In order to block these attacks, it is important to be familiar with the main types of attacks so as to set up preventive measures.

Attacks may be launched for various reasons:

– to obtain access to the system;

– to steal information, such as industrial secrets or intellectual property;

– to gather personal information about a user;

–  to retrieve bank account information;

– to get information about the organization (the user’s company, etc.);
– to disrupt the proper functioning of a service;

– to use the user’s system as a “bounce” for an attack;

– to use the resources of the user’s system, particularly when the network on which it is located has a high bandwidth.

Since you need you know about these attacks to counter them, consider the following types of attacks and learn about them.


A man-in-the-middle attack, as the name implies, generally occurs when attackers are able to place themselves in the middle of two other hosts that are communicating. Ideally, this is done by ensuring that all communication going to or from the target host is routed through the attacker’s host (which can be accomplished if the attacker can compromise the router for the target host). The attacker can then observe all traffic before relaying it and can actually modify or block traffic. To the target host, it appears that communication is occurring normally, since all expected replies are received.

The amount of information that can be obtained in a man-in-the-middle attack will obviously be limited if the communication is encrypted. Even in this case, however, sensitive information can still be obtained, since knowing what communication is being conducted, and between which individuals, may in fact provide information that is valuable in certain circumstances.

Man-in-the-Middle Attacks on Encrypted Traffic

The term “man-in-the-middle attack” is sometimes used to refer to a more specific type of attack—one in which the encrypted traffic issue is addressed. Public-key encryption, requires the use of two keys: your public key, which anybody can use to encrypt or “lock” your message, and your private key, which only you know and which is used to “unlock” or decrypt a message locked with your public key.

Denial-of-Service Attacks

Denial-of-service (DoS) attacks can exploit a known vulnerability in a specific application or operating system, or they can attack features (or weaknesses) in specific protocols or services. In a DoS attack, the attackers attempts to deny authorized users access either to specific information or to the computer system or network itself. This can be accomplished by crashing the system—taking it offline—or by sending so many requests that the machine is overwhelmed.

The purpose of a DoS attack can be simply to prevent access to the target system, or the attack can be used in conjunction with other actions to gain unauthorized access to a computer or network. For example, a SYN flooding attack can be used to prevent service to a system temporarily in order to take advantage of a trusted relationship that exists between that system and another.

SYN flooding is an example of a DoS attack that takes advantage of the way TCP/IP networks were designed to function, and it can be used to illustrate the basic principles of any DoS attack. SYN flooding uses the TCP three-way handshake that establishes a connection between two systems. Under normal circumstances, the first system sends a SYN packet to the system with which it wants to communicate. The second system responds with a SYN/ACK if it is able to accept the request. When the initial system receives the SYN/ACK from the second system, it responds with an ACK packet, and communication can then proceed.

In a SYN flooding attack, the attacker sends fake communication requests to the targeted system. Each of these requests will be answered by the target system, which then waits for the third part of the handshake. Since the requests are fake (a nonexistent IP address is used in the requests, so the target system is responding to a system that doesn’t exist), the target will wait for responses that never come. The target system will drop these connections after a specific time-out period, but if the attacker sends requests faster than the time-out period eliminates them, the system will quickly be filled with requests. The number of connections a system can support is finite, so when more requests come in than can be processed, the system will soon be reserving all its connections for fake requests. At this point, any further requests are simply dropped (ignored), and legitimate users who want to connect to the target system will not be able to do so, because use of the system has been denied to them.

DoS attacks are conducted using a single attacking system. A DoS attack employing multiple attacking systems is known as a distributed denial-of-service (DDoS) attack. The goal of a DDoS attack is also to deny the use of or access to a specific service or system. DDoS attacks were made famous in 2000 with the highly publicized attacks on eBay, CNN, Amazon, and Yahoo!

In a DDoS attack, service is denied by overwhelming the target with traffic from many different systems. A network of attack agents (sometimes called zombies) is created by the attacker, and upon receiving the attack command from the attacker, the attack agents commence sending a specific type of traffic against the target. If the attack network is large enough, even ordinary web traffic can quickly overwhelm the largest of sites, such as those targeted in 2000.

Creating a DDoS network is no simple task. The attack agents are not willing agents—they are systems that have been compromised and on which the DDoS attack software has been installed. To compromise these agents, the attacker has to have gained unauthorized access to the system or tricked authorized users to run a program that installed the attack software. The creation of the attack network may in fact be a multistep process in which the attacker first compromises a few systems that are then used as handlers or masters, which in turn compromise other systems. Once the network has been created, the agents wait for an attack message that will include data on the specific target before launching the attack. One important aspect of a DDoS attack is that with just a few messages to the agents, the attacker can have a flood of messages sent against the targeted system.

Replay Attacks

A replay attack occurs when the attacker captures a portion of a communication between two parties and retransmits it at a later time. For example, an attacker might replay a series of commands and codes used in a financial transaction to cause the transaction to be conducted multiple times. Generally replay attacks are associated with attempts to circumvent authentication mechanisms, such as the capturing and reuse of a certificate or ticket.

Smurf attack

A SMURF attack (named after the program used to perform the attack) is a method by which an attacker can send a moderate amount of traffic and cause a virtual explosion of traffic at the intended target. The method used is as follows: The attacker sends ICMP Echo Request packets where the source IP address has been forged to be that of the target of the attack.

The attacker sends these ICMP datagrams to addresses of remote LANs broadcast addresses, using so-called directed broadcast addresses. These datagrams are thus broadcast out on the LANs by the connected router. All the hosts which are «alive» on the LAN each pick up a copy of the ICMP Echo Request datagram (as they should), and sends an ICMP Echo Reply datagram back to what they think is the source. If many hosts are «alive» on the LAN, the amplification factor can be considerably (100+ is not uncommon).

The attacker can use largish packets (typically up to ethernet maximum) to increase the «effectiveness» of the attack, and the faster network connection the attacker has, the more damage he can inflict on the target and the target’s network.

Not only can the attacker cause problems for the target host, the influx of traffic can in fact be so great as to have a seriously negative effect on the upstream network(s) from the target. In fact, those institutions being abused as amplifier networks can also be similarly affected, in that the Echo Reply packets destined for the target can swamp their network connection.

IP Address Spoofing

IP is designed to work so that the originators of any IP packet include their own IP address in the from portion of the packet. While this is the intent, nothing prevents a system from inserting a different address in from portion of the packet. This is known as IP address spoofing. An IP address can be spoofed for several reasons. In a specific DoS attack known as a smurf attack, the attacker sends a spoofed packet to the broadcast address for a network, which distributes the packet to all systems on that network. In the smurf attack, the packet sent by the attacker to the broadcast address is an echo request with from address forged so that it appears that another system (the target system) has made the echo request. The normal response of a system to an echo request is an echo reply, and it is used in the ping utility to let a user know whether a remote system is reachable and is responding. In the smurf attack, the request is sent to all systems on the network, so all will respond with an echo reply to the target system. The attacker has sent one packet and has been able to generate as many as 254 responses aimed at the target. Should the attacker send several of these spoofed requests, or send them to several different networks, the target can quickly become overwhelmed with the volume of echo replies it receives.

Spoofing and Sequence Numbers

How complicated the spoofing is depends heavily on several factors, including whether the traffic is encrypted and where the attacker is located relative to the target. Spoofing attacks from inside a network, for example, are much easier to perform than attacks from outside of the network, because the inside attacker can observe the traffic to and from the target and can do a better job of formulating the necessary packets.


Phishing (pronounced “fishing”) is a type of social engineering in which an individual attempts to obtain sensitive information from a user by masquerading as a trusted entity in an e-mail or instant message sent to the user. The type of information that the attacker attempts to obtain include usernames, passwords, credit card numbers, or details on the user’s bank account. The message sent often encourages the user to go to a web site that appears to be for a reputable entity such as PayPal or eBay, both of which have frequently been used in phishing attempts. The web site the user actually visits will not be owned by the reputable organization, however, and will ask the user to supply information that can be used in a later attack. Often the message sent to the user will tell a story about the user’s account having been compromised, and for security purposes they are encouraged to enter their account information to verify the details.


Vishing is a variation of phishing that uses voice communication technology to obtain the information the attacker is seeking. Vishing takes advantage of the trust that most people place in the telephone network. Users are unaware that attackers can spoof calls from legitimate entities using voice over IP (VoIP) technology. Voice messaging can also be compromised and used in these attempts. Generally, the attackers are hoping to obtain credit card numbers or other information that can be used in identity theft. The user may receive an e-mail asking him to call a number that is answered by a potentially compromised voice message system. Users may also receive a recorded message that appears to come from a legitimate entity. In both cases, the user will be encouraged to respond quickly and provide the sensitive information so that access to an account is not blocked. If a user ever receives a message that claims to be from a reputable entity and is asking for sensitive information, he should not provide it but instead use theInternet or examine a legitimate account statement to find a phone number that can be used to contact the entity. The user can then verify that the message received was legitimate or report the vishing attempt.

DNS poisoning and ARP poisoning

There has been a long history of attacks on the Domain Name System ranging from bruteforce denial-of-service attacks to targeted attacks requiring specialized software. In July 2008 a new DNS cache-poisoning attack was unveiled that is considered especially dangerous because it does not require substantial bandwidth or processor resources nor does it require sophisticated techniques.

With cache poisoning an attacker attempts to insert a fake address record for an Internet domain into the DNS. If the server accepts the fake record, the cache is poisoned and subsequent requests for the address of the domain are answered with the address of a server controlled by the attacker. For as long as the fake entry is cached by the server (entries usually have a time to live — or TTL — of a couple of hours) subscriber’s browsers or e-mail servers will automatically go to the address provided by the compromised DNS server.

This kind of attack is often categorized as a “pharming” attack and it creates several problems. First, users think they are at a familiar site, but they aren’t. Unlike with a “phishing” attack where an alert user can spot a suspicious URL, in this case the URL is legitimate. Remember, the browser resolves the address of the domain automatically so there is no intervention of any kind on the part of the users and, since nothing unusual has happened, they have no reason to be suspicious.

ARP poisoning

ARP Poisoning, also known as ARP Poison Routing, is a network attack that exploits the transition from Layer 3 to Layer 2 addresses.

ARP (address resolution protocol) operates by broadcasting a message across a network, to determine the Layer 2 address (MAC address) of a host with a predefined Layer 3 address (IP address). The host at the destination IP address sends a reply packet containing its MAC address. Once the initial ARP transaction is complete, the originating device then caches the ARP response, which is used within the Layer 2 header of packets that are sent to a specified IP address.

An ARP Spoofing attack is the egression of unsolicited ARP messages. These ARP messages contain the IP address of a network resource, such as the default gateway, or a DNS server, and replace the MAC address for the corresponding network resource with its own MAC address. Network devices, by design, overwrite any existing ARP information in conjunction with the IP address, with the new, counterfeit ARP information. The attacker then takes the role of man in the middle; any traffic destined for the legitimate resource is sent through the attacking system. As this attack occurs on the lower levels of the OSI model, the end-user is oblivious to the attack occurrence.

ARP Poisoning is also capable of executing Denial of Service (DoS) attacks. The attacking system, instead of posing as a gateway and performing a man in the middle attack, can instead simply drop the packets, causing the clients to be denied service to the attacked network resource. The spoofing of ARP messages is the tributary principal of ARP Poisoning.

Password attacks

Password attacks occur when an account is attacked repeatedly. This is accomplished by using applications known as password crackers, which send possible passwords to the account in a systematic manner. The attacks are initially carried out to gain passwords for an access or modification attack. There are several types of password attacks:

Brute-Force Attack

A brute-force attack is an attempt to guess passwords until a successful guess occurs. As an example of this type of attack, imagine starting to guess with “A” and then going through “z”; when no match is found, the next guess series goes from “AA” to “zz” and then adds a third value (“AAA” to “zzz”). Because of the nature of this routine, this type of attack usually occurs over a long period of time. To make passwords more difficult to guess, they should be much longer than two or three characters (six should be the bare minimum), be complex, and have password lockout policies.

Dictionary Attack

A dictionary attack uses a dictionary of common words to attempt to find the user’s password. Dictionary attacks can be automated, and several tools exist in the public domain to execute them. As an example of this type of attack, imagine guessing words and word combinations found in a standard English-language dictionary.


A hybrid attack typically uses a combination of dictionary entries and brute force. For example, if you know that there is a good likelihood that the employees of a particular company are using derivatives of the company name in their passwords, then you can seed those values into the values attempted.

Birthday Attack

A birthday attack is built on a simple premise. If 25 people are in a room, there is some probability that two of those people will have the same birthday. The probability increases as additional people enter the room. It’s important to remember that probability doesn’t mean that something will occur, only that it’s more likely to occur. To put it another way, if you ask if anyone has a birthday of March 9th, the odds are 1 in 365 (or 25/365 given the number of people in the room), but if you ask if anyone has the same birthday as any other individual, the odds of there being a match increase significantly.

Although two people may not share a birthday in every gathering, the likelihood is fairly high, and as the number of people increases, so too do the odds that there will be a match. A birthday attack works on the same premise: If your key is hashed, the possibility is that given enough time, another value can be created that will give the same hash value. Even encryption such as that with MD5 has been shown to be vulnerable to a birthday attack.

Rainbow Table

A rainbow table attack focuses on identifying a stored value. By using values in an existing table of hashed phrases or words (think of taking a word and hashing it every way you can imagine) and comparing them to values found, a rainbow table attack can reduce the amount of time needed to crack a password significantly. Salt (random bits added to the password) can greatly reduce the ease which rainbow can use tables.

Some systems will identify whether an account ID is valid and whether the password is wrong. Giving the attacker a clue as to a valid account name isn’t a good practice.

Typo squatting/URL

hijacking Typo squatting (also spelled typosquatting) and URL hijacking are one and the same. Difficult to describe as an attack, this is the act of registering domains that are similar to those for a known entity but based on a misspelling or typographical error. com in the hopes that the same reader would misspell the word. Instead of arriving at the safe site of the publisher, they would end up at the other site, which could download Trojans, worms, and viruses—oh my.

The best defense against typo squatting is to register those domains around yours for which a user might intentionally type in a value when trying to locate you. This includes top-level domains as well (.com, .biz, .net, and so on) for all reasonable deviations of your site.

Watering hole attack A watering hole attack can sound a lot more complicated than it really is. The strategy the attacker takes is simply to identify a site that is visited by those they are targeting, poisoning that site, and then waiting for the results.

As an example, suppose an attacker wants to gain unauthorized access to the servers at Spencer Industries, but Spencer’s security is really good. The attacker discovers that Spencer does not host its own email, but instead outsources it to a big cloud provider, and so they focus their attention on the weaker security of the cloud provider. On the cloud provider’s email site, they install the malware du jour, wait until a Spencer employee gets infected, and they suddenly have the access they coveted.

The best defense against a watering hole attack is to make certain that all of your partners are secure. Identify weak links, and bring them up to the same level of security as the rest of your infrastructure.