A denial-of-service attack overwhelms a system’s resources so that it cannot respond to service requests. A DDoS attack is also an attack on system’s resources, but it is launched from a large number of other host machines that are infected by malicious software controlled by the attacker.

Unlike attacks that are designed to enable the attacker to gain or increase access, denial-of-service doesn’t provide direct benefits for attackers. For some of them, it’s enough to have the satisfaction of service denial. However, if the attacked resource belongs to a business competitor, then the benefit to the attacker may be real enough. Another purpose of a DoS attack can be to take a system offline so that a different kind of attack can be launched. One common example is session hijacking, which I’ll describe later.

There are different types of DoS and DDoS attacks; the most common are TCP SYN flood attack, teardrop attack, smurf attack, ping-of-death attack and botnets.

A MitM attack occurs when a hacker inserts itself between the communications of a client and a server. In the following type of MitM attack, an attacker hijacks a session between a trusted client and network server. The attacking computer substitutes its IP address for the trusted client while the server continues the session, believing it is communicating with the client. For instance, the attack might unfold like this:

A client connects to a server.

The attacker’s computer gains control of the client.

The attacker’s computer disconnects the client from the server.

The attacker’s computer replaces the client’s IP address with its own IP address and spoofs the client’s sequence numbers.

The attacker’s computer continues dialog with the server and the server believes it is still communicating with the client.

Phishing attack is the practice of sending emails that appear to be from trusted sources with the goal of gaining personal information or influencing users to do something. It combines social engineering and technical trickery. It could involve an attachment to an email that loads malware onto your computer. It could also be a link to an illegitimate website that can trick you into downloading malware or handing over your personal information.

Spear phishing is a very targeted type of phishing activity. Attackers take the time to conduct research into targets and create messages that are personal and relevant. Because of this, spear phishing can be very hard to identify and even harder to defend against. One of the simplest ways that a hacker can conduct a spear phishing attack is email spoofing, which is when the information in the “From” section of the email is falsified, making it appear as if it is coming from someone you know, such as your management or your partner company. Another technique that scammers use to add credibility to their story is website cloning — they copy legitimate websites to fool you into entering personally identifiable information (PII) or login credentials.

Drive-by download attacks are a common method of spreading malware. Hackers look for insecure websites and plant a malicious script into HTTP or PHP code on one of the pages. This script might install malware directly onto the computer of someone who visits the site, or it might re-direct the victim to a site controlled by the hackers. Drive-by downloads can happen when visiting a website or viewing an email message or a pop-up window. Unlike many other types of cyber security attacks, a drive-by doesn’t rely on a user to do anything to actively enable the attack — you don’t have to click a download button or open a malicious email attachment to become infected. A drive-by download can take advantage of an app, operating system or web browser that contains security flaws due to unsuccessful updates or lack of updates.

Because passwords are the most commonly used mechanism to authenticate users to an information system, obtaining passwords is a common and effective attack approach. Access to a person’s password can be obtained by looking around the person’s desk, ‘‘sniffing’’ the connection to the network to acquire unencrypted passwords, using social engineering, gaining access to a password database or outright guessing. The last approach can be done in either a random or systematic manner:

Brute-force password guessing means using a random approach by trying different passwords and hoping that one work Some logic can be applied by trying passwords related to the person’s name, job title, hobbies or similar items.

In a dictionary attack, a dictionary of common passwords is used to attempt to gain access to a user’s computer and network. One approach is to copy an encrypted file that contains the passwords, apply the same encryption to a dictionary of commonly used passwords, and compare the results.

SQL injection has become a common issue with database-driven websites. It occurs when a malefactor executes a SQL query to the database via the input data from the client to server. SQL commands are inserted into data-plane input (for example, instead of the login or password) in order to run predefined SQL commands. A successful SQL injection exploit can read sensitive data from the database, modify (insert, update or delete) database data, execute administration operations (such as shutdown) on the database, recover the content of a given file, and, in some cases, issue commands to the operating system.

XSS attacks use third-party web resources to run scripts in the victim’s web browser or scriptable application. Specifically, the attacker injects a payload with malicious JavaScript into a website’s database. When the victim requests a page from the website, the website transmits the page, with the attacker’s payload as part of the HTML body, to the victim’s browser, which executes the malicious script. For example, it might send the victim’s cookie to the attacker’s server, and the attacker can extract it and use it for session hijacking. The most dangerous consequences occur when XSS is used to exploit additional vulnerabilities. These vulnerabilities can enable an attacker to not only steal cookies, but also log key strokes, capture screenshots, discover and collect network information, and remotely access and control the victim’s machine.

While XSS can be taken advantage of within VBScript, ActiveX and Flash, the most widely abused is JavaScript — primarily because JavaScript is supported widely on the web.

Eavesdropping attacks occur through the interception of network traffic. By eavesdropping, an attacker can obtain passwords, credit card numbers and other confidential information that a user might be sending over the network. Eavesdropping can be passive or active:

Passive eavesdropping — A hacker detects the information by listening to the message transmission in the network.

Active eavesdropping — A hacker actively grabs the information by disguising himself as friendly unit and by sending queries to transmitters. This is called probing, scanning or tampering.

Detecting passive eavesdropping attacks is often more important than spotting active ones, since active attacks requires the attacker to gain knowledge of the friendly units by conducting passive eavesdropping before.

Data encryption is the best countermeasure for eavesdropping.

Birthday attacks are made against hash algorithms that are used to verify the integrity of a message, software or digital signature. A message processed by a hash function produces a message digest (MD) of fixed length, independent of the length of the input message; this MD uniquely characterizes the message. The birthday attack refers to the probability of finding two random messages that generate the same MD when processed by a hash function. If an attacker calculates same MD for his message as the user has, he can safely replace the user’s message with his, and the receiver will not be able to detect the replacement even if he compares MDs.

Malicious software can be described as unwanted software that is installed in your system without your consent. It can attach itself to legitimate code and propagate; it can lurk in useful applications or replicate itself across the Internet. Here are some of the most common types of malware:

Macro viruses — These viruses infect applications such as Microsoft Word or Excel. Macro viruses attach to an application’s initialization sequence. When the application is opened, the virus executes instructions before transferring control to the application. The virus replicates itself and attaches to other code in the computer system.

File infectors — File infector viruses usually attach themselves to executable code, such as .exe files. The virus is installed when the code is loaded. Another version of a file infector associates itself with a file by creating a virus file with the same name, but an .exe extension. Therefore, when the file is opened, the virus code will execute.

System or boot-record infectors — A boot-record virus attaches to the master boot record on hard disks. When the system is started, it will look at the boot sector and load the virus into memory, where it can propagate to other disks and computers.

Polymorphic viruses — These viruses conceal themselves through varying cycles of encryption and decryption. The encrypted virus and an associated mutation engine are initially decrypted by a decryption program. The virus proceeds to infect an area of code. The mutation engine then develops a new decryption routine and the virus encrypts the mutation engine and a copy of the virus with an algorithm corresponding to the new decryption routine. The encrypted package of mutation engine and virus is attached to new code, and the process repeats. Such viruses are difficult to detect but have a high level of entropy because of the many modifications of their source code. Anti-virus software or free tools like Process Hacker can use this feature to detect them.

Stealth viruses — Stealth viruses take over system functions to conceal themselves. They do this by compromising malware detection software so that the software will report an infected area as being uninfected. These viruses conceal any increase in the size of an infected file or changes to the file’s date and time of last modification.

Trojans — A Trojan or a Trojan horse is a program that hides in a useful program and usually has a malicious function. A major difference between viruses and Trojans is that Trojans do not self-replicate. In addition to launching attacks on a system, a Trojan can establish a back door that can be exploited by attackers. For example, a Trojan can be programmed to open a high-numbered port so the hacker can use it to listen and then perform an attack.

Logic bombs — A logic bomb is a type of malicious software that is appended to an application and is triggered by a specific occurrence, such as a logical condition or a specific date and time.

Worms — Worms differ from viruses in that they do not attach to a host file, but are self-contained programs that propagate across networks and computers. Worms are commonly spread through email attachments; opening the attachment activates the worm program. A typical worm exploit involves the worm sending a copy of itself to every contact in an infected computer’s email address In addition to conducting malicious activities, a worm spreading across the internet and overloading email servers can result in denial-of-service attacks against nodes on the network.

Droppers — A dropper is a program used to install viruses on computers. In many instances, the dropper is not infected with malicious code and, therefore might not be detected by virus-scanning software. A dropper can also connect to the internet and download updates to virus software that is resident on a compromised system.

Ransomware — Ransomware is a type of malware that blocks access to the victim’s data and threatens to publish or delete it unless a ransom is paid. While some simple computer ransomware can lock the system in a way that is not difficult for a knowledgeable person to reverse, more advanced malware uses a technique called cryptoviral extortion, which encrypts the victim’s files in a way that makes them nearly impossible to recover without the decryption key.

Adware — Adware is a software application used by companies for marketing purposes; advertising banners are displayed while any program is running. Adware can be automatically downloaded to your system while browsing any website and can be viewed through pop-up windows or through a bar that appears on the computer screen automatically.

Spyware — Spyware is a type of program that is installed to collect information about users, their computers or their browsing habits. It tracks everything you do without your knowledge and sends the data to a remote user. It also can download and install other malicious programs from the internet. Spyware works like adware but is usually a separate program that is installed unknowingly when you install another freeware applications.