Spam. Spam, or unsolicited commercial e-mail messages, wastes bandwidth and time. The sheer volume of it can be overwhelming, and it can be a vehicle for viruses.

 Spoofing. A couple of kinds of spoofing exist.

IP spoofing means creating packets that look as though they have come from a different IP address.

E-mail spoofing means forging an e-mail message so that the From address does not indicate the true address of the sender.

 Phishing. Typically, an attacker sends an e-mail message that looks very much like it comes from an official source (such as eBay or Microsoft). It is to trick you into giving away personal information.

Viruses. Viruses are programs designed to replicate themselves and potentially cause harmful actions. They are often hidden inside innocuous programs. Viruses try to replicate themselves by infecting other programs on your computer.

Worms. Worms are like viruses in that they try to replicate themselves, but they are often able to do so by sending out e-mail messages themselves rather than simply infecting programs on a single computer.

Trojan horses. These malicious programs pretend to be benign applications. They don’t replicate like viruses and worms but can still cause considerable harm. Often, viruses or worms are smuggled inside a Trojan horse.

Spyware. Spyware refers to small, hidden programs that run on your computer and are used for everything from tracking your online activities to allowing intruders to monitor and access your computer.

 Tampering. Tampering consists of altering the contents of packets as they travel over the Internet or altering data on computer disks after a network has been penetrated.

 Repudiation. Repudiation refers to a user’s ability to falsely deny having performed an action that other parties cannot disprove. For example, a user who deleted a file can successfully deny doing so if no mechanism (such as audit records) can prove otherwise.

 Information disclosure. Information disclosure consists of the exposure of information to individuals who normally would not have access to it. For example, a user on your network might make certain files accessible over the network that should not be shared. Employees also tend to share important information, such as passwords, with people who should not have them.

 Denial of Service. DoS attacks are computerized assaults launched by an attacker in an attempt to overload or halt a network service, such as a Web server or a file server. For example, an attack may cause a server to become so busy attempting to respond that it ignores legitimate requests for connections.

Elevation of privilege. Elevation of privilege is a process by which a user misleads a system into granting unauthorized rights, usually for the purpose of compromising or destroying the system. For example, an attacker might log on to a network by using a guest account, then exploit a weakness in the software that lets the attacker change the guest privileges to administrative privileges.

Rootkit. A rootkit is a software system that consists of a program or combination of several programs designed to hide or obscure the fact that a system has been compromised. Contrary to what its name may imply, a rootkit does not grant a user administrator privileges, as it requires prior access to execute and tamper with system files and processes. An attacker may use a rootkit to replace vital system executables, which may then be used to hide processes and files the attacker has installed, along with the presence of the rootkit. Access to the hardware, e.g., the reset switch, is rarely required, as a rootkit is intended to seize control of the operating system. Typically, rootkits act to obscure their presence on the system through subversion or evasion of standard operating system security scan and surveillance mechanisms such as anti-virus or anti-spyware scan. Often, they are Trojans as well, thus fooling users into believing they are safe to run on their systems. Techniques used to accomplish this can include concealing running processes from monitoring programs, or hiding files or system data from the operating system.[1] Rootkits may also install a "back door" in a system by replacing the login mechanism (such as /bin/login) with an executable that accepts a secret login combination, which, in turn, allows an attacker to access the system, regardless of the changes to the actual accounts on the system.

Rootkits may have originated as regular applications, intended to take control of a failing or unresponsive system, but in recent years have been largely malware to help intruders gain access to systems while avoiding detection. Rootkits exist for a variety of operating systems, such as Microsoft Windows, Linux, Mac OS, and Solaris. Rootkits often modify parts of the operating system or install themselves as drivers or kernel modules, depending on the internal details of an operating system's mechanisms.