Backdoors are the unauthorized open ports created by malicious programs that allow an unauthorized entity to gain access into the system. An important countermeasure is to periodically check these open ports in the system and close the ports that are not used by programs. Port scanning tools will help in this process. While port scanning helps security tests to identify open ports, the scanners can also be used by attackers to find the entry points to system.

Denial-of-Service (DoS) is a type of attack wherein the availability of the system is compromised, and the legitimate users of the system are prevented from accessing their desired resources. A Distributed Denial-of-Service (DDoS) is a type of attack where multiple systems attack a single resource from distributed locations. SYN attacks, Teardrop attack, and Smurf are some examples of Denial-of-Service attacks. A countermeasure to DoS attacks is regularly monitoring the network activities. Also, firewall and Intrusion Protection Systems (IPS) have robust mechanisms to protect against such attacks.

Hijacking is an attack in which the session established by the client to the server is taken over by a malicious person or process. A strong session management and encryption is the countermeasure for such attacks.

The Man-in-the-Middle attack (MIIM) is a type of attack where an attacker hijacks the established session to the server by substituting his public key instead of the client. MIIM attacks are also used to surreptitiously listen to network communications.

TCP hijacking is a type of attack in which the TCP session of the trusted client to the server is hijacked by an attacker.

Malicious codes are prevalent in the information technology environment and are varied in use and purpose. The basic functionality of malicious code is to execute itself in the client machine and compromise security. Important countermeasures are to use and update antivirus systems, firewalls, and intrusion detection systems.

A trojan horse is a type of malicious code that comes disguised inside a trusted program. Once installed, this malicious code can open ports, create backdoors to the system, and do innumerable security breaches. When the Trojan horse is activated on a particular event (such as a particular date), then it is called a logic bomb.

Malicious mobile codes are the ones that are executed in the client system through the network from a remote server.

Password guessing is a type of attack that uses various methods to obtain user passwords. The use of strong password with a combination of alpha, numeric, and special characters is a countermeasure. Besides, adhering to strict password policies, such as frequent change of passwords, length of passwords, and history of passwords, are effective against such attacks.

Dictionary attacks are a type of password-guessing attack that checks the encrypted password database with words found in a dictionary.

Brute force attacks are the means by which the password database is attacked with every possible character combination.

Hybrid attacks combine the dictionary as well as brute force attacks.

Replay attacks are ones in which the session (such as authentication) is captured and replayed against the system. In such attacks, valid authentication tokens are played back at a later time by an unauthorized user. These authentication tokens are commonly obtained through MIIM attacks.

Scanning is an attack to probe the network and system to identify the vulnerabilities for planning a possible attack to compromise.

Vulnerability exploitation is a way of attacking systems by compromising the holes or errors in the operating system or application software to gain access or bypass security controls.

Spoofing is a type of attack to imitate a trusted entity, thereby making the system trust this imitated entity. IP spoofing is an example of such an attack.

Social engineering is a type of attack to obtain credential information, such as passwords, pin numbers, and so on, using social skills such as impersonation or fake e-mails. Generally, social-engineering attacks exploit human nature, such desires to please or be helpful to others, as well as ignorance.

An important countermeasure for the vulnerability compromise of systems is to periodically scan and fix the vulnerabilities in the IT systems using vendor-supplied patches, along with other means of filtering and protection using suitable vulnerability management tools.