In computer security, the term vulnerability is a weakness which allows an attacker to reduce a system's Information Assurance. A computer or a network can be vulnerable due to the following reasons: Complexity: Large, complex systems increase the probability of flaws and unintended access points. Familiarity: Using common, well-known code, software, operating systems, and/or hardware increases the probability an attacker has or can find the knowledge and tools to exploit the flaw. Connectivity: More physical connections, privileges, ports, protocols, and services and time each of those are accessible increase vulnerability. Password management flaws: The computer user uses weak passwords that could be discovered by brute force. The computer user stores the password on the computer where a program can access it. Users re-use passwords between many programs and websites. Fundamental operating system design flaws: The operating system designer chooses to enforce sub optimal policies on user/program management. For example, operating systems with policies such as default permit grant every program and every user full access to the entire computer. This operating system flaw allows viruses and malware to execute commands on behalf of the administrator. Internet Website Browsing: Some Internet websites may contain harmful Spyware or Adware that can be installed automatically on the computer systems. After visiting those websites, the computer systems become infected and personal information will be collected and passed on to third party individuals. Software bugs: The programmer leaves an exploitable bug in a software program. The software bug may allow an attacker to misuse an application. Unchecked user input: The program assumes that all user input is safe. Programs that do not check user input can allow unintended direct execution of commands or SQL statements (known as Buffer overflows, SQL injection or other non-validated inputs).

Answer: B, C are incorrect. Use of common software and common code can make a network vulnerable.