When performing a vulnerability assessment from outside the perimeter, which of the following network devices is MOST likely to skew the scan results?

A firewall is a network security device situated at the perimeter, designed to monitor and control incoming and outgoing traffic based on a defined set of security rules. When a vulnerability assessment is conducted from an external source, the firewall's primary function is to block unsolicited and potentially malicious probes from the scanner. This action inherently skews the results by hiding internal systems and open ports, preventing the scanner from obtaining an accurate picture of the internal network's security posture. The scan report will reflect the hardened state of the firewall, not the true state of the devices behind it, leading to false negatives.

A. Access Point: An access point primarily provides wireless connectivity and is typically located inside the perimeter, not acting as the main filter for external scans.
B. Router: A router's main function is to forward traffic between networks. While it can perform basic filtering with ACLs, a firewall provides more comprehensive, stateful inspection.
D. IDS: An Intrusion Detection System (IDS) is a passive device that monitors and alerts on suspicious activity; it does not block traffic and therefore does not alter scan results.
E. Switch: A switch is an internal network device used to connect endpoints on a local network; it is not positioned at the perimeter to filter external traffic.

1. National Institute of Standards and Technology (NIST) Special Publication 800-115, Technical Guide to Information Security Testing and Assessment. Section 4.3, "Vulnerability Scanning," explains that firewalls and routers can filter scanner traffic, which "can cause the scanner to produce inaccurate results, such as reporting that a host is not online or that a port is closed when it is actually open." This directly confirms that firewalls skew scan results.

2. National Institute of Standards and Technology (NIST) Special Publication 800-41 Rev. 1, Guidelines on Firewalls and Firewall Policy. Section 2.1, "What Is a Firewall?," defines a firewall as a device that enforces an access control policy between networks. Its function of blocking traffic from untrusted networks (like the internet) is the mechanism by which it would skew an external vulnerability scan.

3. Carnegie Mellon University, Software Engineering Institute, CMU/SEI-2005-TN-023, Network Vulnerability Scanners. Section 2.2, "Limitations," states, "Firewalls and intrusion detection systems can detect and/or block network scans, which can result in an incomplete or inaccurate vulnerability assessment." This academic source explicitly identifies firewalls as a primary cause of skewed scan results.