The method described-sending TCP packets with no flags set-is a NULL scan. In TCP header terminology, a NULL packet has all control flags cleared (no SYN, ACK, FIN, RST, PSH, URG). The scanner then interprets the target's response behavior to infer port state. Traditionally, for many TCP/IP stacks, a closed port responds with RST, while an open port may respond with no reply (silence) because the packet does not correspond to any valid state in the TCP state machine. This behavior can vary by OS and filtering devices, but the defining characteristic is the "no flags" probe.
The scenario also highlights stealth: compared to a TCP connect scan, which completes a full three-way handshake and is easily logged, NULL scans can be less conspicuous because they avoid a normal connection setup. Some intrusion detection systems focus heavily on repeated SYN handshakes or completed connections; unusual flag scans may slip through weak detection, though modern IDS/IPS can still detect them.
Why the other options are incorrect:
TCP Connect Scan (A) uses the operating system's connect() call to establish a full TCP connection; it is not a "no flags" technique and is generally noisier.
FIN Scan (B) sends packets with the FIN flag set; it is a different "stealth" scan type, but not "no flags." ACK Scan (D) sends packets with the ACK flag set and is typically used to map firewall rules (filtered vs unfiltered), not to determine open vs closed ports in the same way, and again it is not "no flags." Therefore, the scanning method is C. NULL Scan.

In CEH v13 DNS Enumeration, DNS cache snooping determines whether a DNS resolver has a record cached. When +norecurse is used and the response is NOERROR with no answer, it means the server is authoritative but does not have the record cached.
CEH v13 explains that this suggests no internal client has recently queried the domain, making option D correct.

CEH methodology stresses prioritization based on risk, exploitability, and business impact. High-severity vulnerabilities-especially those related to outdated or unsupported server software-are frequently associated with known, publicly documented exploits. The proper next step after identifying such vulnerabilities is to confirm exploitability safely, typically by researching available exploit code, validating version-specific weaknesses, and determining whether the vulnerability can be successfully leveraged under the defined scope of engagement. CEH highlights that exploitation attempts must be evidence-driven, not arbitrary, and focusing on high-risk vulnerabilities allows testers to demonstrate meaningful security impacts.
Brute-forcing (Option A) is unnecessary and high-noise. Ignoring or deprioritizing the high-risk finding (Options B and C) contradicts CEH risk-based assessment principles. Therefore, verifying exploitability of the high-risk vulnerability is the correct step.

The firewall described is doing more than simple header checks: it is validating session state and also performing some application-level analysis. That combination is characteristic of a Stateful Multilayer Inspection Firewall. This firewall type expands beyond traditional packet filtering by tracking the state of network connections (e.g., TCP handshake progression, established sessions, expected flags and sequence behavior) and applying inspection across multiple OSI layers. Because it understands whether traffic belongs to a legitimate, established session, it can block many spoofed or out-of-context packets that might pass a stateless filter.
The mention of being harder to bypass with fragmentation or tunneling attacks further supports this. Stateless packet-filtering firewalls mainly rely on source/destination IP, port, and protocol fields. Attackers may attempt fragmentation to split payloads across packets and evade simplistic inspection, or use tunneling to encapsulate traffic to hide prohibited content. A stateful multilayer firewall, by maintaining connection tables and reassembling or correlating traffic with session context, is better equipped to detect anomalies that do not align with valid session behavior and to apply deeper inspection policies.
Why the other choices are less fitting:
A Packet Filtering Firewall (A) focuses on basic header fields and is typically stateless, matching the opposite of what Mia observed.
An Application-Level Firewall (C) (proxy firewall) can do deep application inspection, but the scenario emphasizes a multilayer approach combining state tracking with application-level checks-more aligned with stateful multilayer inspection than a pure application proxy model.
A Circuit-Level Gateway Firewall (D) validates session establishment (e.g., TCP handshakes) and creates virtual circuits, but it generally does not perform meaningful application-level analysis of the content.
Therefore, the best match is B. Stateful Multilayer Inspection Firewall.