hardMultiple Choice
ISC2 CC A company uses a stateful firewall Practice Question
A company uses a stateful firewall. A user reports that an application requiring multiple dynamic ports is not working. The firewall logs show that packets from the server are being dropped. What is the most likely cause?
⚠ Common exam trap
ISC2 often tests the misconception that a stateful firewall automatically handles all dynamic port protocols, when in fact it requires proper ALG configuration to inspect control channel payloads for port negotiation.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall is only allowing packets that match an established session, but the application's control channel is not being tracked correctly.
A stateful firewall tracks sessions by monitoring the control channel. For applications like FTP that use dynamic ports, the firewall must inspect the control channel to dynamically open pinholes for data channels. If the control channel is not tracked correctly, the firewall will not recognize the data channel packets as part of an established session and will drop them, even if the initial control session was allowed.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The firewall is only allowing packets that match an established session, but the application's control channel is not being tracked correctly.
Why this is correct
A stateful firewall permits return traffic only for sessions it has tracked. If the application's control channel is not correctly inspected, the firewall never records the dynamic data-port sessions, so server packets arrive without a matching state entry and are dropped.
- ✗
The firewall does not have a rule allowing inbound traffic for the application.
Why it's wrong here
Stateful inspection automatically permits return traffic for sessions the client initiated, so a missing inbound rule is not the cause; the server's replies are dropped because their dynamic ports were never opened. It is tempting because inbound rules commonly block traffic, and would be correct for a stateless firewall.
- ✗
The firewall is not inspecting application-layer data.
Why it's wrong here
Application-layer inspection governs content filtering, not port-level session tracking; the drops concern dynamic port replies, which stateful inspection should already handle. It is tempting because deep packet inspection sounds thorough, and would be correct if the application failed on payload or protocol anomalies rather than ports.
- ✗
The firewall's state table is full and cannot track new sessions.
Why it's wrong here
A full state table would drop packets across all sessions, not specifically the server's dynamic-port replies; the stem's pattern points to return traffic being blocked. It is tempting because state exhaustion is a real firewall failure, and would be correct if logs showed widespread drops across unrelated flows.
Go deeper
Related to this question
Learn chapter
Access Control Fundamentals
Key term
User
A user is any person, system, or device that interacts with an IT service, resource, or identity system, typically authenticated through credentials and authorized to perform specific actions.
Key term
Firewall
A firewall is a network security system that monitors and controls incoming and outgoing traffic based on predetermined security rules to protect trusted internal networks from untrusted external networks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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