Troubleshoot HTTPS Traffic Denied Because of App-ID Misidentification
This PCNSE practice question tests your understanding of securing traffic and app-id. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
user@fw> show running security-policy
rule 1: name "Allow-Web" from trust to untrust source any destination any application web-browsing service application-default action allow
rule 2: name "Allow-SSL" from trust to untrust source any destination any application ssl service application-default action allow
rule 3: name "Block-Other" from trust to untrust source any destination any application any service any action deny log-start
rule 4: name "Allow-All" from trust to trust source any destination any application any service any action allow
Refer to the exhibit. An administrator notices that HTTPS traffic to a specific website is being denied. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Exhibit
user@fw> show running security-policy
rule 1: name "Allow-Web" from trust to untrust source any destination any application web-browsing service application-default action allow
rule 2: name "Allow-SSL" from trust to untrust source any destination any application ssl service application-default action allow
rule 3: name "Block-Other" from trust to untrust source any destination any application any service any action deny log-start
rule 4: name "Allow-All" from trust to trust source any destination any application any service any action allow
A
The HTTPS traffic is being identified as web-browsing instead of ssl, so it does not match rule 2 and is denied by rule 3.
If App-ID misidentifies HTTPS traffic as web-browsing, it fails to match rule 2 and is blocked.
B
Rule 2 does not have a service set to application-default, so it cannot match the traffic.
Why wrong: Rule 2 does have service set to application-default, which expects port 443 for ssl.
C
The traffic is from trust to trust, matching rule 4, but still denied.
Why wrong: The traffic is from trust to untrust, not trust to trust.
D
The traffic requires a specific service other than application-default.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The HTTPS traffic is being identified as web-browsing instead of ssl, so it does not match rule 2 and is denied by rule 3.
Option A is correct because when App-ID identifies HTTPS traffic as web-browsing (HTTP over port 443) instead of ssl, the traffic does not match rule 2 (which requires the 'ssl' application). Consequently, it falls through to rule 3, which denies the traffic. This misidentification often occurs when the SSL handshake is incomplete or when decryption is not configured, causing the firewall to classify the traffic based on the port rather than the application signature.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 HTTPS traffic is being identified as web-browsing instead of ssl, so it does not match rule 2 and is denied by rule 3.
Why this is correct
If App-ID misidentifies HTTPS traffic as web-browsing, it fails to match rule 2 and is blocked.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Rule 2 does not have a service set to application-default, so it cannot match the traffic.
Why it's wrong here
Rule 2 does have service set to application-default, which expects port 443 for ssl.
✗
The traffic is from trust to trust, matching rule 4, but still denied.
Why it's wrong here
The traffic is from trust to untrust, not trust to trust.
✗
The traffic requires a specific service other than application-default.
Why it's wrong here
The correct service is already set.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common trap is to assume that the service setting (application-default) is required for App-ID to match traffic, when the real issue is application misidentification due to incomplete SSL inspection or port-based fallback.
Detailed technical explanation
How to think about this question
Under the hood, App-ID uses multiple identification methods including protocol decoding, SSL handshake inspection, and heuristics. When HTTPS traffic is not decrypted (e.g., due to lack of SSL Forward Proxy), the firewall may fall back to port-based identification, classifying it as web-browsing (application 'web-browsing') instead of 'ssl'. This is a common scenario where the application signature for SSL requires a successful TLS handshake to be identified; if the handshake is not observed (e.g., due to asymmetric routing or session reuse), the traffic is misclassified. In real-world deployments, this often happens with traffic to sites that use HSTS or non-standard ports, leading to unexpected denials.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the PCNSE exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Securing Traffic and App-ID — This question tests Securing Traffic and App-ID — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The HTTPS traffic is being identified as web-browsing instead of ssl, so it does not match rule 2 and is denied by rule 3. — Option A is correct because when App-ID identifies HTTPS traffic as web-browsing (HTTP over port 443) instead of ssl, the traffic does not match rule 2 (which requires the 'ssl' application). Consequently, it falls through to rule 3, which denies the traffic. This misidentification often occurs when the SSL handshake is incomplete or when decryption is not configured, causing the firewall to classify the traffic based on the port rather than the application signature.
What should I do if I get this PCNSE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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