The answer is that the firewall rule fails because the source instance uses the default compute engine service account, not the specified api-sa. In Google Cloud, a firewall rule using sourceServiceAccount matches traffic based on the service account attached to the source virtual machine, not the application-level service account or the instance’s tags. Even though the instance is tagged ‘api-instances’ and runs the API server, the rule explicitly requires the source’s attached service account to be api-sa; since the default service account is in use, the HTTPS traffic is implicitly denied. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding that sourceServiceAccount overrides network tags for source matching—a common trap where candidates confuse the instance’s tag with its identity. Remember the memory tip: “Tags tell you where, service accounts tell you who.” If the source’s attached service account doesn’t match the rule, the traffic is blocked regardless of tags.
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Refer to the exhibit. A developer created the firewall rule to allow HTTPS traffic from the API service account to instances tagged 'api-instances'. However, HTTPS requests from the API server (which runs on an instance with tag 'api-instances' and uses the default compute engine service account) are failing. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The source instance is using the default compute engine service account, not the specified api-sa.
Option B is correct because the firewall rule explicitly specifies the source as the API service account (api-sa), but the API server is using the default compute engine service account. In Google Cloud, firewall rules that use sourceServiceAccounts match traffic based on the service account attached to the source instance, not the instance's tags or the service account used by the application. Since the source instance's service account does not match the rule's specified service account, the HTTPS traffic is implicitly denied.
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 rule should have a higher priority to override the default implicit deny.
Why it's wrong here
The rule's priority (1000) is already higher than the implicit deny (65535); priority is not the issue.
✓
The source instance is using the default compute engine service account, not the specified api-sa.
Why this is correct
This service account mismatch means the source does not match the rule's sourceServiceAccounts, so the traffic is not allowed.
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.
✗
The direction should be EGRESS because the traffic originates from the API server.
Why it's wrong here
The direction is correct as INGRESS on the target; the rule is applied to the target instance's incoming traffic.
✗
The rule should use sourceTags instead of sourceServiceAccounts to match the source instance.
Why it's wrong here
The rule already uses sourceServiceAccounts; changing to sourceTags would not solve the service account mismatch.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between service account identity and instance tags, tricking candidates into thinking that tagging the source instance or using the default service account will satisfy a rule that explicitly specifies a different service account.
Detailed technical explanation
How to think about this question
Google Cloud VPC firewall rules evaluate traffic based on the source's service account when sourceServiceAccounts is used, regardless of the instance's tags or the application's identity. The default compute engine service account (project-number-compute@developer.gserviceaccount.com) is distinct from a custom service account like api-sa. This is a common pitfall when mixing identity-based and tag-based firewall rules, as the rule's source must match the service account attached to the source instance's metadata, not the service account used by the running process.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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.
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The source instance is using the default compute engine service account, not the specified api-sa. — Option B is correct because the firewall rule explicitly specifies the source as the API service account (api-sa), but the API server is using the default compute engine service account. In Google Cloud, firewall rules that use sourceServiceAccounts match traffic based on the service account attached to the source instance, not the instance's tags or the service account used by the application. Since the source instance's service account does not match the rule's specified service account, the HTTPS traffic is implicitly denied.
What should I do if I get this PCSE 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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