Why Lambda Resource Policy Needs aws:SourceArn for S3 Invocation
This DVA-C02 practice question tests your understanding of development with aws services. 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.
Exhibit
Refer to the exhibit.
{
"Statement": [
{
"Effect": "Allow",
"Action": "lambda:InvokeFunction",
"FunctionName": "arn:aws:lambda:us-east-1:123456789012:function:my-function",
"Principal": "s3.amazonaws.com"
}
]
}
A developer is setting up an S3 bucket to trigger an AWS Lambda function when a new object is created. After configuring the event notification, the Lambda function is not invoked. The developer checks the Lambda resource-based policy, which is shown in the exhibit. What is the issue?
Exhibit
Refer to the exhibit.
{
"Statement": [
{
"Effect": "Allow",
"Action": "lambda:InvokeFunction",
"FunctionName": "arn:aws:lambda:us-east-1:123456789012:function:my-function",
"Principal": "s3.amazonaws.com"
}
]
}
A
The policy is missing a condition that specifies the S3 bucket ARN.
To allow S3 to invoke the function, the policy should include a condition like 'ArnLike' with the bucket ARN to prevent other buckets from triggering the function.
B
The Action is incorrect; it should be 'lambda:Invoke' instead of 'lambda:InvokeFunction'.
Why wrong: The correct action is 'lambda:InvokeFunction'.
C
The principal is incorrect; it should be the S3 bucket ARN.
Why wrong: The principal should be 's3.amazonaws.com'.
D
The policy is missing the 'Version' field.
Why wrong: The 'Version' field is recommended but not strictly required; the missing condition is the key issue.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The policy is missing a condition that specifies the S3 bucket ARN.
The Lambda resource-based policy must include a condition that restricts the `aws:SourceArn` to the specific S3 bucket ARN to prevent the 'confused deputy' problem. Without this condition, S3 is not authorized to invoke the Lambda function, even if the event notification is configured correctly. The policy shown allows invocation from any S3 bucket, which AWS S3 requires to be scoped down for security reasons.
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 policy is missing a condition that specifies the S3 bucket ARN.
Why this is correct
To allow S3 to invoke the function, the policy should include a condition like 'ArnLike' with the bucket ARN to prevent other buckets from triggering the function.
Related concept
Read the scenario before looking for a memorised answer.
✗
The Action is incorrect; it should be 'lambda:Invoke' instead of 'lambda:InvokeFunction'.
Why it's wrong here
The correct action is 'lambda:InvokeFunction'.
✗
The principal is incorrect; it should be the S3 bucket ARN.
Why it's wrong here
The principal should be 's3.amazonaws.com'.
✗
The policy is missing the 'Version' field.
Why it's wrong here
The 'Version' field is recommended but not strictly required; the missing condition is the key issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on the principal or action fields and overlook the critical `aws:SourceArn` condition, which AWS explicitly requires for S3-triggered Lambda functions to prevent security vulnerabilities.
Detailed technical explanation
How to think about this question
The 'confused deputy' problem occurs when a downstream service (S3) is tricked into invoking a Lambda function on behalf of a malicious actor. By adding a condition with `aws:SourceArn` set to the specific bucket ARN, you ensure that only that bucket can trigger the function. Under the hood, S3 sends a `CreateEvent` to Lambda, and Lambda checks the resource-based policy for both the principal (`s3.amazonaws.com`) and the source ARN condition before allowing the invocation.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
Quick reference
AWS S3 Storage Class Comparison
Storage Class
Min Duration
Retrieval
Use Case
S3 Standard
None
Immediate
Frequently accessed data
S3 Standard-IA
30 days
Immediate
Infrequent access, rapid retrieval
S3 One Zone-IA
30 days
Immediate
Non-critical infrequent data
S3 Intelligent-Tiering
None
Immediate–hours
Unknown or changing access patterns
S3 Glacier Instant
90 days
Milliseconds
Archive with instant retrieval
S3 Glacier Flexible
90 days
Minutes–hours
Archive, flexible retrieval
S3 Glacier Deep Archive
180 days
Hours
Long-term compliance archive
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.
Development with AWS Services — This question tests Development with AWS Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The policy is missing a condition that specifies the S3 bucket ARN. — The Lambda resource-based policy must include a condition that restricts the `aws:SourceArn` to the specific S3 bucket ARN to prevent the 'confused deputy' problem. Without this condition, S3 is not authorized to invoke the Lambda function, even if the event notification is configured correctly. The policy shown allows invocation from any S3 bucket, which AWS S3 requires to be scoped down for security reasons.
What should I do if I get this DVA-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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