Grant S3 access through the Lambda function’s execution role, and allow only the S3 actions and bucket or object resources the code actually uses. Bucket-level operations such as listing need a bucket ARN; object operations such as reading or writing data need object ARNs, ideally restricted to the required key prefixes. If S3 also triggers the function, configure that invocation permission separately: it does not grant the function permission to call S3.
How Lambda gets permission to call S3
When a Lambda function runs, Lambda assumes its configured execution role. The role’s trust policy must allow the Lambda service principal, lambda.amazonaws.com, to assume it. The role’s permissions policies then determine which AWS resources the function can access. See AWS’s guide to defining Lambda function permissions with an execution role.
The execution role also needs basic permissions for the function’s CloudWatch Logs activity. Keep those logging permissions distinct from the S3 permissions you add; granting S3 access does not provide logging access.
Inventory the S3 operations your code needs
Start with the S3 API calls made by the function, including calls made by libraries or helper code. Identify the operation, the bucket, and—where applicable—the object key or key prefix. For each call, determine whether it needs to list a bucket, read object data or metadata, write an object, delete an object, or perform another operation.
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- Do not add permissions merely because they might be convenient later. A function that reads known keys may not need permission to list a bucket.
- Account for every intended code path, including scheduled jobs, retries, and error handling that may make additional S3 calls.
- Use the S3 API-to-IAM mapping to identify the required action for each operation; similar-sounding API operations can have different permission requirements.
AWS recommends narrowing permissions granted during development before production. Its Lambda guidance covers managing permissions in AWS Lambda.
Match each action to the correct resource ARN
IAM evaluates both the requested action and the resource named in the policy. S3 bucket-level actions and object-level actions do not use interchangeable resource ARNs. Consult AWS’s required permissions for Amazon S3 API operations for the specific API calls your function makes.
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- Bucket-level operations: Use the bucket ARN for operations that act on the bucket, such as listing its contents. The bucket ARN has the form
arn:aws:s3:::BUCKET_NAME. - Object-level operations: Use object resources for operations on data or metadata of objects. An object ARN has the form
arn:aws:s3:::BUCKET_NAME/OBJECT_KEY. - Key-prefix restrictions: If the function only needs a subset of the objects, scope its object resources to that prefix—for example,
arn:aws:s3:::BUCKET_NAME/incoming/*—rather than all objects in the bucket.
A bucket ARN alone does not authorize every object operation, and an object ARN does not replace the bucket-level permission needed for listing. The correct action-resource pairing depends on the particular S3 API operation, so verify each one in AWS’s permission mapping.
Build and attach the execution-role policy
- Inventory calls and scope: Record each S3 operation, the bucket it uses, and the narrowest object prefix the workload can use.
- Map operations to IAM permissions: Use AWS’s S3 operation-to-permission reference to identify the required action and whether it needs a bucket or object resource.
- Add only the required permissions: Attach an identity-based policy to the Lambda execution role. Include the necessary S3 actions and their matching resource ARNs, along with the role’s separate basic logging permissions.
- Check the full authorization path: Review bucket policies, access-point policies, cross-account requirements, encryption configuration, and explicit denies. These can affect the result; any additional permissions depend on the workload and configuration.
- Validate and exercise the function: Use IAM Access Analyzer policy validation to identify relevant findings, then run the intended application paths in the target account and verify both allowed and disallowed access.
This is a procedure, not a universal policy document: the least-privilege policy varies with the function’s actual API calls, object keys, account boundaries, and bucket configuration.
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Keep S3 invocation permission separate
The execution role controls what the running function can call, including S3. A Lambda resource-based policy controls which principals or services may invoke the function. If an S3 event notification invokes the function, the permission allowing that invocation is separate from the execution role’s permission to read or otherwise access S3. Granting one does not grant the other.
Account for access points and other policy layers
If the function accesses data through an S3 access point, authorization must allow the request through the relevant access-point policy and the underlying bucket authorization. Access-point restrictions apply to traffic through that access point; they do not automatically change permissions for direct bucket access. AWS explains the relationship in its guide to configuring IAM policies for using access points.
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For cross-account access or encryption configurations, inspect the policies and permissions involved in that specific setup. There is no single additional permission that applies to every Lambda-to-S3 workload.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use broad policies and activity analysis carefully
Broad managed policies can make initial setup easy but may exceed the needs of a particular function. AWS notes that managed policies might not be least privilege for a specific use case; AmazonS3FullAccess, for example, grants full S3 access. Avoid leaving such access in place simply because the function works.
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IAM Access Analyzer can use CloudTrail access activity to generate a policy template that helps identify permissions to retain or remove. Treat observed activity as a starting point, not proof that all necessary permissions have been discovered: rarely used paths may not have run during the observation period. AWS also documents S3 Access Grants as an access-management option.
Choose an access-management pattern that fits the workload
For small-to-medium numbers of datasets, AWS describes IAM identity policies and bucket policies as a straightforward approach. More granular or scaled access patterns may call for access points or S3 Access Grants. Consider who owns each policy, whether access crosses accounts, how many datasets or clients need distinct access, and whether clients use direct bucket access or access points. The right choice depends on those operational needs; it does not remove the need to grant only the actions and resources the function requires.
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