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Amazon S3

Your Lambda Has Admin on S3 and Doesn’t Know It

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A Lambda function’s access to Amazon S3 comes from its IAM execution role. If that role grants broad S3 permissions, the function may be able to do more than its workload requires—but that alone does not make an S3 bucket public. Review the role’s policies, narrow actions and resources to the function’s needs, and inspect bucket-level access controls separately when public or cross-account exposure is the concern.

Why does my Lambda have admin access to S3?

Lambda assumes an execution role when a function runs. The permissions granted to that role determine which AWS resources the function can access. A role with broad S3 permissions can therefore give the function broad S3 capabilities, even if the function’s code uses only a small subset of them. AWS explains the execution-role model in its Lambda execution role documentation.

That is an identity-permission issue, not proof that a bucket is public. A role policy describes what the function is allowed to request as an AWS identity; bucket policies, ACLs, and access-point policies are separate controls that can allow or restrict access to S3 resources. AWS recommends using least privilege—granting only the permissions a workload needs—and notes that broad or wildcarded policies can expand access beyond what was intended. See Defining Lambda function permissions with an execution role and the IAM Access Analyzer policy-check reference.

How do I check what the function can access?

  1. Find the execution role. In the AWS Lambda console, open Functions, select the function, and review Configuration → Permissions. Follow the execution-role link to IAM.
  2. Review every policy attached to the role. Check both managed and inline policies, and look for S3 actions, wildcard actions, and resource entries that use *. Also check any applicable permissions boundary or organization-level control: a role policy grants permissions, while other controls may limit the effective permissions.
  3. Compare grants with the workload. Identify which S3 operations the code actually performs—such as reading, writing, listing, or deleting—and which bucket and object paths those operations need. Include event-driven flows, deployment or maintenance tasks, and scheduled jobs, not just the common request path.
  4. Keep non-S3 needs in view. Every Lambda function needs an execution role, and AWS’s basic execution-role guidance includes permissions for CloudWatch logging. Avoid removing permissions the function needs to emit logs while tightening S3 access. See AWS’s execution-role guidance.

AWS recommends adjusting development and production roles so they include only the permissions required by the function. The exact minimum depends on the function’s code and operating requirements; a policy that is narrow for one workload may break another.

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How do I limit an AWS Lambda function to one S3 bucket?

Reduce both the actions and the resources in the role’s S3 permissions. Do not grant a broad set of S3 actions if the function only reads objects, and do not use a resource wildcard when the workload only needs a particular bucket or object path. For example, distinguish a function that reads specific objects from one that writes or deletes objects; do not assume those operations belong together.

Use the role policy editor in IAM to make a scoped change, then review the policy before saving. The policy should reflect the function’s actual operations and target resources. AWS’s policy validation guidance can help identify broad or wildcarded permissions, but the reviewed AWS materials do not establish a complete action-by-action S3 policy example for every Lambda workload. Consult AWS’s policy checks and validate the specific actions and resource scope your function needs.

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  • Action scope: include only the read, write, list, or delete operations required by the workload.
  • Resource scope: target the necessary bucket or object path rather than all S3 resources when possible.
  • Workload scope: account for secondary flows, scheduled work, and operational tasks, not only the usual invocation.

Can IAM Access Analyzer help identify the right permissions?

Yes. IAM Access Analyzer can use CloudTrail activity over a selected date range to generate a policy template based on permissions observed in use. AWS recommends it as one aid for identifying permissions needed by a Lambda execution role; the generated template is evidence to review, not a guarantee that it captures every legitimate requirement. See AWS’s Lambda execution-role guidance and Generating policies with IAM Access Analyzer.

Activity-based recommendations have a coverage limitation: AWS says its role-permission recommendations use the last 30 days of activity. A quarterly process or other infrequent task may not appear in that window and could look unused. Before removing a permission based on observed activity, compare the observation period with the function’s schedule, code paths, and documented requirements. The 30-day figure is the recommendation window described by AWS, not evidence that a policy is complete after a month.

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How do I check whether the bucket is public or shared?

Inspect S3’s resource-side access controls separately from the Lambda role. AWS IAM Access Analyzer for S3 can review bucket policies, ACLs, and access-point policies to identify public or shared access. This matters when the concern is whether other accounts or the public can access a bucket; narrowing the Lambda role and checking bucket exposure address different questions. See Using IAM Access Analyzer for S3.

If an analyzer finding indicates unintended access, use it to identify the policy responsible, change that policy, and rescan to verify the finding’s status. AWS describes this finding-and-remediation process in its IAM Access Analyzer documentation.

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What should I verify after changing the policy?

  • Exercise the function’s expected S3 read, write, listing, or other required flows.
  • Check scheduled and infrequent jobs against the permissions removed, especially work that would not have run during the activity observation window.
  • Confirm the function can still write its required CloudWatch logs.
  • Review relevant IAM Access Analyzer findings and, for public or shared bucket access, S3 analyzer findings after policy changes.

A policy change should be judged against the intended workload and the resource-side access controls—not merely by whether the usual invocation still succeeds.

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