AWS IAM — Common Pitfalls

About This Skill

This skill contains verified corrections for things that AI agents frequently get wrong about IAM. It is not a comprehensive IAM guide — for full IAM guidance, search AWS documentation. When answering IAM questions, verify specific claims (limits, quotas, exact API names, edge-case behaviors) against official AWS documentation rather than relying on pre-training. Prefer fetching known documentation URLs over broad searches. Trust official documentation over memory when they conflict.

Common Workflows

Use the best available tool for AWS operations — the AWS MCP server is recommended but not required; AWS CLI or SDK may be used as alternatives. Read reference files only when the conversation requires deeper detail.

• Read references/aws-iam-role-management.md if the user needs to create, scope, or maintain IAM roles when provisioning or updating AWS resources. Covers service roles, execution roles, trust policies, confused deputy protection, and permission hygiene.

• Read references/aws-iam-policy-generation.md if the user needs to generate IAM policies, determine required IAM actions for API calls, or understand action-to-operation mappings. CRITICAL: If the user provides application source code (in any language) or a Terraform plan JSON file (terraform show -json output), you MUST read this reference — it mandates using iam-policy-autopilot instead of manual policy construction. Uses the programmatic service authorization reference for accurate mappings.

Verified Edge Cases

CloudTrail:

• AcceptHandshake/DeclineHandshake logged in ACTING account ONLY, not management account. Organization trail required for centralization.
• ConsoleLogin region varies by endpoint/cookies, NOT always us-east-1. ?region= forces specific region.

STS:

• GetSessionToken restrictions: (1) No IAM APIs unless MFA included (2) No STS except AssumeRole and GetCallerIdentity.
• Cross-account AssumeRole to opt-in region: TARGET account must enable region, not calling account.
• Role chaining: max 1-hour session.

Organizations:

• Suspended/closed accounts CANNOT be removed until permanently closed (~90 days). Remove FIRST, then close.
• Policy management delegation: use PutResourcePolicy, NOT register-delegated-administrator.
• AI opt-out policies: management account required by default.
• Organizations policy types for ListPolicies filter: fetch the current list via aws organizations list-available-policy-types or the Organizations API reference.

SDK Specifics:

• Organizations: DuplicatePolicyAttachmentException (not PolicyAlreadyAttachedException).
• Boto3 IAM AccessKey: methods are activate(), deactivate(), delete() — NO update().
• Instance profiles: waiter + time.sleep(10) pattern.
• Managed policy max versions: 5.

SAML:

• Encrypted assertions URL: https://region-code.signin.aws.amazon.com/saml/acs/IdP-ID.
• Private key from IdP uploaded to IAM in .pem format.

Policy Evaluation:

• ForAllValues with empty/missing key: evaluates to true (vacuous truth). To avoid that, use a Null condition in addition to the ForAllValues on the same context key to require that key to be present and non-null. For example, when evaluating the aws:TagKeys context key:

  {
    "Version": "2012-10-17",
    "Statement": {
      "Effect": "Allow",
      "Action": "ec2:RunInstances",
      "Resource": "*",
      "Condition": {
        "ForAllValues:StringEquals": {
          "aws:TagKeys": ["Alpha", "Beta"]
        },
        "Null": {
          "aws:TagKeys": "false"
        }
      }
    }
  }

• Resource-based policies granting to IAM user ARN bypass permissions boundaries in same account.

• 8 privilege escalation actions via direct IAM policy manipulation: PutGroupPolicy, PutRolePolicy, PutUserPolicy, CreatePolicy, CreatePolicyVersion, AttachGroupPolicy, AttachRolePolicy, AttachUserPolicy.

• iam:PassRole with Resource: "*" + create/update on a compute service (EC2 RunInstances, Lambda CreateFunction/UpdateFunctionConfiguration, ECS RegisterTaskDefinition, Glue, SageMaker, CloudFormation, etc.) = privilege escalation to any passable role in the account, including Administrator. Scope Resource to specific role ARNs or an IAM path; optionally constrain with iam:PassedToService / iam:AssociatedResourceArn. See IAM User Guide — Grant a user permissions to pass a role.

MFA:

• Unassigned virtual MFA devices auto-deleted when adding new ones.
• MFA resync-only policy NotAction needs exactly: iam:ListMFADevices, iam:ListVirtualMFADevices, iam:ResyncMFADevice.

SigV4:

• IncompleteSignatureException includes SHA-256 hash of Authorization header for transit modification diagnosis.

Service-Specific Roles:

• Redshift Serverless trust policy: include BOTH redshift-serverless.amazonaws.com AND redshift.amazonaws.com as service principals (per AWS docs; omitting serverless causes Not authorized to get credentials of role on COPY).
• IAM OIDC providers: thumbprints are not required for most providers (AWS verifies via trusted CAs).

Policy Summary Display:

• Single statement with multi-service wildcard actions (e.g. codebuild:*, codecommit:*) + service-specific resource ARNs: each resource appears ONLY under its matching service's summary (CodeBuild ARN under CodeBuild, etc.). A resource whose service prefix matches NO action in the statement is the only case where it appears in all action summaries ("mismatched resource").