AWS IAM — Common Pitfalls Skill

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 least-privilege IAM policies, determine required IAM actions for API calls, or understand action-to-operation mappings. CRITICAL: If the user provides source code (Python, Go, TypeScript, JavaScript, Java), 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").

Agent Skills: AWS IAM — Common Pitfalls

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