IAM Policy Variable Username Resolver

The Incident (What does the error mean?)

You'll typically hit this when a role-assumed principal tries to access a resource scoped to their "username" path:

User: arn:aws:sts::123456789012:assumed-role/MyRole/session-name is not authorized
to perform: s3:GetObject on resource:
arn:aws:s3:::my-bucket/home/${aws:username}/*
because no identity-based policy allows it.

Or worse — no error at all. The variable resolves to an empty string, and your resource ARN becomes arn:aws:s3:::my-bucket/home//*, which matches nothing, silently denying all access.

The immediate consequence: Your S3 home-directory isolation pattern, DynamoDB per-user partition scoping, or SSM parameter namespace isolation is completely broken. Every role-based principal is either fully denied or — depending on your wildcard structure — potentially granted access to paths they shouldn't touch.

The Attack Vector / Blast Radius

This is a data isolation failure, not just a broken permission. Here's the blast radius:

Scenario A — Silent Deny (Broken App): A developer assumes arn:aws:iam::123456789012:role/AppRole. The policy intends to scope them to s3://my-bucket/home/appuser/. Because ${aws:username} resolves to "", the effective resource becomes s3://my-bucket/home//. Zero objects match. Your application throws 403s in production at 2 AM.

Scenario B — Privilege Escalation via Empty Variable: If your bucket policy uses a Deny statement scoped to home/${aws:username}/* expecting to block cross-user access, an empty resolution means the Deny never fires. A compromised role session can now traverse other users' home prefixes if any Allow exists at a broader scope.

Scenario C — Federated Identity Mismatch: With AWS SSO / IAM Identity Center, the principal is always a role. ${aws:username} is never populated. Teams that copy IAM User policies directly into permission sets ship broken isolation to every SSO user in the org.

The core problem: AWS documentation lists aws:username as resolving only for IAM Users. For all other principal types, the value is an empty string. This is not an error AWS surfaces at policy creation time — it fails silently at authorization time.

Prevention in CI/CD

This class of bug ships because no linter catches a syntactically valid policy variable that resolves incorrectly at runtime. Fix that gap:

1. Checkov — Block aws:username in non-user policies:

Write a custom Checkov check (CKV_CUSTOM_IAM_001) that scans all IAM policy JSON files and fails the build if ${aws:username} appears in a Resource ARN while the policy is attached to a Role or a permission boundary.

checkov -d ./iam-policies --check CKV_CUSTOM_IAM_001

2. OPA/Conftest — Rego policy to catch empty-resolving variables:

deny[msg] {
  resource := input.Statement[_].Resource
  contains(resource, "${aws:username}")
  # Flag any policy not explicitly scoped to IAM User principals
  not input.Statement[_].Principal.AWS == "arn:aws:iam::*:user/*"
  msg := "aws:username variable used in non-user-scoped policy. Use aws:PrincipalTag/Username instead."
}

conftest test iam-policy.json --policy opa/iam_variable_check.rego

3. Terraform — aws_iam_policy_document validation:

Add a precondition block in Terraform to assert that any policy document containing aws:username is only attached to aws_iam_user resources, not aws_iam_role.

lifecycle {
  precondition {
    condition     = !can(regex("\\$\\{aws:username\\}", data.aws_iam_policy_document.this.json)) || var.principal_type == "user"
    error_message = "aws:username policy variable is only valid for IAM Users. Use aws:PrincipalTag/Username for roles."
  }
}

4. AWS Access Analyzer — Enable policy validation:

AWS Access Analyzer's policy validation checker (ValidatePolicy API) will flag unresolvable policy variables as findings of type SUGGESTION. Integrate this into your pipeline:

aws accessanalyzer validate-policy \
  --policy-document file://my-policy.json \
  --policy-type IDENTITY_POLICY

Treat any SECURITY_WARNING or SUGGESTION finding related to policy variables as a pipeline hard-fail.