Fixing IAM Policy Variable ${aws:username} Not Resolving in Resource ARNs

Threat/Impact Level: HIGH | Exploitability/Downtime Risk: HIGH | Time to Fix: 15 mins

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TL;DR

• What broke: ${aws:username} is an IAM policy variable that only resolves for IAM Users authenticated via long-term credentials. It returns an empty string — or outright fails to match — for IAM Roles, federated identities (SAML/OIDC), and AWS service principals, silently breaking your resource-level isolation.
• How to fix it: Replace ${aws:username} with the correct scoping mechanism for your principal type: ${aws:userid} for unique ID matching, or a Condition block using aws:PrincipalArn for role-based access.
• Shortcut: Use our Client-Side Sandbox above to auto-refactor this — paste your broken policy and get corrected code without leaking your ARNs to any external server.

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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.

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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.

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How to Fix It (The Solution)

Basic Fix — Switch to ${aws:userid} for Unique Principal Scoping

aws:userid resolves for both IAM Users and Role sessions. For a role session it returns AROAEXAMPLEID:session-name. Use this when you need a unique, non-empty identifier in the ARN path.

{
  "Effect": "Allow",
  "Action": ["s3:GetObject", "s3:PutObject"],
  "Resource": [
-   "arn:aws:s3:::my-bucket/home/${aws:username}/*"
+   "arn:aws:s3:::my-bucket/home/${aws:userid}/*"
  ]
}

⚠️ Caveat: aws:userid for role sessions includes the role ID prefix (AROAID:session). Your S3 prefix structure must be built around this value. Pre-provision prefixes programmatically or use a Lambda authorizer to normalize session names.

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Enterprise Best Practice — Condition-Key Scoping with aws:PrincipalArn

Stop embedding identity into resource ARNs entirely. Use Condition blocks. This is the correct pattern for role-based and federated principals.

{
  "Effect": "Allow",
  "Action": ["s3:GetObject", "s3:PutObject", "s3:DeleteObject"],
- "Resource": "arn:aws:s3:::my-bucket/home/${aws:username}/*",
- "Condition": {}
+ "Resource": "arn:aws:s3:::my-bucket/home/*",
+ "Condition": {
+   "StringLike": {
+     "aws:PrincipalArn": "arn:aws:iam::123456789012:role/UserRole-*"
+   },
+   "StringEquals": {
+     "s3:prefix": ["home/${aws:PrincipalTag/Username}/"]
+   }
+ }
}

The PrincipalTag pattern is the correct enterprise solution. Tag your IAM roles or Identity Center permission sets with a Username tag. AWS propagates principal tags into the session context. Then use ${aws:PrincipalTag/Username} as your scoping variable — it works for roles, federated users, and SSO sessions.

# IAM Role trust policy — attach session tag
{
  "Effect": "Allow",
  "Action": "sts:AssumeRole",
  "Condition": {
+   "StringEquals": {
+     "sts:TransitiveTagKeys": ["Username"]
+   }
  }
}

# Resource policy using the tag
{
  "Effect": "Allow",
  "Action": ["s3:*"],
- "Resource": "arn:aws:s3:::my-bucket/home/${aws:username}/*"
+ "Resource": "arn:aws:s3:::my-bucket/home/${aws:PrincipalTag/Username}/*"
}

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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.