not authorized to perform sts:assumerolewithwebidentity

3 min read 22-11-2024

The error "Not authorized to perform sts:AssumeRoleWithWebIdentity" typically arises when an application or service attempts to assume an IAM role using a web identity provider (like AWS IAM roles for applications, Google Cloud, or Azure Active Directory) but lacks the necessary permissions. This comprehensive guide will walk you through understanding the error, common causes, and effective troubleshooting steps.

Understanding the sts:AssumeRoleWithWebIdentity Action

The sts:AssumeRoleWithWebIdentity action is a crucial part of federated identity management in cloud environments. It allows an application or service to temporarily assume the permissions of an IAM role without needing explicit long-term credentials. This is a secure and efficient way to grant access to resources. The error indicates the identity provider's token isn't authorized to perform this action.

Common Causes of the "Not Authorized" Error

Several factors can lead to this frustrating error. Let's examine the most prevalent ones:

1. Incorrect IAM Role Configuration:

Missing Permissions: The IAM role you're trying to assume might not have the sts:AssumeRoleWithWebIdentity permission explicitly granted. Check the role's policy document to ensure it includes this permission. You might need to attach a managed policy like AWSServiceRoleForAmazonEC2 or create a custom policy.
Incorrect Trust Relationship: The trust policy associated with the IAM role must explicitly grant permission to the specific web identity provider you're using. This policy defines which identities are allowed to assume the role. It needs to include the provider's ARN (Amazon Resource Name). Double-check that the ARN is correct and properly formatted within the trust policy.

2. Problems with the Web Identity Provider:

Invalid Token: The token issued by your web identity provider (e.g., Google Cloud, Azure AD) might be invalid, expired, or incorrectly formatted. Verify the token's validity and ensure it's properly generated and passed to the AWS STS (Security Token Service). Examine the provider's documentation for token generation and expiration details.
Insufficient Permissions from the Provider: The identity provider itself might not have the necessary permissions to issue a token that allows the sts:AssumeRoleWithWebIdentity action. Review the provider's settings and ensure it's configured correctly to grant access to the required AWS resources.

3. Network Connectivity Issues:

Network Restrictions: Firewalls or network security groups might be blocking communication between your application and the AWS STS endpoint. Ensure that your application can access the STS endpoint over the required ports and protocols.

4. Incorrect Role ARN:

Typos or Incorrect ARN: A simple typo in the ARN (Amazon Resource Name) of the IAM role can prevent successful role assumption. Verify the ARN's accuracy against the IAM console.

Troubleshooting Steps

Let's outline a systematic approach to resolving this error:

Verify IAM Role Permissions: Begin by carefully reviewing the IAM role's policy. Ensure the sts:AssumeRoleWithWebIdentity permission is explicitly granted.
Examine the Trust Relationship: Check the trust policy associated with the IAM role. Make sure it includes the correct ARN of your web identity provider and allows the sts:AssumeRoleWithWebIdentity action. A common mistake is an incorrect provider ARN.
Validate the Web Identity Provider Token: Use tools provided by your identity provider to confirm the token's validity, expiration date, and format.
Check Network Connectivity: Temporarily disable firewalls or network security groups to rule out network restrictions. This should only be done in a controlled test environment.
AWS CLI Debugging: Utilize the AWS CLI to simulate the role assumption process. This allows for more precise error identification. For example:

aws sts assume-role-with-web-identity \
    --web-identity-token <your_web_identity_token> \
    --role-arn <your_role_arn> \
    --role-session-name <your_session_name>

Cloud Provider Documentation: Refer to the documentation for your specific web identity provider (e.g., Google Cloud, Azure AD) for detailed instructions and troubleshooting guidance.

Prevention and Best Practices

Regular Audits: Regularly audit your IAM roles and trust policies to identify and fix any inconsistencies or outdated configurations.
Principle of Least Privilege: Grant only the necessary permissions to your IAM roles to minimize the potential impact of security breaches.
Thorough Testing: Always thoroughly test your application's ability to assume roles before deploying it to a production environment.

By carefully following these troubleshooting steps and best practices, you can effectively resolve the "Not authorized to perform sts:AssumeRoleWithWebIdentity" error and ensure seamless integration with your web identity provider. Remember to always prioritize secure configuration and adherence to the principle of least privilege.