Description

The sts:ExternalId is a parameter used in AWS Security Token Service (STS) to provide an additional layer of security when granting cross-account access. It helps prevent the confused deputy problem, where a third-party service might be tricked into performing actions on behalf of an attacker.

Features

• Confused Deputy Prevention: Helps prevent unauthorized cross-account access by requiring an additional identifier.
• Enhanced Security: Adds an extra layer of security by ensuring that the requestor knows the external ID.
• Policy Integration: Can be integrated into IAM roles and policies to enforce the use of sts:ExternalId.

Limitations

• Configuration Complexity: Requires careful configuration and management to ensure proper use and security.
• Dependency on Correct Implementation: Effectiveness depends on correct implementation by both the AWS account owner and the third-party service.

Constraints

• Unique Value: The sts:ExternalId should be unique and known only to the AWS account owner and the trusted third party.
• Mandatory for Certain Policies: Some IAM role policies may require the use of sts:ExternalId to be effective.
• Character Limits: The sts:ExternalId must adhere to character limits defined by AWS, typically allowing alphanumeric characters and specific special characters.
• Consistent Implementation: Must be consistently implemented across all relevant IAM roles and policies to ensure security.

Use Cases

• Third-Party Services: When granting AWS account access to third-party services (e.g., CI/CD tools), using sts:ExternalId ensures only authorized requests are allowed.
• Cross-Account Roles: For organizations with multiple AWS accounts, sts:ExternalId helps secure cross-account role assumption by internal services or users.
• Multi-Tenant Applications: SaaS providers can use sts:ExternalId to securely manage access to individual tenant resources within AWS.