Introduction to Tokens in Azure AD
In Azure Active Directory (Azure AD), tokens are essential for secure authentication and authorization. When a user or application successfully authenticates, Azure AD issues a token. This token acts as a credential that can be presented to resources (like APIs or other applications) to prove the identity and permissions of the presenter. Understanding the types of tokens, their structure, and how to validate them is crucial for building secure cloud applications.
Types of Tokens
Azure AD primarily issues two types of tokens:
- Access Tokens: These tokens are used by clients to access protected resources (APIs). They contain claims about the user or application and the permissions granted.
- ID Tokens: These tokens are issued to the user at the end of an authentication flow. They contain claims about the authenticated user, such as their name, email, and object ID. ID tokens are typically used by the client application to identify the user.
JWT: The Common Token Format
Both Access Tokens and ID Tokens in Azure AD are typically formatted as JSON Web Tokens (JWTs). A JWT is a
compact, URL-safe means of representing claims to be transferred between two parties. A JWT consists of three
parts separated by dots (.):
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c- Header: Contains metadata about the token, such as the signing algorithm used.
- Payload: Contains the claims, which are statements about an entity (typically, the user) and additional data.
- Signature: Used to verify that the sender of the JWT is who it says it is and to ensure that the message was not changed along the way.
Key Security Considerations
Securing tokens involves several critical aspects:
1. Token Validation
When a resource receives a token, it must validate it thoroughly. This includes:
- Verifying the signature to ensure the token hasn't been tampered with.
- Checking the audience (
aud) claim to ensure the token was intended for this resource. - Validating the issuer (
iss) claim to confirm it was issued by a trusted authority (Azure AD). - Checking the expiration time (
exp) to ensure the token is still valid. - Validating the token version (
ver) if applicable.
2. Scopes and Permissions
Access tokens contain information about the permissions (scopes) the client application has been granted. Resources should always check that the required scopes are present in the token before granting access to a requested resource or operation.
3. Token Lifetime and Refresh Tokens
Access tokens have a limited lifetime (typically 1 hour). To maintain user sessions without requiring re-authentication for every request, Azure AD issues refresh tokens. Refresh tokens are used to obtain new access tokens.
- Keep refresh tokens secure, as they allow for the issuance of new access tokens.
- Implement appropriate refresh token rotation and revocation mechanisms.
4. Avoiding Sensitive Data in Tokens
While tokens contain claims, they are not encrypted by default (only signed). Avoid placing highly sensitive information
directly in the payload of ID or Access tokens if it's not necessary for the intended recipient. Use claims like sub (subject ID) and reference other sensitive data via secure API calls if needed.
5. Secure Storage
Tokens should be stored securely on the client. Avoid storing them in local storage for web applications due to cross-site scripting (XSS) vulnerabilities. Consider using HTTP-only cookies or in-memory storage for web applications, and platform-specific secure storage for mobile applications.
Conclusion
Mastering token security in Azure AD is fundamental to building robust and secure cloud-native applications. By understanding JWTs, implementing rigorous validation, managing scopes effectively, and adhering to secure storage practices, you can significantly enhance the security posture of your applications.