Given a new critical microservice at Uber, how would a security engineer design a robust authentication and authorization system to protect it from common threats and ensure proper access control?
Designing a robust authentication and authorization system for a new critical Uber microservice involves several layers of defense, focusing on both user-facing and service-to-service interactions. For user authentication, an OAuth 2.0 and OpenID Connect (OIDC) flow is ideal, leveraging an existing Identity Provider (IdP) if available. This separates identity management from the microservice itself. Authorization should then be handled using a combination of role-based access control (RBAC) and attribute-based access control (ABAC) to enforce fine-grained permissions.
Secure Token Management
For user sessions, after successful authentication, the IdP issues a JSON Web Token (JWT) containing claims about the user and their permissions. This token should be short-lived for security, with a refresh token mechanism for extending sessions without repeated logins. The microservice should validate these JWTs using public keys from the IdP, ensuring their integrity and authenticity before processing any request. Implement token revocation mechanisms for compromised or explicitly logged-out sessions.
Internal microservices require their own authentication and authorization. A common approach is to use mTLS (mutual TLS) for authentication, where services verify each other’s certificates. For authorization, a dedicated Authorization Service can issue short-lived JWTs to calling services, scope-restricted to specific actions or resources. This adheres to the principle of least privilege, ensuring one service cannot access data or functionality it does not explicitly need.
Best practice
Adopt the principle of least privilege across the entire system. Each user, service, or component should only have the minimum necessary permissions to perform its designated function. Regularly audit access policies and token configurations, and enforce strong password policies, multi-factor authentication (MFA) for users, and secure secret management for services. Automated static and dynamic analysis should be integrated into the CI/CD pipeline to identify common vulnerabilities early.
Edge case interviewers probe for
Interviewers often ask about handling token revocation efficiently in a distributed system, especially for short-lived access tokens without immediate database lookups. Strategies like maintaining a distributed blacklist (or denylist) of revoked tokens, or short access token lifetimes coupled with frequently rotating refresh tokens, are key solutions. Discuss how to mitigate replay attacks and ensure freshness of authorization decisions.
Common mistake
A common mistake is treating all services or users as equally privileged, granting overly broad permissions. Another is insecurely storing secrets (API keys, private keys) directly in code repositories or environment variables without proper encryption or rotation. Failing to implement robust token validation, particularly signature verification and expiry checks, leaves the system vulnerable to tampered or expired tokens. Relying solely on network segmentation without application-level authentication is also a critical oversight.
What the interviewer is checking
The interviewer is assessing your holistic understanding of security in a distributed environment, your ability to design layered defenses, and your familiarity with industry standards like OAuth 2.0, OIDC, and JWTs. They want to see your practical knowledge of threat models (e.g., replay attacks, token theft), your proactive approach to security (e.g., least privilege, secure coding), and your capacity to think about operational aspects like token revocation and secret management.
Imagine a very exclusive nightclub, “The Microservice Mansion.” When you first arrive, a strict bouncer at the main entrance needs to confirm who you are. You show your ID, maybe answer a security question. This is like authentication: proving your identity to the system. Once the bouncer verifies you, he gives you a special, unique wristband. This wristband is your “access token” — it shows that you have been checked in and are a legitimate guest.
Now, with your wristband, you can enter the club. But not all areas are open to everyone. Some rooms are for VIPs only, some for performers, and others for general admission. Your wristband might have different colors or markings indicating which rooms you are allowed into. This is authorization: what you are allowed to do or where you can go after your identity is confirmed. So, the bartender checking your wristband before serving a drink, or a security guard blocking entry to the VIP lounge, is enforcing authorization rules based on your access level.
Why interviewers ask this
Interviewers ask this to gauge your foundational understanding of security principles, especially in modern distributed systems. They want to see if you can design a secure system from the ground up, identifying potential vulnerabilities and implementing robust controls for both user and internal service interactions. It demonstrates your ability to apply theoretical knowledge to a practical, architectural challenge.
What a strong answer signals
A strong answer demonstrates a comprehensive understanding of authentication protocols (e.g., OAuth 2.0, OIDC, mTLS), token security (JWTs, refresh tokens, revocation), and authorization models (RBAC, ABAC). It signals a proactive security mindset, an awareness of common threats, and the ability to articulate trade-offs and best practices in a clear, structured manner, emphasizing layered security and the principle of least privilege.
Common follow-ups
- How would you handle token revocation for a short-lived access token across many microservices without a centralized database lookup for every request?
- What are the differences between RBAC and ABAC, and when would you choose one over the other for fine-grained authorization?
- Describe how you would implement multi-factor authentication (MFA) for user access without significantly impacting user experience.
Advanced variation
Design a federated identity solution that allows users from partner organizations to seamlessly access specific functionalities within the new Uber microservice, leveraging their existing corporate credentials, while maintaining strict access control and auditability. Consider challenges like attribute mapping, trust relationships, and cross-domain authorization policies.
Consider a new Uber “Driver Incentives” microservice that needs to calculate and disburse bonuses. Initially, developers might secure access with a single API key embedded in calling services. A more robust approach would involve setting up mTLS between the Incentives service and other internal services (like “Trip History” or “Driver Profile”), ensuring mutual authentication. For authorization, define specific API scopes (e.g., incentives:read, incentives:calculate, incentives:disburse) and issue short-lived JWTs from a dedicated Authorization Service to calling services, allowing them only the necessary permissions. This prevents a compromised “Trip History” service from accidentally or maliciously disbursing funds.
import jwt
from jwt.exceptions import InvalidTokenError, ExpiredSignatureError
def validate_service_token(token: str, jwk_set: dict, audience: str, issuer: str) -> dict | None:
"""Validates an incoming JWT token for a microservice."""
try:
decoded = jwt.decode(
token,
jwk_set, # JSON Web Key Set from Identity Provider
algorithms=['RS256', 'ES256'],
audience=audience,
issuer=issuer,
options={"require": ["exp", "aud", "iss"]} # Ensure critical claims exist
)
return decoded
except ExpiredSignatureError:
print("Token expired.")
return None
except InvalidTokenError as e:
print(f"Invalid token: {e}")
return None
# Example usage in a microservice endpoint:
def handle_request(request_token: str):
if not request_token:
return {"status": 401, "message": "No token provided"}
# jwk_set, audience, issuer would be loaded from secure config or IdP discovery
JWK_SET = { "keys": [] } # Placeholder for actual JWKS
SERVICE_AUDIENCE = "uber-rides-api"
IDP_ISSUER = "https://auth.uber.com"
payload = validate_service_token(request_token, JWK_SET, SERVICE_AUDIENCE, IDP_ISSUER)
if payload:
print(f"Authorized request from user: {payload.get('sub')} with scopes: {payload.get('scope')}")
# Proceed with business logic based on 'scope' or other claims
return {"status": 200, "data": "Sensitive Data"}
else:
return {"status": 403, "message": "Unauthorized access"}- 1Authentication verifies identity, while authorization determines what actions an authenticated entity can perform within the system.
- 2Leverage established standards like OAuth 2.0 and OpenID Connect with an Identity Provider for robust user authentication.
- 3Utilize JSON Web Tokens (JWTs) for secure, stateless session management, ensuring proper validation and timely revocation mechanisms.
- 4Implement mutual TLS (mTLS) and scoped tokens for strong service-to-service authentication and fine-grained authorization in microservices.
- 5Always apply the principle of least privilege, granting only the minimum necessary permissions to users and services to reduce attack surface.