Microsoft·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say

Apr 2026

Summary

Microsoft system design round focused almost entirely on security for a Copilot-style API service. Dense question, lots of ground to cover, and I felt like I was playing catch-up the whole time.

Questions Asked (1)

Design a Copilot-style API service with security as the primary concern. Walk through authentication and authorization end-to-end, including how clients get and present credentials, how access tokens are signed and verified, key rotation, token scoping, and protections against misuse like rate limiting, replay attacks, and abuse detection. Also address how you'd scale this while keeping the same security guarantees.

System DesignAPI & IntegrationsTechnical Trade-offs

Author's notes

This is a lot to fit into one question.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and assumptions, then walk through the end-to-end security flow from client registration to token usage, covering authentication, authorization, token lifecycle, and abuse protections. Finally, discuss scaling strategies that preserve security guarantees, emphasizing trade-offs and Microsoft-specific technologies.

Pro tip: Anchor your design in industry standards like OAuth 2.0 and OpenID Connect, and mention Microsoft Entra ID (Azure AD) as the identity provider to show familiarity with Microsoft's ecosystem. Also, proactively discuss trade-offs between security and performance, demonstrating a balanced engineering mindset.

1. Clarify Requirements and Assumptions

Ask clarifying questions about client types, expected scale, compliance needs, and existing infrastructure. State assumptions to scope the design.

2. Design Authentication and Authorization Flow

Describe how clients register, obtain credentials, and authenticate. Explain OAuth 2.0 flows (e.g., authorization code with PKCE), token issuance, and scopes for authorization.

3. Detail Token Security and Lifecycle

Cover token signing (e.g., JWT with RS256), verification, key rotation using JWKS, token scoping, expiration, and revocation mechanisms.

4. Implement Abuse Protections

Explain rate limiting, replay attack prevention (e.g., nonces, short-lived tokens), and abuse detection using anomaly detection and logging.

5. Address Scaling with Security

Discuss horizontal scaling of auth services, distributed rate limiting, caching public keys, and maintaining security at scale via automation and monitoring.

Key Points to Mention

OAuth 2.0 and OpenID Connect standards, with Microsoft Entra ID as the identity provider
JWT signing and verification using asymmetric keys (RS256) and JWKS for key rotation
Token scoping and least privilege to limit access
Rate limiting strategies (e.g., token bucket, sliding window) and distributed rate limiting
Replay attack prevention via short-lived tokens, nonces, and token binding
Abuse detection using machine learning, logging, and real-time monitoring

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.