I started with the obvious stuff, redirect URI manipulation and token leakage, but then kind of stalled when they pushed me on more subtle flows.
Start by clarifying the scope: are we attacking OAuth as an attacker (security assessment) or designing a secure OAuth implementation? Then walk through the OAuth flow step-by-step, identifying potential vulnerabilities at each stage, and discuss mitigation strategies and trade-offs. Emphasize a systematic, defense-in-depth approach.
Pro tip: Focus on the most common OAuth vulnerabilities like CSRF, token leakage, and open redirects, and relate them to real-world incidents (e.g., Facebook's OAuth flaws). Demonstrating awareness of both attacker and defender perspectives shows depth.
Determine whether the question is about attacking (penetration testing) or securing an OAuth implementation. Clarify the OAuth version (2.0 vs 1.0a) and the specific flow (authorization code, implicit, etc.).
Outline the chosen OAuth flow (e.g., authorization code grant) and identify all parties: client, authorization server, resource server, and user. Highlight key steps where data is exchanged.
For each step, brainstorm potential attacks: CSRF on the authorization endpoint, token leakage via referrer or logs, open redirectors, insufficient redirect_uri validation, and scope escalation.
Suggest defenses like PKCE, state parameter, strict redirect_uri matching, short-lived tokens, and token binding. Discuss trade-offs between security and usability (e.g., PKCE adds complexity but prevents code interception).
Conclude with a prioritized list of recommendations, emphasizing the most critical vulnerabilities and how to address them systematically.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.