Explain the OAuth 2.0 authorization code flow with PKCE.
Interviewer Expectations for this Question
01
Core Competency
Assesses fundamental understanding of Authentication & Authorization conventions, runtime behavior, and memory/performance considerations.
02
Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
It is the recommended flow for web, mobile and single-page applications.
- The client generates a random
code_verifier, hashes it to acode_challenge, and redirects the user to the authorization endpoint with the client id, redirect URI, scope, state and challenge. - The user authenticates and consents.
- The authorization server redirects back with an authorization code and the original
state. - The client exchanges the code at the token endpoint, sending the
code_verifier. - The server verifies the verifier against the stored challenge and returns tokens.
GET /authorize?response_type=code&client_id=app
&code_challenge=...&code_challenge_method=S256&state=xyz
PKCE prevents an attacker who intercepts the authorization code from redeeming it, because they lack the verifier. Always validate state for CSRF, use exact registered redirect URIs, and avoid the deprecated implicit flow that exposes tokens in the URL.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.