Securing Your OAuth Callback Endpoint

Why Callbacks Are Targets

Your OAuth callback endpoint receives authorization codes from identity providers. If an attacker can manipulate this flow, they can hijack user accounts.

Essential Security Measures

1. Validate the State Parameter

The state parameter prevents CSRF attacks. Generate a cryptographically random value, store it in the session, and verify it matches on callback.

// Before redirect
const state = crypto.randomBytes(32).toString("hex");
session.oauthState = state;

// On callback
if (req.query.state !== session.oauthState) {
  throw new SecurityError("Invalid state parameter");
}

2. Use PKCE for All Clients

Proof Key for Code Exchange prevents authorization code interception. It's required for mobile apps and SPAs, but you should use it everywhere.

// Generate PKCE verifier and challenge
const verifier = crypto.randomBytes(32).toString("base64url");
const challenge = crypto
  .createHash("sha256")
  .update(verifier)
  .digest("base64url");

3. Validate Redirect URIs Strictly

Never accept arbitrary redirect URIs. Whitelist exact URIs - no wildcards, no partial matches, no open redirects.

4. Exchange Codes Immediately

Authorization codes should be single-use and short-lived. Exchange them for tokens immediately upon receipt - don't queue them.

5. Verify Token Claims

After receiving tokens, verify:

• The aud (audience) matches your client ID
• The iss (issuer) is the expected IdP
• The nonce matches what you sent (for OIDC)
• The token hasn't expired

Common Vulnerabilities

• Open redirects: Allowing arbitrary post-login destinations
• Token leakage: Exposing tokens in URLs or referrer headers
• Missing state validation: Enabling CSRF attacks
• Code reuse: Not invalidating codes after use

Get the callback right, and you've secured the most critical part of your OAuth flow.