Diagnosing PKCE Authorization Code Exchange Failures in OAuth 2.0

Recognizable condition

When redeeming an authorization code at the token endpoint, the server returns invalid_grant or invalid_request errors, often indicating a problem with the PKCE code_verifier parameter.

Cause / diagnostic table

Observed error	Likely cause
invalid_request	Missing code_verifier in the token request
invalid_grant	code_verifier does not match the code_challenge sent earlier
invalid_grant	Authorization code already used or expired
invalid_grant	Clock skew causing the verifier/challenge to be considered stale

Ordered checks

1. Verify that the client generated a random code_verifier and included its SHA‑256 Base64URL‑encoded code_challenge in the initial authorization request.
2. Confirm that the token endpoint receives the exact same raw code_verifier value (no URL‑encoding or truncation).
3. Check that the authorization code has not been previously redeemed and is still within its validity window (commonly 10 minutes).
4. Ensure the authorization server’s system clock is synchronized (e.g., via NTP) to avoid premature expiration of the challenge.

Fixes tied to findings

• Missing verifier – Modify the client to create a cryptographically random code_verifier (43‑128 characters) before the auth request, store it securely, and send it in the token request.
• Mismatch – Re‑compute the challenge from the stored verifier (BASE64URL(SHA256(verifier))) and compare it to the value sent in the authorization request; correct any encoding error.
• Reused code – Discard the used authorization code and initiate a new authorization flow; do not attempt to redeem the same code twice.
• Clock skew – Sync the server clocks (NTP) or, if you control the authorization server, increase the allowed code lifetime in its configuration.

Example diagnostic flow (illustrative)

# 1. Generate verifier and challenge (run on client, no special privileges)
VERIFIER=$(openssl rand -base64 32 | tr -d '=+/' | cut -c1-128)
CHALLENGE=$(echo -n "$VERIFIER" | openssl dgst -sha256 -binary | openssl base64 -A | tr '+/' '-_' | tr -d '=')

# 2. Authorization request (browser or curl)
curl -i "https://auth.example.com/authorize?response_type=code&client_id=myapp&redirect_uri=https%3A%2F%2Fmyapp.example.com%2Fcallback&scope=read&code_challenge=$CHALLENGE&code_challenge_method=S256"

# After user approval, you receive a redirect with ?code=AUTH_CODE

# 3. Token request (run on client, include verifier)
curl -i -X POST "https://auth.example.com/token" -d "grant_type=authorization_code" -d "code=AUTH_CODE" -d "redirect_uri=https%3A%2F%2Fmyapp.example.com%2Fcallback" -d "client_id=myapp" -d "code_verifier=$VERIFIER"

If the response contains "access_token" without an error field, the PKCE flow succeeded. If you see invalid_grant, repeat the checks above.

Verification steps

1. Capture a successful token exchange with the commands above and log the code_verifier and code_challenge values (do not store them in persistent logs).
2. Inspect the authorization server’s token‑endpoint logs for the presence of the exact code_verifier parameter and a grant_type=authorization_code request that returns HTTP 200 with an access token.
3. Run a negative test: omit code_verifier or send an altered value and confirm the server returns invalid_request or invalid_grant as expected.

Escalation criteria

• Persistent invalid_grant errors after verifying the verifier integrity across multiple clients.
• Repeated token‑endpoint failures that are not resolved by client‑side checks.
• These patterns may indicate an authorization‑server misconfiguration, clock‑synchronization issue, or a potential security incident; escalate to the identity‑provider operations team for log review and possible key rotation.

Limitations and practical checks

This guide assumes the authorization server implements PKCE as defined in RFC 7636 and that the authorization code lifetime is configurable. If the server does not expose logs, work with the provider’s support to obtain token‑endpoint audit trails. Always avoid logging the raw code_verifier or authorization code in production systems.