401 Unauthorized on token exchange with PKCE in Genesys Cloud SPA

NICE CXone API & Integrations
1

Need some help troubleshooting the Authorization Code flow with PKCE for a single-page application against Genesys Cloud.

Background

Implementing a custom SPA login widget. Using code_challenge_method=S256 with SHA-256 hash for the verifier.

Issue

The authorization request succeeds and returns the code. However, the subsequent POST to /oauth/token fails with a 401 Unauthorized response.

{
 "error": "invalid_grant",
 "error_description": "The provided authorization grant is invalid, expired, or revoked"
}

Troubleshooting

  • Verified the code_verifier matches the base64url-encoded SHA-256 hash of the code_challenge.
  • Confirmed the redirect_uri matches exactly.
  • The code is used only once.

Is there a specific format requirement for the code_verifier in Genesys Cloud that differs from standard RFC 7636 implementation?

2

It depends, but generally… The documentation states: “The code_verifier parameter must match the code_challenge exactly.” I am seeing discrepancies in how the verifier is generated in browser-based JS versus my backend Python services. Try these adjustments:

  • Verify the SHA-256 hash generation. Ensure you are using base64url encoding without padding. The docs specify: “Use URL-safe Base64 encoding without padding characters.”
  • Check the expiration timestamp. If the code is too old, the token endpoint returns 401. The documentation notes: “Authorization codes expire after 10 minutes.”
  • Inspect the redirect_uri. It must match the registered URI exactly, including the trailing slash. The API is strict about this: “The redirect URI must match exactly.”

Here is the Python snippet I use to generate the challenge:

import hashlib
import base64

def make_code_challenge(code_verifier):
 digest = hashlib.sha256(code_verifier.encode('utf-8')).digest()
 return base64.urlsafe_b64encode(digest).decode('utf-8').rstrip('=')

Why does the 401 persist if the challenge matches? I suspect the scope parameter might be missing openid.

3

You need to ensure your code_verifier is strictly base64url encoded without padding, as Genesys rejects standard base64. Use this payload structure for the token exchange to avoid 401 errors.

{
 "grant_type": "authorization_code",
 "code": "<auth_code>",
 "code_verifier": "<base64url_no_padding_verifier>",
 "redirect_uri": "https://your-app.com/callback"
}
4

Yep, this is a known issue… docs state “implicit grant tokens are opaque” so local signature verification is invalid. use the PureCloudPlatformClientV2 SDK to handle auth automatically instead of manually constructing the token exchange payload. I tried doing the base64url encoding manually in JS but it always failed because of padding. the suggestion above about the JSON payload is correct but you still need the right headers. here is the working curl command that actually gets a 200 OK for me. it uses the application client credentials directly which is easier for testing.

curl -X POST https://api.mypurecloud.com/api/v2/authorize \
 -H "Content-Type: application/x-www-form-urlencoded" \
 -d "grant_type=client_credentials&client_id=YOUR_CLIENT_ID&client_secret=YOUR_CLIENT_SECRET&scope=login:read"

if you are building a flow later, just use the OAuth2 Data Action. it handles the PKCE flow internally and saves you from writing custom expressions to parse the JSON response. the docs say “Data Actions manage the lifecycle of the token” so let the platform do the heavy lifting.