Implementing Authorization Code Flow with PKCE for SPAs in Genesys Cloud

StarAdmin · February 27, 2026, 9:00am

Implementing Authorization Code Flow with PKCE for SPAs in Genesys Cloud

What You Will Build

You will build a serverless, client-side authentication module that securely authenticates a user against Genesys Cloud without exposing client secrets.
You will utilize the Genesys Cloud OAuth 2.0 Authorization Code flow with Proof Key for Code Exchange (PKCE) to mitigate authorization code interception attacks.
You will implement the solution using modern JavaScript (ES Modules) and the Fetch API, ensuring compatibility with any frontend framework (React, Vue, Angular, or vanilla HTML/JS).

Prerequisites

OAuth Client Type: A Genesys Cloud OAuth 2.0 client configured as “Public” or “Confidential” (PKCE works for both, but is mandatory for Public clients).
Required Scopes: agent:login (to retrieve the user identity) and analytics:events:read (example scope for subsequent API calls).
SDK Version: No specific SDK is required for the auth flow itself as it uses standard HTTP endpoints. Subsequent API calls may use genesys-cloud-purecloud-platform-client-v2 (JS).
Runtime: Any modern web browser (Chrome, Firefox, Safari, Edge) supporting ES6+ and the Fetch API.
Dependencies: None. This implementation relies on native browser APIs.

Authentication Setup

The Authorization Code flow with PKCE is the industry standard for Single-Page Applications (SPAs). Unlike the Implicit flow (which is deprecated), this flow exchanges a short-lived authorization code for an access token. The PKCE mechanism ensures that even if the authorization code is intercepted during the redirect, it cannot be used by an attacker without the corresponding verifier.

Step 1: Generate PKCE Parameters

Before redirecting the user to Genesys Cloud, you must generate a cryptographic challenge. This consists of a code_verifier (kept secret in the browser memory) and a code_challenge (sent to Genesys Cloud).

The code_challenge is derived from the code_verifier using SHA-256 hashing and Base64 URL encoding.

// authUtils.js

/**
 * Generates a cryptographically random string of bytes.
 * @param {number} length - The length of the string.
 * @returns {string} A random ASCII string.
 */
async function generateRandomString(length = 64) {
    const array = new Uint8Array(length);
    window.crypto.getRandomValues(array);
    return Array.from(array, byte => byte.toString(16).padStart(2, '0')).join('');
}

/**
 * Encodes a string using Base64 URL encoding.
 * @param {string} input - The string to encode.
 * @returns {string} The Base64 URL encoded string.
 */
function base64URLEncode(str) {
    return btoa(str)
        .replace(/\+/g, '-')
        .replace(/\//g, '_')
        .replace(/=/g, '');
}

/**
 * Creates the PKCE code challenge from a code verifier.
 * @param {string} verifier - The code verifier.
 * @returns {Promise<string>} The SHA-256 hashed and Base64URL encoded challenge.
 */
async function createCodeChallenge(verifier) {
    const encoder = new TextEncoder();
    const data = encoder.encode(verifier);
    const hashBuffer = await window.crypto.subtle.digest('SHA-256', data);
    const hashArray = Array.from(new Uint8Array(hashBuffer));
    const hashBase64 = hashArray.map(byte => String.fromCharCode(byte)).join('');
    return base64URLEncode(hashBase64);
}

/**
 * Generates both the code_verifier and code_challenge.
 * @returns {Promise<{ codeVerifier: string, codeChallenge: string }>}
 */
export async function generatePKCEParams() {
    const codeVerifier = await generateRandomString(128);
    const codeChallenge = await createCodeChallenge(codeVerifier);
    return { codeVerifier, codeChallenge };
}

Why this matters: If you skip PKCE, an attacker intercepting the redirect URL could steal the authorization code. With PKCE, the attacker possesses the code but lacks the code_verifier generated in the user’s browser, making the code useless.

Step 2: Initiate the Authorization Request

Construct the Genesys Cloud authorization URL. You must store the codeVerifier in a secure location (such as sessionStorage or a memory variable) because you will need it later to exchange the code for tokens.

// authFlow.js

import { generatePKCEParams } from './authUtils.js';

const GENESYS_CLOUD_URL = 'https://api.mypurecloud.com'; // Change to your environment
const CLIENT_ID = 'YOUR_CLIENT_ID_HERE';
const REDIRECT_URI = 'http://localhost:3000/callback'; // Must match exactly in Genesys Admin
const SCOPES = 'agent:login analytics:events:read';

export async function initiateLogin() {
    const { codeVerifier, codeChallenge } = await generatePKCEParams();

    // Store the verifier securely. In a real app, use sessionStorage or a secure in-memory store.
    // Do NOT use localStorage for sensitive auth data if possible, due to XSS risks.
    sessionStorage.setItem('pkce_code_verifier', codeVerifier);

    const params = new URLSearchParams({
        response_type: 'code',
        client_id: CLIENT_ID,
        redirect_uri: REDIRECT_URI,
        scope: SCOPES,
        state: generateRandomString(16), // Anti-CSRF token
        code_challenge: codeChallenge,
        code_challenge_method: 'S256'
    });

    const authUrl = `${GENESYS_CLOUD_URL}/oauth/authorize?${params.toString()}`;

    // Redirect the user to Genesys Cloud
    window.location.href = authUrl;
}

Critical Parameter Details:

response_type: Must be code. Never use token (Implicit flow).
code_challenge_method: Must be S256. This tells Genesys Cloud that the challenge was hashed using SHA-256.
state: A random string to prevent Cross-Site Request Forgery (CSRF). You must validate this state parameter upon return.

Step 3: Handle the Callback and Exchange Code for Tokens

When Genesys Cloud redirects the user back to your REDIRECT_URI, the URL will contain a code parameter and the state. You must now send this code to the /oauth/token endpoint to get the actual access token.

// tokenExchange.js

const GENESYS_CLOUD_URL = 'https://api.mypurecloud.com';
const CLIENT_ID = 'YOUR_CLIENT_ID_HERE';
const REDIRECT_URI = 'http://localhost:3000/callback';

/**
 * Exchanges the authorization code for an access token.
 * @param {string} code - The authorization code from the URL.
 * @param {string} state - The state parameter for CSRF validation.
 * @returns {Promise<Object>} The token response containing access_token, refresh_token, etc.
 */
export async function exchangeCodeForToken(code, state) {
    // 1. Retrieve the stored code verifier
    const codeVerifier = sessionStorage.getItem('pkce_code_verifier');
    
    if (!codeVerifier) {
        throw new Error('PKCE code verifier not found. Authentication session expired or tampered.');
    }

    // 2. Clear the verifier from storage as it is no longer needed
    sessionStorage.removeItem('pkce_code_verifier');

    const body = new URLSearchParams({
        grant_type: 'authorization_code',
        client_id: CLIENT_ID,
        redirect_uri: REDIRECT_URI,
        code: code,
        code_verifier: codeVerifier
    });

    try {
        const response = await fetch(`${GENESYS_CLOUD_URL}/oauth/token`, {
            method: 'POST',
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded'
            },
            body: body
        });

        if (!response.ok) {
            const errorData = await response.json();
            throw new Error(`OAuth Token Exchange Failed: ${response.status} - ${errorData.error_description || errorData.error}`);
        }

        const tokenData = await response.json();
        
        // Store tokens securely (preferably in memory or httpOnly cookies if backend proxy is used)
        // For this SPA example, we use sessionStorage, but be aware of XSS risks.
        sessionStorage.setItem('genesys_access_token', tokenData.access_token);
        sessionStorage.setItem('genesys_refresh_token', tokenData.refresh_token);
        sessionStorage.setItem('genesys_token_expiry', Date.now() + (tokenData.expires_in * 1000));

        return tokenData;

    } catch (error) {
        console.error('Token exchange error:', error);
        throw error;
    }
}

/**
 * Handles the callback URL parsing.
 */
export function handleCallback() {
    const urlParams = new URLSearchParams(window.location.search);
    const code = urlParams.get('code');
    const state = urlParams.get('state');
    const error = urlParams.get('error');

    if (error) {
        console.error('Authorization Error:', urlParams.get('error_description'));
        return;
    }

    if (!code || !state) {
        console.error('Invalid callback: missing code or state.');
        return;
    }

    // Validate state if you stored it in Step 2
    // if (state !== sessionStorage.getItem('pkce_state')) { throw new Error('State mismatch'); }

    exchangeCodeForToken(code, state)
        .then(() => {
            // Redirect to the main application dashboard
            window.location.href = '/dashboard';
        })
        .catch(err => {
            console.error('Login failed:', err);
            alert('Authentication failed. Please try again.');
        });
}

Expected Response Body:

{
  "access_token": "eyJraWQiOiIxNjQ5MjM...",
  "token_type": "Bearer",
  "expires_in": 3600,
  "refresh_token": "dGhpcyBpcyBhIHJlZnJlc2ggdG9rZW4...",
  "scope": "agent:login analytics:events:read"
}

Step 4: Using the Access Token for API Calls

Once you have the access token, you can make authenticated requests to Genesys Cloud APIs. The token should be included in the Authorization header as a Bearer token.

// apiClient.js

const GENESYS_CLOUD_URL = 'https://api.mypurecloud.com';

/**
 * Makes an authenticated GET request to a Genesys Cloud API.
 * @param {string} endpoint - The API endpoint (e.g., '/api/v2/users/me').
 * @returns {Promise<Object>} The JSON response.
 */
export async function getAuthenticatedData(endpoint) {
    const accessToken = sessionStorage.getItem('genesys_access_token');
    
    if (!accessToken) {
        throw new Error('No access token found. Please log in.');
    }

    // Check if token is expired
    const expiry = parseInt(sessionStorage.getItem('genesys_token_expiry') || '0');
    if (Date.now() > expiry) {
        // In a real app, trigger a refresh token flow here
        throw new Error('Access token expired. Refresh required.');
    }

    try {
        const response = await fetch(`${GENESYS_CLOUD_URL}${endpoint}`, {
            method: 'GET',
            headers: {
                'Authorization': `Bearer ${accessToken}`,
                'Content-Type': 'application/json'
            }
        });

        if (!response.ok) {
            if (response.status === 401) {
                // Token might be invalid or expired
                sessionStorage.removeItem('genesys_access_token');
                sessionStorage.removeItem('genesys_refresh_token');
                throw new Error('Unauthorized: Token invalid or expired.');
            }
            const errorData = await response.json();
            throw new Error(`API Error: ${response.status} - ${JSON.stringify(errorData)}`);
        }

        return await response.json();

    } catch (error) {
        console.error('API Request Failed:', error);
        throw error;
    }
}

// Example Usage: Get current user profile
export async function getCurrentUser() {
    return await getAuthenticatedData('/api/v2/users/me');
}

OAuth Scope Requirement: The endpoint /api/v2/users/me requires the agent:login scope. If you attempt to call an endpoint requiring a different scope (e.g., analytics:events:read) without including it in the initial authorization request, Genesys Cloud will return a 403 Forbidden error.

Step 5: Refreshing Tokens

Access tokens expire (default 1 hour). You must use the refresh_token to obtain a new access token without requiring the user to log in again.

// refreshToken.js

const GENESYS_CLOUD_URL = 'https://api.mypurecloud.com';
const CLIENT_ID = 'YOUR_CLIENT_ID_HERE';

/**
 * Refreshes the access token using the refresh token.
 * @returns {Promise<Object>} New token data.
 */
export async function refreshAccessToken() {
    const refreshToken = sessionStorage.getItem('genesys_refresh_token');
    
    if (!refreshToken) {
        throw new Error('No refresh token available.');
    }

    const body = new URLSearchParams({
        grant_type: 'refresh_token',
        client_id: CLIENT_ID,
        refresh_token: refreshToken
    });

    try {
        const response = await fetch(`${GENESYS_CLOUD_URL}/oauth/token`, {
            method: 'POST',
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded'
            },
            body: body
        });

        if (!response.ok) {
            const errorData = await response.json();
            throw new Error(`Token Refresh Failed: ${response.status} - ${errorData.error_description}`);
        }

        const tokenData = await response.json();
        
        // Update stored tokens
        sessionStorage.setItem('genesys_access_token', tokenData.access_token);
        if (tokenData.refresh_token) {
            sessionStorage.setItem('genesys_refresh_token', tokenData.refresh_token);
        }
        sessionStorage.setItem('genesys_token_expiry', Date.now() + (tokenData.expires_in * 1000));

        return tokenData;

    } catch (error) {
        console.error('Refresh error:', error);
        // If refresh fails, the user session is likely dead. Clear tokens and redirect to login.
        sessionStorage.removeItem('genesys_access_token');
        sessionStorage.removeItem('genesys_refresh_token');
        throw error;
    }
}

Complete Working Example

Below is a single-file HTML implementation combining all steps for testing purposes. Save this as index.html and open it in a browser. Ensure you replace YOUR_CLIENT_ID_HERE and REDIRECT_URI with your actual Genesys Cloud OAuth application settings.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Genesys Cloud PKCE Auth Demo</title>
    <style>
        body { font-family: sans-serif; padding: 20px; max-width: 800px; margin: 0 auto; }
        .btn { padding: 10px 20px; background: #007bff; color: white; border: none; cursor: pointer; border-radius: 4px; }
        .btn:hover { background: #0056b3; }
        .hidden { display: none; }
        pre { background: #f4f4f4; padding: 10px; overflow-x: auto; }
        .error { color: red; }
        .success { color: green; }
    </style>
</head>
<body>

    <h1>Genesys Cloud PKCE Authentication</h1>

    <div id="login-screen">
        <p>Click below to log in to Genesys Cloud.</p>
        <button id="login-btn" class="btn">Login with Genesys</button>
    </div>

    <div id="dashboard-screen" class="hidden">
        <h2>User Profile</h2>
        <pre id="user-profile">Loading...</pre>
        <button id="logout-btn" class="btn" style="background: #dc3545;">Logout</button>
    </div>

    <div id="error-screen" class="hidden">
        <h2 class="error">Authentication Error</h2>
        <p id="error-message"></p>
        <button id="retry-btn" class="btn">Retry</button>
    </div>

    <script type="module">
        const GENESYS_CLOUD_URL = 'https://api.mypurecloud.com';
        const CLIENT_ID = 'YOUR_CLIENT_ID_HERE'; // REPLACE THIS
        const REDIRECT_URI = window.location.origin + window.location.pathname; // Current page as callback

        // --- PKCE Utilities ---
        async function generateRandomString(length = 64) {
            const array = new Uint8Array(length);
            window.crypto.getRandomValues(array);
            return Array.from(array, byte => byte.toString(16).padStart(2, '0')).join('');
        }

        function base64URLEncode(str) {
            return btoa(str).replace(/\+/g, '-').replace(/\//g, '_').replace(/=/g, '');
        }

        async function createCodeChallenge(verifier) {
            const encoder = new TextEncoder();
            const data = encoder.encode(verifier);
            const hashBuffer = await window.crypto.subtle.digest('SHA-256', data);
            const hashArray = Array.from(new Uint8Array(hashBuffer));
            const hashBase64 = hashArray.map(byte => String.fromCharCode(byte)).join('');
            return base64URLEncode(hashBase64);
        }

        async function generatePKCEParams() {
            const codeVerifier = await generateRandomString(128);
            const codeChallenge = await createCodeChallenge(codeVerifier);
            return { codeVerifier, codeChallenge };
        }

        // --- Auth Flow ---
        async function initiateLogin() {
            const { codeVerifier, codeChallenge } = await generatePKCEParams();
            sessionStorage.setItem('pkce_code_verifier', codeVerifier);
            
            const params = new URLSearchParams({
                response_type: 'code',
                client_id: CLIENT_ID,
                redirect_uri: REDIRECT_URI,
                scope: 'agent:login',
                state: await generateRandomString(16),
                code_challenge: codeChallenge,
                code_challenge_method: 'S256'
            });

            window.location.href = `${GENESYS_CLOUD_URL}/oauth/authorize?${params.toString()}`;
        }

        async function exchangeCodeForToken(code) {
            const codeVerifier = sessionStorage.getItem('pkce_code_verifier');
            if (!codeVerifier) throw new Error('PKCE verifier missing');
            sessionStorage.removeItem('pkce_code_verifier');

            const body = new URLSearchParams({
                grant_type: 'authorization_code',
                client_id: CLIENT_ID,
                redirect_uri: REDIRECT_URI,
                code: code,
                code_verifier: codeVerifier
            });

            const response = await fetch(`${GENESYS_CLOUD_URL}/oauth/token`, {
                method: 'POST',
                headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
                body: body
            });

            if (!response.ok) {
                const err = await response.json();
                throw new Error(err.error_description || 'Token exchange failed');
            }

            const data = await response.json();
            sessionStorage.setItem('genesys_access_token', data.access_token);
            sessionStorage.setItem('genesys_refresh_token', data.refresh_token);
            sessionStorage.setItem('genesys_token_expiry', Date.now() + (data.expires_in * 1000));
            return data;
        }

        async function fetchUserProfile() {
            const token = sessionStorage.getItem('genesys_access_token');
            if (!token) return null;

            const response = await fetch(`${GENESYS_CLOUD_URL}/api/v2/users/me`, {
                headers: { 'Authorization': `Bearer ${token}` }
            });

            if (response.status === 401) {
                sessionStorage.clear();
                return null;
            }

            if (!response.ok) throw new Error('Failed to fetch user profile');
            return await response.json();
        }

        // --- UI Logic ---
        const loginScreen = document.getElementById('login-screen');
        const dashboardScreen = document.getElementById('dashboard-screen');
        const errorScreen = document.getElementById('error-screen');
        const loginBtn = document.getElementById('login-btn');
        const logoutBtn = document.getElementById('logout-btn');
        const retryBtn = document.getElementById('retry-btn');
        const userProfilePre = document.getElementById('user-profile');
        const errorMessage = document.getElementById('error-message');

        function showScreen(screen) {
            loginScreen.classList.add('hidden');
            dashboardScreen.classList.add('hidden');
            errorScreen.classList.add('hidden');
            screen.classList.remove('hidden');
        }

        async function checkAuthState() {
            const urlParams = new URLSearchParams(window.location.search);
            const code = urlParams.get('code');
            const error = urlParams.get('error');

            if (error) {
                errorMessage.textContent = urlParams.get('error_description');
                showScreen(errorScreen);
                return;
            }

            if (code) {
                try {
                    await exchangeCodeForToken(code);
                    // Clean URL
                    window.history.replaceState({}, document.title, window.location.pathname);
                    await loadDashboard();
                } catch (err) {
                    errorMessage.textContent = err.message;
                    showScreen(errorScreen);
                }
                return;
            }

            // Check for existing session
            const token = sessionStorage.getItem('genesys_access_token');
            const expiry = parseInt(sessionStorage.getItem('genesys_token_expiry') || '0');

            if (token && Date.now() < expiry) {
                await loadDashboard();
            } else {
                showScreen(loginScreen);
            }
        }

        async function loadDashboard() {
            try {
                const user = await fetchUserProfile();
                if (!user) {
                    showScreen(loginScreen);
                    return;
                }
                userProfilePre.textContent = JSON.stringify(user, null, 2);
                showScreen(dashboardScreen);
            } catch (err) {
                errorMessage.textContent = err.message;
                showScreen(errorScreen);
            }
        }

        loginBtn.addEventListener('click', initiateLogin);
        logoutBtn.addEventListener('click', () => {
            sessionStorage.clear();
            showScreen(loginScreen);
        });
        retryBtn.addEventListener('click', () => {
            window.location.href = window.location.pathname;
        });

        // Initialize
        checkAuthState();

    </script>
</body>
</html>

Common Errors & Debugging

Error: `invalid_grant`

What causes it: The authorization code has already been used, expired, or the code_verifier does not match the code_challenge sent during the initial request.
How to fix it: Ensure you are not reusing the authorization code. Verify that the code_verifier stored in sessionStorage exactly matches the one used to generate the code_challenge. Check that the redirect_uri in the token exchange matches the one used in the authorization request exactly (including trailing slashes).

Error: `invalid_client`

What causes it: The client_id provided is invalid, not found, or the client is not configured to allow the redirect_uri specified.
How to fix it: Go to Genesys Cloud Admin > Platform > OAuth 2.0. Verify the Client ID. Ensure the redirect_uri is explicitly listed in the “Allowed Redirect URIs” list for that client.

Error: `access_denied`

What causes it: The user denied the authorization request, or the scope requested is not permitted for the client or the user’s role.
How to fix it: Check the browser console for any JavaScript errors during the redirect. Verify that the OAuth client has the necessary permissions for the requested scopes.

Error: `429 Too Many Requests`

What causes it: You have exceeded the rate limit for the /oauth/token endpoint.
How to fix it: Implement exponential backoff logic. Do not retry immediately. Wait 1 second, then 2, then 4, etc., before retrying the token exchange.

Implementing Authorization Code Flow with PKCE for SPAs in Genesys Cloud

Implementing Authorization Code Flow with PKCE for SPAs in Genesys Cloud

What You Will Build

Prerequisites

Authentication Setup

Step 1: Generate PKCE Parameters

Step 2: Initiate the Authorization Request

Step 3: Handle the Callback and Exchange Code for Tokens

Step 4: Using the Access Token for API Calls

Step 5: Refreshing Tokens

Complete Working Example

Common Errors & Debugging

Error: invalid_grant

Error: invalid_client

Error: access_denied

Error: 429 Too Many Requests

Official References

Error: `invalid_grant`

Error: `invalid_client`

Error: `access_denied`

Error: `429 Too Many Requests`