Support PKCE for GitHub Auth (#265381)

Fixes https://github.com/microsoft/vscode/issues/264795

extensions/github-authentication/src/flows.ts

@@ -9,6 +9,7 @@ import { Log } from './common/logger';
 import { Config } from './config';
 import { UriEventHandler } from './github';
 import { fetching } from './node/fetch';
+import { crypto } from './node/crypto';
 import { LoopbackAuthServer } from './node/authServer';
 import { promiseFromEvent } from './common/utils';
 import { isHostedGitHubEnterprise } from './common/env';
@@ -112,11 +113,44 @@ interface IFlow {
 	trigger(options: IFlowTriggerOptions): Promise<string>;
 }
 
+/**
+ * Generates a cryptographically secure random string for PKCE code verifier.
+ * @param length The length of the string to generate
+ * @returns A random hex string
+ */
+function generateRandomString(length: number): string {
+	const array = new Uint8Array(length);
+	crypto.getRandomValues(array);
+	return Array.from(array)
+		.map(b => b.toString(16).padStart(2, '0'))
+		.join('')
+		.substring(0, length);
+}
+
+/**
+ * Generates a PKCE code challenge from a code verifier using SHA-256.
+ * @param codeVerifier The code verifier string
+ * @returns A base64url-encoded SHA-256 hash of the code verifier
+ */
+async function generateCodeChallenge(codeVerifier: string): Promise<string> {
+	const encoder = new TextEncoder();
+	const data = encoder.encode(codeVerifier);
+	const digest = await crypto.subtle.digest('SHA-256', data);
+
+	// Base64url encode the digest
+	const base64String = btoa(String.fromCharCode(...new Uint8Array(digest)));
+	return base64String
+		.replace(/\+/g, '-')
+		.replace(/\//g, '_')
+		.replace(/=+$/, '');
+}
+
 async function exchangeCodeForToken(
 	logger: Log,
 	endpointUri: Uri,
 	redirectUri: Uri,
 	code: string,
+	codeVerifier: string,
 	enterpriseUri?: Uri
 ): Promise<string> {
 	logger.info('Exchanging code for token...');
@@ -130,7 +164,8 @@ async function exchangeCodeForToken(
 		['code', code],
 		['client_id', Config.gitHubClientId],
 		['redirect_uri', redirectUri.toString(true)],
-		['client_secret', clientSecret]
+		['client_secret', clientSecret],
+		['code_verifier', codeVerifier]
 	]);
 	if (enterpriseUri) {
 		body.append('github_enterprise', enterpriseUri.toString(true));
@@ -199,13 +234,19 @@ class UrlHandlerFlow implements IFlow {
 			}),
 			cancellable: true
 		}, async (_, token) => {
+			// Generate PKCE parameters
+			const codeVerifier = generateRandomString(64);
+			const codeChallenge = await generateCodeChallenge(codeVerifier);
+
 			const promise = uriHandler.waitForCode(logger, scopes, nonce, token);
 
 			const searchParams = new URLSearchParams([
 				['client_id', Config.gitHubClientId],
 				['redirect_uri', redirectUri.toString(true)],
 				['scope', scopes],
-				['state', encodeURIComponent(callbackUri.toString(true))]
+				['state', encodeURIComponent(callbackUri.toString(true))],
+				['code_challenge', codeChallenge],
+				['code_challenge_method', 'S256']
 			]);
 			if (existingLogin) {
 				searchParams.append('login', existingLogin);
@@ -236,7 +277,7 @@ class UrlHandlerFlow implements IFlow {
 				? Uri.parse(`${proxyEndpoints.github}login/oauth/access_token`)
 				: baseUri.with({ path: '/login/oauth/access_token' });
 
-			const accessToken = await exchangeCodeForToken(logger, endpointUrl, redirectUri, code, enterpriseUri);
+			const accessToken = await exchangeCodeForToken(logger, endpointUrl, redirectUri, code, codeVerifier, enterpriseUri);
 			return accessToken;
 		});
 	}
@@ -283,10 +324,16 @@ class LocalServerFlow implements IFlow {
 			}),
 			cancellable: true
 		}, async (_, token) => {
+			// Generate PKCE parameters
+			const codeVerifier = generateRandomString(64);
+			const codeChallenge = await generateCodeChallenge(codeVerifier);
+
 			const searchParams = new URLSearchParams([
 				['client_id', Config.gitHubClientId],
 				['redirect_uri', redirectUri.toString(true)],
 				['scope', scopes],
+				['code_challenge', codeChallenge],
+				['code_challenge_method', 'S256']
 			]);
 			if (existingLogin) {
 				searchParams.append('login', existingLogin);
@@ -329,6 +376,7 @@ class LocalServerFlow implements IFlow {
 				baseUri.with({ path: '/login/oauth/access_token' }),
 				redirectUri,
 				codeToExchange,
+				codeVerifier,
 				enterpriseUri);
 			return accessToken;
 		});