Recover OAuth login when callback lands in a different browser

When the OIDC callback lands in a browser that didn't start the login
flow (e.g. "Open in Safari" from an in-app browser), both the CSRF
state validation and the PKCE code verifier cookie check fail, and the
user dead-ends on a 403. Restart the login flow once in the new browser
instead, guarded by a short-lived one-shot cookie to prevent redirect
loops, and best-effort preserve the original return path (re-sanitized)
via the login route's ?next= param.

Re-applies gary149/chat-ui#18 which had targeted the wrong repository.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QWamsbC2xoGRoQazysMEVp

Author: claude

src/lib/server/auth.ts

@@ -67,7 +67,7 @@ export const sameSite = z
 	.default(!secure || dev || config.ALLOW_INSECURE_COOKIES === "true" ? "lax" : "none")
 	.parse(config.COOKIE_SAMESITE === "" ? undefined : config.COOKIE_SAMESITE);
 
-function sanitizeReturnPath(path: string | undefined | null): string | undefined {
+export function sanitizeReturnPath(path: string | undefined | null): string | undefined {
 	if (!path) {
 		return undefined;
 	}
@@ -80,6 +80,32 @@ function sanitizeReturnPath(path: string | undefined | null): string | undefined
 	return path;
 }
 
+/**
+ * One-shot guard used when restarting the OAuth flow after a callback that was started
+ * in another browser (e.g. "Open in Safari" from an in-app browser). Prevents redirect loops.
+ */
+const loginRetryCookieName = "hfChat-loginRetry";
+
+export function hasLoginRetryCookie(cookies: Cookies): boolean {
+	return cookies.get(loginRetryCookieName) === "1";
+}
+
+export function setLoginRetryCookie(cookies: Cookies) {
+	cookies.set(loginRetryCookieName, "1", {
+		path: "/",
+		// `strict` would keep this cookie from being sent on the cross-site IdP -> callback
+		// navigation, which is exactly where the loop guard must be observable
+		sameSite: sameSite === "strict" ? "lax" : sameSite,
+		secure,
+		httpOnly: true,
+		maxAge: 5 * 60,
+	});
+}
+
+export function clearLoginRetryCookie(cookies: Cookies) {
+	cookies.delete(loginRetryCookieName, { path: "/" });
+}
+
 export function refreshSessionCookie(cookies: Cookies, sessionId: string) {
 	cookies.set(config.COOKIE_NAME, sessionId, {
 		path: "/",

src/routes/login/callback/+server.ts

@@ -1,5 +1,12 @@
 import { error, redirect } from "@sveltejs/kit";
-import { getOIDCUserData, validateAndParseCsrfToken } from "$lib/server/auth";
+import {
+	clearLoginRetryCookie,
+	getOIDCUserData,
+	hasLoginRetryCookie,
+	sanitizeReturnPath,
+	setLoginRetryCookie,
+	validateAndParseCsrfToken,
+} from "$lib/server/auth";
 import { z } from "zod";
 import { base } from "$app/paths";
 import { config } from "$lib/server/config";
@@ -47,14 +54,32 @@ export async function GET({ url, locals, cookies, request, getClientAddress }) {
 	const csrfToken = Buffer.from(state, "base64").toString("utf-8");
 
 	const validatedToken = await validateAndParseCsrfToken(csrfToken, locals.sessionId);
+	const codeVerifier = cookies.get("hfChat-codeVerifier");
 
-	if (!validatedToken) {
-		throw error(403, "Invalid or expired CSRF token");
-	}
+	if (!validatedToken || !codeVerifier) {
+		// The `state` token and PKCE code verifier are bound to the browser that started the
+		// login flow, so either check can fail on its own; when the callback lands in a
+		// different browser (e.g. "Open in Safari" from an in-app browser) both do. Restart
+		// the flow once in this browser instead of dead-ending on a 403.
+		if (!hasLoginRetryCookie(cookies)) {
+			setLoginRetryCookie(cookies);
+
+			// Best-effort recovery of the return path from the (unverified) state payload;
+			// it is re-sanitized to an in-app absolute path before use.
+			let next: string | undefined;
+			try {
+				next = sanitizeReturnPath(JSON.parse(csrfToken)?.data?.next);
+			} catch {
+				next = undefined;
+			}
+
+			return redirect(302, `${base}/login${next ? `?next=${encodeURIComponent(next)}` : ""}`);
+		}
 
-	const codeVerifier = cookies.get("hfChat-codeVerifier");
-	if (!codeVerifier) {
-		throw error(403, "Code verifier cookie not found");
+		throw error(
+			403,
+			validatedToken ? "Code verifier cookie not found" : "Invalid or expired CSRF token"
+		);
 	}
 
 	const { userData, token } = await getOIDCUserData(
@@ -93,6 +118,8 @@ export async function GET({ url, locals, cookies, request, getClientAddress }) {
 		ip: getClientAddress(),
 	});
 
+	clearLoginRetryCookie(cookies);
+
 	// Prefer returning the user to their original in-app path when provided.
 	// `validatedToken.next` is sanitized server-side to avoid protocol-relative redirects.
 	const next = validatedToken.next;