Sending a cookie as a response with Firebase Callable Functions - node.js

Question

I am trying to send a cookie with options set to it as a response using a Firebase callable cloud function (https.onCall). I see in the Firebase docs that this can be done with express:
(The below is taken directly form the Firebase docs)
app.post('/sessionLogin', (req, res) => {
// Get the ID token passed and the CSRF token.
const idToken = req.body.idToken.toString();
const csrfToken = req.body.csrfToken.toString();
// Guard against CSRF attacks.
if (csrfToken !== req.cookies.csrfToken) {
res.status(401).send('UNAUTHORIZED REQUEST!');
return;
}
// Set session expiration to 5 days.
const expiresIn = 60 * 60 * 24 * 5 * 1000;
// Create the session cookie. This will also verify the ID token in the process.
// The session cookie will have the same claims as the ID token.
// To only allow session cookie setting on recent sign-in, auth_time in ID token
// can be checked to ensure user was recently signed in before creating a session cookie.
getAuth()
.createSessionCookie(idToken, { expiresIn })
.then(
(sessionCookie) => {
// Set cookie policy for session cookie.
const options = { maxAge: expiresIn, httpOnly: true, secure: true };
res.cookie('session', sessionCookie, options);
res.end(JSON.stringify({ status: 'success' }));
},
(error) => {
res.status(401).send('UNAUTHORIZED REQUEST!');
}
);
});
I have implemented the callable function, but I do now know how to attach the options to my cookie string.
The below is my code:
// I want the return type to be a Promise of a cookie object, not a string
export const setCookie = https.onCall(async (context: https.CallableContext): Promise<string> => {
try {
console.log(context);
const auth: Auth = getAuth();
const idToken: DecodedIdToken = await auth.verifyIdToken(context.instanceIdToken!); // https://firebase.google.com/docs/auth/admin/verify-id-tokens#web
console.log("idToken: ", idToken);
const cookie: string = await auth.createSessionCookie(idToken.uid, { expiresIn: 300000 });
const options = {
maxAge: 300000,
httpOnly: true,
secure: true,
sameSite: "strict",
};
// res.cookie("session", cookie, options);
return cookie; // should be assigned to __session cookie with domain .web.app
// httpOnly=true, secure=true and sameSite=strict set.
} catch (error) {
console.log("ERROR FOUND: ", error);
throw new https.HttpsError("unknown", "Error found in setCookie");
}
});
Is there any way I can do this using a Callable Firebase Cloud Function? All the documentation and resources I have found require express to send an cookie with Node.
Thanks!

Answer 1

The documentation you're linking to assumes you are writing standard nodejs backend code using express. However, your code is using a callable type function. They are not the same and do not have the same capabilities. Callable functions don't let you set cookies in the response. You can only send a JSON payload back to the client; the SDK handles all of the HTTP headers and they are outside of your control.
Perhaps you should look into using a standard HTTP type function (onRequest), where you do have some control over the headers in the response.