Manual JWT Verification

Your Clerk-generated session tokens are essentially JWTs which are signed using your instance's private key and can be verified using your instance's public key. Depending on your architecture, these tokens will be in your backend requests either via a cookie named __session or via the Authorization header.

For every request, you must validate its token to make sure it hasn't expired and it is authentic (i.e. no malicious user tried to tamper it). If these validations pass, then it means that the user is authenticated to your application and you should consider them signed in.

Retrieve the session token

Retrieve the session token from either __session cookie for a same origin request or from the Authorization header for cross origin requests.

Get your instance's Public Key

There are three ways to obtain your public key:

Using the Backend API in JSON Web Key Set (JWKS) format at the following endpoint https://api.clerk.dev/v1/jwks.
Using the Frontend API in JSON Web Key Set (JWKS) format at the following endpoint https://<YOUR_FRONTEND_API>/.well-known/jwks.json. This can be obtained from the Dashboard API Keys > Advanced > JWT Public Key.
Using the public key provided by Clerk in the dashboard under API Keys > Advanced > JWT Public Key.

Verify the token signature

To verify the token signature, you should:

Use the above Public Key to verify the token's signature
Validate that the token is not expired, by checking the exp and nbf claims
If the azp claim exists, validate that equals any of your known Origins that are permitted to generate those tokens. This is an extra security check that we highly recommend that you do.

If the above process is successful, it means that the user is signed in to your application and you can consider them authenticated. You can also retrieve the session ID and user ID out of the token's claims.

Example usage

pages/api/token-example.ts
import type { NextApiRequest, NextApiResponse } from "next";
import Cookies from "cookies";
import jwt from "jsonwebtoken";
 
export default async function (req: NextApiRequest, res: NextApiResponse) {
  const publicKey = process.env.CLERK_PEM_PUBLIC_KEY;
  const cookies = new Cookies(req, res);
  const sessToken = cookies.get("__session");
  const token = req.headers.authorization;
  if (sessToken === undefined && token === undefined) {
    res.status(401).json({ error: "not signed in" });
    return;
  }
  try {
    let decoded = "";
    if (token) {
      decoded = jwt.verify(token, publicKey);
      res.status(200).json({ sessToken: decoded });
      return;
    } else {
      decoded = jwt.verify(sessToken, publicKey);
      res.status(200).json({ sessToken: decoded });
      return;
    }
  } catch (error) {
    res.status(400).json({
      error: "Invalid Token",
    });
    return;
  }
}

This will return the following JSON which you can then use to validate the azp and the expiration. You then have a valid session and user_id that you can use in your application.

{
  "sessToken": {
    "azp": "http://localhost:3000",
    "exp": 1687906422,
    "iat": 1687906362,
    "iss": "https://magical-marmoset-51.clerk.accounts.dev",
    "nbf": 1687906352,
    "sid": "sess_2Ro7e2IxrffdqBboq8KfB6eGbIy",
    "sub": "user_2RfWKJREkjKbHZy0Wqa5qrHeAnb"
  }
}