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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 the token to ensure it hasn't expired or been tampered with (i.e., it's authentic and secure). If these validations succeed, then the user is authenticated to your application and should be considered signed in. The authenticateRequest() method from the JavaScript Backend SDK handles these validations for you. Alternatively, you can manually verify the token without using the SDK. See the following sections for more information.

Use authenticateRequest() to verify a session token

The authenticateRequest() method from the JavaScript Backend SDK accepts the request object and authenticates the session token in it.

The following example uses the authenticateRequest() method to verify the session token. It also performs networkless authentication by passing jwtKey. This verifies if the user is signed into the application. For more information, including usage with higher-level SDKs, see the authenticateRequest() reference.

import { createClerkClient } from '@clerk/backend'

export async function GET(req: Request) {
  const clerkClient = createClerkClient({
    secretKey: process.env.CLERK_SECRET_KEY,
    publishableKey: process.env.CLERK_PUBLISHABLE_KEY,
  })

  const { isSignedIn } = await clerkClient.authenticateRequest(req, {
    jwtKey: process.env.CLERK_JWT_KEY,
    authorizedParties: ['https://example.com'],
  })

  if (!isSignedIn) {
    return Response.json({ status: 401 })
  }

  // Add logic to perform protected actions

  return Response.json({ message: 'This is a reply' })
}

Manually verify a session token

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

Use one of the three ways to obtain your public key:

  1. Use the Backend API in JSON Web Key Set (JWKS) format at the following endpoint https://api.clerk.com/v1/jwks.
  2. Use your Frontend API URL in JWKS format, also known as JWKS URL. The format is https://<YOUR_FRONTEND_API>/.well-known/jwks.json. To retrieve your JWKS URL, navigate to the API keys page in the Clerk Dashboard and select Show JWT public key.
  3. Use your PEM Public Key. To retrieve it, navigate to the API keys page in the Clerk Dashboard and select Show JWT Public Key.

Verify the token signature

To verify the token signature:

  1. Use your instance's public key to verify the token's signature.
  2. Validate that the token isn't expired by checking the exp (expiration time) and nbf (not before) claims.
  3. Validate that the azp (authorized parties) claim equals any of your known origins permitted to generate those tokens. For better security, it's highly recommended to explicitly set the authorizedParties option when authorizing requests. The value should be a list of domains allowed to make requests to your application. Not setting this value can open your application to CSRF attacks. For example, if you're permitting tokens retrieved from http://localhost:3000, then the azp claim should equal http://localhost:3000. You can also pass an array of strings, such as ['http://localhost:4003', 'https://clerk.dev']. If the azp claim doesn't exist, you can skip this step.

Finished

If the above process succeeds, the user is considered signed in to your application and authenticated. You can also retrieve the session ID and user ID from of the token's claims.

Example

The following example manually verifies a session token.

import Cookies from 'cookies'
import jwt from 'jsonwebtoken'

export default async function (req: Request, res: Response) {
  // Your public key should be set as an environment variable
  const publicKey = process.env.CLERK_PEM_PUBLIC_KEY
  // Retrieve session token from either `__session` cookie for a same-origin request
  // or from the `Authorization` header for cross-origin requests
  const cookies = new Cookies(req, res)
  const tokenSameOrigin = cookies.get('__session')
  const tokenCrossOrigin = req.headers.authorization

  if (!tokenSameOrigin && !tokenCrossOrigin) {
    res.status(401).json({ error: 'Not signed in' })
    return
  }

  try {
    let decoded
    const options = { algorithms: ['RS256'] } // The algorithm used to sign the token. Optional.
    const permittedOrigins = ['http://localhost:3000', 'https://example.com'] // Replace with your permitted origins

    if (tokenSameOrigin) {
      decoded = jwt.verify(tokenSameOrigin, publicKey, options)
    } else {
      decoded = jwt.verify(tokenCrossOrigin, publicKey, options)
    }

    // Validate the token's expiration (exp) and not before (nbf) claims
    const currentTime = Math.floor(Date.now() / 1000)
    if (decoded.exp < currentTime || decoded.nbf > currentTime) {
      throw new Error('Token is expired or not yet valid')
    }

    // Validate the token's authorized party (azp) claim
    if (decoded.azp && !permittedOrigins.includes(decoded.azp)) {
      throw new Error("Invalid 'azp' claim")
    }

    res.status(200).json({ sessionToken: decoded })
  } catch (error) {
    res.status(400).json({
      error: error.message,
    })
  }
}

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