Supabase Edge Functions: Disable Legacy JWT Verification By Default

Hey everyone! Are you running into issues with your Supabase edge functions breaking after updates? You're not alone. It seems like the default behavior of Supabase is to enable Verify JWT with legacy secret when new functions are created or existing ones are edited. While this might seem helpful initially, it's actually a deprecated method and can cause headaches, especially for functions accessed from outside Supabase.

The recommendation from Supabase is to keep this setting OFF and instead implement JWT verification and authorization logic directly within your function's code. This approach is more secure and aligns with modern best practices. However, the current default behavior is causing functions to break, requiring manual intervention to disable the legacy secret verification. Let's dive into why this is happening, the implications, and what can be done about it.

Understanding JWT and Legacy Secrets

Before we delve deeper, let's quickly recap what JWTs are and how legacy secrets come into play.

What is JWT?

JSON Web Tokens (JWTs) are a standard for securely transmitting information between parties as a JSON object. Think of them as digital identity cards. They are commonly used for authentication and authorization in web applications. A JWT typically consists of three parts:

1. Header: Specifies the type of token and the hashing algorithm used.
2. Payload: Contains the claims (statements) about the user or entity. This might include the user's ID, roles, permissions, and other relevant information.
3. Signature: Ensures the token's integrity and verifies that it hasn't been tampered with. The signature is calculated using the header, payload, and a secret key.

When a user logs in, the server generates a JWT and sends it to the client. The client then includes this JWT in subsequent requests, allowing the server to verify the user's identity and grant access to protected resources.

Legacy Secrets and Why They're Deprecated

The legacy secret method involves using a single, shared secret key to sign JWTs. While simple to implement, this approach has significant security drawbacks. If the secret key is compromised, all JWTs signed with that key become invalid, potentially granting unauthorized access. This is a critical vulnerability that can lead to severe security breaches.

Supabase recommends against using legacy secrets for several reasons:

• Security Risks: A single compromised secret key jeopardizes the entire system.
• Lack of Granularity: Legacy secrets don't allow for fine-grained control over access. It's an all-or-nothing approach.
• Scalability Issues: Managing a single secret key across a large system becomes challenging as the application grows.

Instead, Supabase advocates for a more robust approach where JWT verification and authorization logic are handled within the edge functions themselves. This provides greater flexibility, security, and scalability.

The Problem: Default Legacy Secret Verification

The core issue is that Supabase's default setting for new and updated edge functions is to enable Verify JWT with legacy secret. This means that even if you intend to implement a more secure JWT verification method within your function, the legacy secret verification will be active, potentially causing conflicts and unexpected behavior. This is especially problematic for edge functions that are accessed from outside the Supabase ecosystem, as they may not be designed to handle legacy secret verification.

Why This Breaks Edge Functions

When Verify JWT with legacy secret is enabled, Supabase attempts to validate the JWT using the project's shared secret. If the JWT is not signed with this secret (which is often the case when using custom authentication flows or external identity providers), the verification will fail, and the function will likely return an error. This can manifest in various ways:

• 401 Unauthorized Errors: The most common symptom is the function returning a 401 status code, indicating that the request is not authorized.
• Function Errors: The function might throw an error internally due to the failed JWT verification, leading to unexpected behavior.
• Inconsistent Behavior: The function might work intermittently, depending on whether the JWT happens to be signed with the legacy secret (e.g., during development or testing).

The Manual Workaround: Disabling Legacy Verification

Currently, the workaround is to manually disable Verify JWT with legacy secret for each new or updated edge function. This involves navigating to the function's settings in the Supabase dashboard and toggling the setting off. While this solves the immediate problem, it's a tedious and error-prone process, especially when dealing with a large number of functions. It also requires developers to remember this step every time they create or update a function, increasing the risk of overlooking it and causing issues in production.

The Solution: Requesting a System Prompt Update

The ideal solution is for Supabase to update the system prompt so that Verify JWT with legacy secret is disabled by default. This would prevent the issue from occurring in the first place and eliminate the need for manual intervention. By default disabling the legacy verification, developers will be encouraged to implement more secure and flexible JWT handling within their functions.

Benefits of Disabling Legacy Verification by Default

• Improved Security: Encourages the use of more secure JWT verification methods.
• Reduced Errors: Prevents unexpected errors caused by legacy secret verification conflicts.
• Simplified Development: Streamlines the development process by eliminating the need for manual configuration.
• Better Alignment with Best Practices: Aligns with Supabase's recommendation to handle JWT verification within functions.

How to Request a System Prompt Update

The most effective way to request this change is to communicate directly with the Supabase team. This can be done through their official channels, such as their GitHub repository, Discord server, or support channels. When making the request, it's important to clearly articulate the problem, explain the impact on developers, and highlight the benefits of disabling legacy verification by default.

Implementing JWT Verification in Your Edge Functions

Now that we've discussed the issue with legacy secret verification, let's explore how to implement JWT verification directly within your edge functions. This approach provides greater flexibility, security, and control over your authentication and authorization mechanisms.

Steps for Implementing JWT Verification

1. Obtain the JWT: The first step is to retrieve the JWT from the request. Typically, the JWT is included in the Authorization header as a Bearer token (e.g., Authorization: Bearer <JWT>).
2. Verify the JWT Signature: Use a JWT library (such as jsonwebtoken in Node.js or similar libraries in other languages) to verify the JWT's signature. This ensures that the token hasn't been tampered with and that it was issued by a trusted authority.
3. Validate the JWT Claims: After verifying the signature, validate the claims within the JWT. This includes checking the expiration time (exp), the issuer (iss), the audience (aud), and any other custom claims relevant to your application.
4. Authorize the Request: Based on the validated claims, determine whether the user is authorized to access the requested resource. This might involve checking the user's roles, permissions, or other attributes.

Example Code (Node.js with jsonwebtoken)

Here's a basic example of how to verify a JWT in a Node.js edge function using the jsonwebtoken library:

import { serve } from 'std/server';
import { verify } from 'jsonwebtoken';

const JWT_SECRET = Deno.env.get('JWT_SECRET');

serve(async (req) => {
  const authHeader = req.headers.get('authorization');
  if (!authHeader) {
    return new Response('Unauthorized', { status: 401 });
  }

  const token = authHeader.split(' ')[1];
  if (!token) {
    return new Response('Unauthorized', { status: 401 });
  }

  try {
    const decoded = verify(token, JWT_SECRET);
    // Access decoded JWT payload (e.g., decoded.userId, decoded.roles)
    console.log('Decoded JWT:', decoded);

    // Perform authorization logic based on decoded claims
    if (/* Authorization check */) {
      // ...
      return new Response('Authorized');
    } else {
      return new Response('Forbidden', { status: 403 });
    }
  } catch (error) {
    console.error('JWT Verification Error:', error);
    return new Response('Unauthorized', { status: 401 });
  }
});

This example demonstrates the core steps of JWT verification: extracting the token, verifying the signature using a secret key, and accessing the decoded payload for authorization. Remember to replace JWT_SECRET with your actual secret key and implement appropriate authorization logic based on your application's requirements.

Conclusion

The current default behavior of enabling Verify JWT with legacy secret in Supabase edge functions is causing unnecessary friction and potential security risks. By requesting a system prompt update to disable this setting by default, we can encourage the use of more secure JWT verification methods and streamline the development process. In the meantime, remember to manually disable legacy verification for new and updated functions and implement robust JWT handling within your function code. Let's work together to make Supabase edge functions more secure and developer-friendly!