Why You Should Never Put Sensitive Data in Unencrypted JWTs

In the world of web development, JSON Web Tokens (JWTs) have become a popular choice for securely transmitting information between parties. Their simplicity and ease of use make them a go-to solution for implementing authentication and authorization mechanisms. However, with great power comes great responsibility, and one critical mistake that developers often make is storing sensitive data in unencrypted JWTs.

Let’s explore why this is a risky practice and what you can do to safeguard your applications.

Understanding JWTs: A Quick Overview

JWTs are compact, URL-safe tokens that consist of three parts: a header, a payload, and a signature. The header typically contains information about the token type and the signing algorithm. The payload contains the claims, which are pieces of information about the user or the token itself, such as user IDs, roles, and expiration times. Finally, the signature is used to verify the integrity of the token and ensure that it hasn’t been tampered with.

Here’s an example of a JWT:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VySWQiOjEsInJvbGUiOiJhZG1pbiIsImV4cCI6MTYwOTU4ODQ4M30.r3ijKxl1sFNiQoq6zA2bniSm-3E2LFrwQJ4O9Ko4R4c

When decoded, it reveals:

Header:

{
   "alg": "HS256",
   "typ": "JWT"
}

Payload:

{
   "userId": 1,
   "role": "admin",
   "exp": 1609588483
}

The Problem: Unencrypted JWTs

The main issue with JWTs is that they are not encrypted by default; they are only encoded in base64. This means that anyone who intercepts a JWT can easily decode its payload and see the data inside. While the signature ensures that the token hasn’t been altered, it does nothing to protect the confidentiality of the data.

For example, if you were to store sensitive information such as a user’s email, address, or even passwords in a JWT, an attacker who manages to capture the token can effortlessly decode and access this information.

Why This Is Dangerous

• Exposes Sensitive Information: If a JWT is intercepted in transit or retrieved from local storage, any sensitive information it contains can be exposed. This could lead to data breaches, identity theft, or other malicious activities.
• Person-in-the-Middle Attacks: In cases where an attacker can eavesdrop on network traffic, an unencrypted JWT is like an open book. Even if the connection is encrypted with HTTPS, there are still risks, especially if your application uses insecure configurations.
• Insecure Local Storage: Storing JWTs in places like local storage or session storage, which are vulnerable to cross-site scripting (XSS) attacks, can lead to token theft. If these tokens contain sensitive information, the attacker gains access to that data as well.
• Token Lifespan Mismanagement: If sensitive information is stored in a JWT with a long expiration time, it prolongs the period during which this information can be exposed if the token is compromised.

Best Practices for Handling Sensitive Data in JWTs

• Avoid Storing Sensitive Data in JWTs: The simplest and most effective approach is to never include sensitive information in JWTs. Use JWTs for non-sensitive data, such as user IDs or roles, and store sensitive information securely on the server-side. When needed, you can reference this data using a secure method.
• Use Encrypted JWTs (JWE): If you must include sensitive data in a token, use JSON Web Encryption (JWE) instead of a regular JWT. JWEs are encrypted versions of JWTs that ensure the payload is not visible to anyone without the decryption key.
• Keep Tokens Short-Lived: Reduce the risk by limiting the lifespan of your JWTs. Short-lived tokens minimize the window of opportunity for an attacker to misuse a compromised token.
• Implement Secure Transmission: Always transmit JWTs over secure channels (HTTPS). This prevents them from being intercepted by attackers in transit.
• Store Tokens Securely: When storing JWTs on the client-side, use secure storage mechanisms like HttpOnly cookies, which are not accessible via JavaScript and are less vulnerable to XSS attacks.
• Rotate and Revoke Tokens: Implement a strategy for rotating and revoking tokens. This limits the damage in case a token is compromised.

Conclusion

JWTs are powerful tools for managing authentication and authorization in web applications, but they come with inherent risks if not used properly. Storing sensitive information in unencrypted JWTs is a dangerous practice that can lead to serious security vulnerabilities. By following best practices such as avoiding the inclusion of sensitive data, using encrypted tokens, and securing token storage, you can significantly reduce the risks associated with JWTs.

Remember, in the realm of cybersecurity, prevention is always better than cure. Protect your users and your application by handling JWTs with care.