---
title: "JWT Security Best Practices"
description: "A security hardening guide for JWT authentication in distributed systems"
keywords: ["jwt", "security", "authentication", "access token", "refresh token", "signature", "HS256", "RS256", "token revocation", "token storage"]
category: "jwt"
tags: ["jwt", "security", "access token", "refresh token", "token revocation"]
---

## JWT Security Best Practices

JSON Web Tokens (JWTs) are a popular and widely-used method for securing APIs and user sessions. However, JWT-based authentication systems can be vulnerable to various attacks if not implemented securely. This guide outlines several best practices to harden the security of your JWT-based authentication system.

### Choose RS256 over HS256 for Distributed Systems

The JWT specification supports two main signing algorithms: HMAC (HS256) and RSA (RS256). While HS256 is simpler to implement, it is not well-suited for distributed systems where the secret key needs to be shared across multiple servers.

In a distributed environment, using HS256 means that the secret key must be available on all servers that need to validate the JWT. This creates a security risk, as a compromise of any one server would expose the secret key and allow attackers to forge valid JWTs.

Instead, we recommend using the RS256 algorithm, which uses public/private key pairs for signing and verification. This allows you to keep the private signing key secure on a central server, while distributing the public key to all servers that need to validate JWTs. This significantly reduces the attack surface and makes it much harder for an attacker to forge valid tokens.

### Use Short Expiry Times

JWTs should have a relatively short expiry time to limit the window of opportunity for an attacker to use a compromised token. A good practice is to use:

- **Access Tokens**: 15 minutes or less
- **Refresh Tokens**: 7 days or less

Short expiry times force users to re-authenticate more frequently, reducing the potential damage if a token is stolen. Refresh tokens can be used to obtain new access tokens without requiring the user to re-authenticate, but they should also have a limited lifespan.

### Secure Token Storage

Where you store your JWT tokens is crucial for security. Avoid storing access tokens in client-side storage like localStorage or cookies, as these can be accessed by JavaScript and are vulnerable to Cross-Site Scripting (XSS) attacks.

Instead, store access tokens in an HttpOnly, Secure cookie. This prevents the token from being accessed by client-side scripts and ensures it is only sent over HTTPS. Refresh tokens can be stored in a more secure server-side session store.

### Implement Token Revocation Strategies

To mitigate the risk of compromised tokens, you should implement a token revocation strategy. This can be done in two ways:

1. **Blacklist**: Maintain a server-side blacklist of revoked tokens. When a user logs out or their session is terminated, add the token to the blacklist to prevent further use.

2. **Token Versioning**: Assign a version number to each token. When a user's session is terminated, increment the version number, effectively invalidating all previous tokens.

Implementing token revocation ensures that even if a token is compromised, it can be quickly disabled to prevent unauthorized access.

### Prevent Common JWT Attacks

There are several common attacks that target vulnerabilities in JWT-based authentication systems. Be sure to protect against these:

1. **None Algorithm Attack**: Ensure that your JWT validation code does not allow the "none" algorithm, which would allow an attacker to forge a valid token without a signature.

2. **Key Confusion Attack**: Properly validate the "alg" claim in the JWT header to ensure it matches the expected algorithm (e.g., RS256). This prevents an attacker from substituting a different signing algorithm.

3. **Token Sidejacking**: Use the "aud" (audience) claim to ensure the token is intended for your application, and the "iss" (issuer) claim to ensure it was issued by your authentication server.

4. **Replay Attacks**: Use the "jti" (JWT ID) claim to uniquely identify each token and prevent replay attacks.

Implementing these protections in your JWT validation logic and middleware will help ensure the overall security of your authentication system.

By following these best practices, you can significantly improve the security of your JWT-based authentication system and protect your application and users from a variety of attacks.