JSON Web Tokens (JWTs) have become the go-to method for authentication in modern applications. They are lightweight, easy to use across different platforms, and perfect for securing APIs. However, many developers misuse JWTs, which leads to vulnerabilities and bugs in production systems.
In this article, we’ll explore the top 5 JWT mistakes developers make and share how you can avoid them. If you’re working with tokens, this guide will help you strengthen your security practices—and at the end, we’ll also show you a handy JWT Generator / Builder you can use to test and debug your tokens.
1. Confusing Encoding with Encryption
One of the most common mistakes is assuming that a JWT is encrypted. In reality, JWTs are encoded, not encrypted.
- Encoding means the data is transformed into a different format, but anyone who has the token can decode it and read the content.
- Encryption, on the other hand, actually hides the content so that only the intended recipient can decrypt and read it.
If you put sensitive information like passwords, private keys, or personal user data inside a JWT, you’re exposing that data to anyone who intercepts the token.
✅ How to Avoid:
Never store confidential data in JWT payloads. Use JWTs only for storing claims like user IDs, roles, or permissions. If you need confidentiality, use encrypted JWTs (JWE) instead of the standard signed JWTs (JWS).
2. Using Weak or “None” Algorithms
Another mistake is leaving the algorithm field (alg) set to none or using weak algorithms like HS256 without proper key management. Attackers can exploit these weak spots to forge tokens and bypass authentication.
✅ How to Avoid:
- Always use strong algorithms such as
RS256(RSA with SHA-256). - Make sure your server enforces algorithm validation and rejects tokens with unexpected algorithms.
A good JWT Generator will allow you to choose the right algorithm and test your tokens safely before implementation.
3. Ignoring Token Expiration
JWTs often come with an expiration claim (exp), but many developers forget to implement it correctly. Without expiration, tokens can be reused indefinitely—even by hackers who have stolen them.
✅ How to Avoid:
- Always set a reasonable expiration time (
exp) for your tokens. - For high-security systems, keep token lifespans short and use refresh tokens for re-authentication.
By generating JWTs with correct expiration using a JWT Generator / Builder, you can easily test how your system handles expired tokens.
4. Storing JWTs in Unsafe Places
Where you store your JWT matters. Many developers put tokens in localStorage, which can be accessed by malicious JavaScript if the site has a Cross-Site Scripting (XSS) vulnerability.
✅ How to Avoid:
- Prefer storing tokens in HTTP-only cookies, which cannot be accessed via JavaScript.
- If you must use localStorage, implement strong Content Security Policies (CSP) to reduce XSS risks.
5. Not Validating the Signature Properly
Some developers simply decode the JWT and trust the payload without validating the signature. This is a critical mistake, as attackers can tamper with the payload and send a forged token.
✅ How to Avoid:
- Always verify the signature on the server side.
- Ensure that the token was signed using the secret or public key that you control.
Using a JWT Generator / Builder, you can create tokens with different signing algorithms and test your signature validation logic effectively.
Final Thoughts
JWTs are powerful, but like any tool, they can become a security liability if misused. By avoiding these five mistakes—confusing encoding with encryption, using weak algorithms, ignoring expiration, storing tokens unsafely, and skipping signature validation—you can ensure that your authentication system stays secure.
And here’s the best part: you don’t have to do everything manually. You can instantly create, test, and debug tokens using our free tools:
👉 Try the JWT Encoder (JWT Generator / Builder)
👉 Try the JWT Decoder
Whether you’re building an authentication system or debugging an existing one, these tools will save you time and help you avoid costly mistakes.
Frequently Asked Questions (FAQs)
1. What is a JWT Generator?
A JWT Generator (or JWT Builder) is an online tool that allows developers to create signed JWT tokens by providing a payload, header, and secret key.
2. Is JWT secure?
Yes, JWTs are secure if used correctly. Always validate the signature, use strong algorithms, and avoid storing sensitive data in the payload.
3. Where should I store my JWTs?
The safest option is an HTTP-only cookie, as it protects against JavaScript-based attacks like XSS.
4. Can JWTs be revoked?
JWTs are stateless by design, so revocation requires additional mechanisms like blacklists or short expiration times combined with refresh tokens.