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JWT Inspector — Structural Validity & Common Misconfiguration Checks

Paste a JWT to check it for structural validity and the specific misconfigurations that cause real security incidents — an alg:none header (a known bypass some libraries historically accepted), a missing or absent expiry claim, and use of a weak algorithm — without needing the signing secret, since these are structural checks rather than a full signature verification.

How to use JWT Inspector

1. Paste the JWT.
2. Review the structural validity check.
3. Check the flagged misconfiguration warnings.
4. Fix any issues in how the token is issued.

Frequently asked questions

What is the alg:none vulnerability?

Some early JWT libraries would accept a token whose header declares alg:none as valid with no signature at all — an attacker who knows this can forge any token. Modern libraries reject it, but this checks that yours does.

Does this verify the signature?

No — full signature verification requires the secret or public key, which this tool does not ask for. It checks structure and known misconfiguration patterns instead.

Why check for a missing exp claim?

A token with no expiry is valid forever once issued, which is rarely intentional and a common oversight worth catching before it reaches production.

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Also searched for: jwt security checker · jwt alg none vulnerability check · validate jwt structure online

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