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How do I decode a JWT?
A conventional signed JWT in compact form has three dot-separated, base64url-encoded segments: a header, a payload, and a signature. The header and payload can often be decoded without a key. Encrypted or nested JWTs use different structures, so do not assume every token has three readable sections. The format and claims are defined in RFC 7519.
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- Capture the exact token from the failing request in a safe development environment. Treat a live bearer token as a credential: do not paste it into a public tool or include it in shared logs.
- Check its shape. Count the dot-separated segments and compare them with the form your application expects. A different shape may mean the token is encrypted, nested, malformed, or not the type of token the service expects.
- Decode the header and payload with a debugger or a local tool. Inspect
algand, if present,kidin the header; in the payload, look atiss,sub,aud,exp,nbf,iat, and any application-specific claims. - Compare the contents with the receiving service’s configuration. Check its trusted issuer and key source, accepted algorithm, expected audience, time policy, token type, and required permissions.
- Reproduce the check through the application’s established JWT library or middleware. A browser debugger is useful for inspection, not a substitute for the server’s validation path.
- Record the failed rule, not the credential. A claim name or validation error is usually enough to identify the problem; avoid logging the full token.
The jwt.io JWT debugger provides a visual way to inspect a token and an optional signature-verification workflow. Use it as a debugging aid, and keep real credentials out of public tools.
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Does decoding a JWT verify it?
No. Decoding is a representation change: it makes encoded header and payload data readable. It does not prove who issued the token, whether the signature is genuine, or whether the token is meant for the API receiving it. A token can decode cleanly and still be forged, expired, issued by an untrusted party, or intended for another service.
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JWTs can be signed, integrity-protected, or encrypted. In a signed JWT, the payload is not necessarily secret; anyone who has the token may be able to read its claims. Handle real tokens as sensitive even when their contents appear harmless. The jwt.io introduction to JWTs explains the basic format and distinction between token contents and protection.
Why is my JWT not working?
Start with the application’s token profile rather than assuming there is one universal definition of a valid JWT. RFC 8725, the IETF’s February 2020 Best Current Practice, says: “Each application of JWTs defines a profile specifying the required and optional JWT claims and the validation rules associated with them.” In practice, the receiving service’s documented expectations decide which claims and checks are required.
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- Expiration:
expis the time after which the token must not be accepted, subject to the implementation’s allowed clock-skew policy. Check the service’s time handling as well as the claim value. - Not-before time: If
nbfis present, a token may not be accepted until that time under the service’s rules. Compare it with the current time and the service’s clock-skew policy. - Audience mismatch:
audidentifies the intended recipient or recipients. A mismatch can mean the token belongs to another API, or that the service’s configuration does not match the token profile. RFC 8725 requires audience checking when a token can be intended for multiple relying parties. - Issuer or key mismatch:
issidentifies the issuer, but a claim alone does not establish trust. The verification key must be trusted and bound to that issuer. RFC 8725 states, in this context, “If they do not, the application MUST reject the JWT.” - Algorithm or key selection: Compare the header’s
algand anykidwith the algorithm policy and trusted key source configured by the service. Do not let an untrusted token choose what algorithms the application will accept. - Authorization claims: A token can have a valid signature and still lack the required scope, role, subject, or other application-specific permission. Authentication and authorization are separate checks.
These clues narrow the investigation, but the right issuer, audience, keys, token type, and permission rules must come from the application that receives the token. A signature that verifies is necessary in many signed-token flows; by itself, it does not prove the token is intended for this API or authorized for the requested action.
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Use the receiving application’s maintained JWT library or framework middleware, configured with that service’s trusted keys and policy. Auth0’s JWT validation documentation says: “We strongly recommend that you use middleware or one of the existing open source third-party libraries to parse and validate JWTs.”
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At a high level, validation means checking the cryptographic signature with a trusted key and an explicitly permitted algorithm, then applying the token profile’s claim and authorization rules. Obtain keys from the configured trusted source for the expected issuer; do not treat a key identifier in the token as proof that a key is trustworthy. RFC 8725 sets out these best practices, including issuer and audience considerations.
Do not build production validation by decoding segments and comparing values yourself. Libraries and middleware can enforce the cryptographic and parsing details; your application still needs to configure the expected issuer, audience, allowed algorithms, time policy, and required claims correctly.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which JWT debugging tool should I use?
| Tool category | Best use | What it can establish | Key limitation |
|---|---|---|---|
| Browser-based visual debugger | Quickly inspect a token’s encoded header and payload during debugging. | Readable contents; some tools also offer an optional signature-verification workflow. | A displayed decode is not production validation, and a tool’s verification result is only meaningful with appropriate trusted keys and settings. |
| JWT library or framework middleware | Parse and validate tokens in the application that receives them. | Signature checks and configured claim-policy enforcement, when correctly set up. | Must fit the application’s framework and be configured for its trusted keys, accepted algorithms, and token profile. |
Choose based on the task: use a debugger to inspect data, and the application’s own validation path to determine whether it accepts the token. The sources support this category-level distinction, not a current ranking of individual vendors.
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