Structure: JWTs have 3 parts: Header, Payload, and Signature, separated by dots.
Header: Contains the algorithm (alg) and token type (typ).
Payload: Contains claims like subject (sub), expiration (exp), and custom data.
Security: Never trust JWT claims without verifying the signature server-side.
Showing 8 of 94 related tools
Get up and running in 30 seconds
Copy and paste a complete JWT token (including header.payload.signature) from HTTP headers, API responses, cookies, or authentication systems. The tool automatically validates the JWT structure.
Instantly see decoded header (algorithm and token type), payload (claims and data), and signature. All three JWT components are Base64-decoded and formatted as readable JSON.
Review standard claims (iss, sub, aud, exp, iat) and custom claims. Check token expiration, issued time, and audience. Validate claim structure matches API specifications.
The tool validates JWT format and structure but does NOT verify cryptographic signatures. Use for debugging and inspection only - never for production security validation.
Understanding JWT tokens for developers
A JWT (JSON Web Token) decoder is a development tool that decodes and displays the contents of JWT tokens used for authentication and authorization in modern web applications. JWTs are compact, URL-safe tokens that encode JSON data in three Base64-encoded sections: header, payload, and signature. A JWT decoder parses these sections, revealing the token's algorithm, claims, expiration, and other metadata essential for debugging authentication flows.
JWT tokens are the de facto standard for stateless authentication in REST APIs, OAuth 2.0, OpenID Connect (OIDC), and single sign-on (SSO) systems. Developers encounter JWTs daily when working with authentication, API security, microservices, and third-party integrations. Understanding JWT structure and claims is critical for debugging auth issues, implementing security correctly, and integrating with identity providers.
A JWT consists of three Base64 URL-encoded sections separated by dots (.): header.payload.signature
Header: Contains token type (typ: "JWT") and signing algorithm (alg: "HS256", "RS256", etc.). The algorithm specifies how the signature is generated and verified. Common algorithms: HS256 (HMAC SHA-256), RS256 (RSA SHA-256), ES256 (ECDSA SHA-256).
Payload: Contains claims - JSON key-value pairs with user data, permissions, and metadata. Standard claims include iss (issuer), sub (subject/user ID), aud (audience), exp (expiration time), iat (issued at time), and nbf (not before time). Custom claims can include user roles, permissions, or any application-specific data.
Signature: Cryptographic signature that validates the token hasn't been tampered with. Generated by signing the header and payload with a secret key (HS256) or private key (RS256). Only systems with the secret/public key can verify the signature. The signature ensures token integrity and authenticity.
Debugging authentication flows requires inspecting JWT contents to verify claims are correct. Common issues: expired tokens (exp claim in the past), wrong audience (aud doesn't match API), missing permissions (custom role claims incorrect), or algorithm mismatches (API expects RS256 but receives HS256).
API integration with identity providers (Auth0, Okta, AWS Cognito, Firebase Auth) involves validating JWT structure matches specifications. JWT decoders help verify tokens contain expected claims, use correct algorithms, and follow provider documentation.
Security analysis requires examining JWT tokens for vulnerabilities: weak algorithms (none, HS256 with weak secrets), overly permissive claims (admin roles for regular users), excessive token lifetimes (exp too far in future), or sensitive data exposure (personal information in payload).
Development and testing workflows use JWT decoders to inspect tokens during development, verify token generation logic produces correct claims, and troubleshoot authentication errors in development or staging environments.
Unlike opaque tokens (random strings requiring database lookup), JWTs are self-contained - all data is in the token. This enables stateless authentication where servers validate tokens without database queries, improving scalability and performance.
Session IDs stored in cookies require server-side session storage. JWTs can be stored client-side (localStorage, cookies) and validated without server state. This makes JWTs ideal for distributed systems and microservices.
API keys are static and permanent. JWTs are temporary (with exp claim) and can include dynamic claims (user permissions that change). JWTs support fine-grained authorization through custom claims.
Authentication: After login, server generates JWT with user ID and roles. Client includes JWT in Authorization header for subsequent API requests. Server validates JWT signature and checks claims to authenticate and authorize requests.
Single Sign-On (SSO): User logs into identity provider, receives JWT. Multiple applications trust the same JWT for authentication. OIDC ID tokens are JWTs containing user profile information.
API Security: Microservices validate JWTs to authenticate requests without querying a central auth database. JWT claims include permissions for fine-grained access control.
Information Exchange: JWTs securely transmit information between parties. The signature ensures the data hasn't been tampered with. Used for sharing user profile data, permissions, or session information.
NEVER use client-side JWT decoding for production security validation. This tool decodes and displays JWT contents but does NOT verify cryptographic signatures. Anyone can create a JWT with fake claims - only signature verification proves authenticity.
In production, always verify JWT signatures on the server using proper cryptographic libraries (jsonwebtoken in Node.js, PyJWT in Python, jose in Go). Client-side decoding is for debugging and development only, never for security decisions.
Storing JWTs in localStorage exposes them to XSS attacks. Prefer httpOnly cookies for production. Never store sensitive secrets in JWT payloads - they're Base64-encoded (readable by anyone), not encrypted.
How developers use JWT decoders daily
Debug failed authentication by decoding JWTs to verify claims are correct. Check token expiration, validate issuer and audience, ensure required claims are present. Essential for troubleshooting 401 Unauthorized and 403 Forbidden errors.
Integrate with identity providers (Auth0, Okta, AWS Cognito, Google, Microsoft) by decoding their JWT ID tokens and access tokens. Verify token structure matches provider documentation and contains expected claims for user authentication.
Validate JWTs in microservices architectures where multiple services verify tokens independently. Decode tokens to ensure they contain required claims for service-to-service communication and user authorization across distributed systems.
Audit JWT implementations for security vulnerabilities: weak algorithms, excessive token lifetimes, sensitive data exposure, or missing security claims. Essential for security reviews, penetration testing, and compliance audits.
Master JWT decoding and analysis
This JWT decoder parses and displays all three sections of JWT tokens with real-time validation and formatted JSON output. All processing happens client-side - your tokens never leave your browser.
Paste a complete JWT token (including all three sections: header.payload.signature) into the input field. The tool automatically detects the JWT format and decodes each section instantly. Results appear as formatted JSON for easy reading.
JWT tokens typically start with "eyJ" (Base64 encoding of {"alg":...). If your token doesn't start this way, verify it's a valid JWT and not a different token format (opaque token, session ID, API key).
The three sections are separated by dots (.). All three must be present for valid JWT structure, though the signature section may be empty for unsigned tokens (alg: "none", which is insecure).
Header: Shows the signing algorithm (alg) and token type (typ). Common algorithms: HS256 (symmetric HMAC), RS256 (asymmetric RSA), ES256 (asymmetric ECDSA). The kid (key ID) field indicates which key was used for signing, enabling key rotation.
Payload: Contains claims - the actual data in the token. Standard claims follow IETF RFC 7519 specification: iss (issuer), sub (subject), aud (audience), exp (expiration), iat (issued at), nbf (not before), jti (JWT ID). Custom claims can include user roles, permissions, tenant IDs, or any application-specific data.
Signature: Displayed as Base64-encoded string. The signature cannot be meaningfully decoded - it's a cryptographic hash. Signature verification requires the secret key (HS256) or public key (RS256) and happens on the server, not in this tool.
The exp (expiration) claim is a Unix timestamp (seconds since Jan 1, 1970). The tool converts this to human-readable date/time. If exp is in the past, the token is expired and should be rejected by APIs.
Compare exp to current time. Expired tokens cause 401 Unauthorized errors. Check iat (issued at) and exp difference to see token lifetime. Typical lifetimes: 15 minutes (access tokens), 7 days (refresh tokens), 1 hour (ID tokens).
The nbf (not before) claim indicates when the token becomes valid. If nbf is in the future, the token is not yet valid. This prevents clock skew issues in distributed systems.
iss (issuer): Should match your identity provider's URL. Mismatch indicates token from wrong source or forgery attempt.
aud (audience): Should match your API or application identifier. Prevents tokens issued for one API being used on another.
sub (subject): User identifier (UUID, email, username). Use this to identify the authenticated user.
scope or permissions: Custom claims for authorization. Check these match expected user permissions.
This tool decodes JWTs but does NOT verify signatures. Anyone can create a JWT with fake claims. Only cryptographic signature verification proves a token is authentic and hasn't been tampered with.
NEVER use client-side JWT decoding for production authentication or authorization decisions. Always verify signatures on the server using proper cryptographic libraries:
Never store sensitive data in JWT payloads. JWTs are Base64-encoded (easily readable), not encrypted. Anyone with access to the token can decode and read the payload. Store only minimal data: user ID, roles, permissions. Never include passwords, API keys, PII (social security numbers, credit cards), or confidential data.
Invalid signature: Server rejects token because signature doesn't match. Check algorithm (HS256 vs RS256), verify correct secret/public key is used, ensure header and payload weren't modified.
Token expired: exp claim is in the past. Refresh the token or re-authenticate the user.
Invalid audience: aud claim doesn't match API's expected audience. Token was issued for a different application or service.
Algorithm mismatch: API expects RS256 but receives HS256 (or vice versa). Align token generation and validation algorithms.
Malformed token: Missing sections (not 3 parts separated by dots), invalid Base64 encoding, or corrupted during transmission.
HTTP Authorization Header: Remove "Bearer " prefix before pasting. Only paste the token itself: eyJhbGciOi...
Cookies: Extract JWT value from browser DevTools → Application → Cookies. Look for cookies named "token", "auth_token", "jwt", or similar.
API Responses: After login, APIs return tokens in response body: {"token": "eyJ..."}. Copy just the token value.
Browser Storage: Open DevTools → Application → Local Storage or Session Storage. Look for JWT tokens stored by the application.
Everything you need to know about JWT tokens
Your JWT data never leaves your browser
Your JWT tokens never leave your browser. This decoder operates entirely client-side using JavaScript's atob() (Base64 decoding) function built into your web browser. There are no server uploads, no backend processing, and no data transmission to any external services.
This makes the tool safe for decoding development and staging tokens. For production tokens with real user sessions, use local tools or your own JWT decoder to ensure absolute security.
This tool does NOT verify JWT signatures. It only decodes Base64-encoded sections. Anyone can create a JWT with fake claims. Never use client-side JWT decoding for authentication or authorization decisions in production.
Always verify signatures on the server using cryptographic libraries (jsonwebtoken, PyJWT, golang-jwt). Server-side validation with proper key management is the only secure way to authenticate JWTs.
Never store JWTs in localStorage (vulnerable to XSS). Prefer httpOnly, secure, SameSite cookies that JavaScript cannot access. Never expose JWTs in URLs or GET parameters (logged in server logs, browser history).
Performance metrics and capabilities