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JWT Debugger

Decode and debug JSON Web Tokens with header and payload analysis.

JWT Debugger

Paste your JWT token to decode and analyze

Paste your JWT token above to decode the header, payload, and signature

Header

Header will appear here...

Payload

Payload will appear here...

Signature

Signature will appear here...

Common JWT Claims

issIssuer (who created the token)
subSubject (who the token represents)
audAudience (who the token is intended for)
expExpiration time (Unix timestamp)
nbfNot Before time (token is invalid before this)
iatIssued At time (when token was created)
jtiJWT ID (unique identifier for the token)

About JWT Debugger

What is a JWT?

JWT (JSON Web Token) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. JWTs are digitally signed using a secret key or a public/private key pair, ensuring that the information contained within them cannot be tampered with or forged. These tokens are commonly used for authentication, authorization, and information exchange in web applications, APIs, and distributed systems. JWTs consist of three parts: a header, a payload, and a signature, all encoded in Base64Url format and separated by dots.

How JWT Tokens Work

JWT tokens operate on a stateless authentication mechanism where all necessary information is contained within the token itself. When a user logs in, the server creates a JWT containing user information and signs it with a secret key. The client stores this token and includes it in subsequent requests. The server can then verify the token's authenticity by checking the signature and extracting the user information without needing to query a database. This makes JWTs ideal for scalable, distributed systems where maintaining session state across multiple servers would be complex and resource-intensive.

Common Uses for JWT Tokens

Authentication

JWTs are widely used for user authentication in web applications and APIs. After successful login, a JWT is issued containing user identity information, which is then used to authenticate subsequent requests without requiring repeated login credentials.

Authorization

JWTs can contain role and permission information, allowing applications to make authorization decisions based on the token's payload. This eliminates the need for separate authorization checks against a database.

API Security

REST APIs and microservices use JWTs to secure endpoints and ensure that only authenticated and authorized clients can access protected resources. The stateless nature makes them perfect for distributed architectures.

Single Sign-On (SSO)

JWTs enable single sign-on solutions where users can authenticate once and access multiple applications or services without re-entering credentials. This is commonly used in enterprise environments and modern web applications.

Information Exchange

JWTs can securely transmit information between parties in a standardized format. This is useful for passing user context, preferences, or other data between different parts of an application or between different services.

Session Management

Modern web applications use JWTs for session management instead of traditional server-side sessions. This approach reduces server memory usage and enables better scalability across multiple servers.

JWT Structure and Components

Every JWT consists of three distinct parts that work together to create a secure, self-contained token. Understanding the structure is essential for proper implementation and debugging of JWT-based systems.

Header (Algorithm & Token Type)
  • • Specifies the signing algorithm (HS256, RS256, etc.)
  • • Indicates the token type (JWT)
  • • May include additional metadata
  • • Base64Url encoded
  • • Example: {"alg": "HS256", "typ": "JWT"}
Payload (Claims)
  • • Contains the actual data (claims)
  • • Registered claims (iss, exp, sub, etc.)
  • • Public claims (custom data)
  • • Private claims (application-specific)
  • • Base64Url encoded
Signature (Verification)
  • • Created using header + payload + secret
  • • Ensures token integrity
  • • Prevents tampering and forgery
  • • Algorithm-specific generation
  • • Base64Url encoded
Complete Format
  • • Three parts separated by dots
  • • header.payload.signature
  • • Compact and URL-safe
  • • Self-contained information
  • • Stateless verification

Understanding JWT Claims and Standards

Registered Claims (RFC 7519)

The JWT specification defines several standard claims that are commonly used across applications. These include "iss" (issuer), "sub" (subject), "aud" (audience), "exp" (expiration time), "nbf" (not before), "iat" (issued at), and "jti" (JWT ID). These claims provide essential information for token validation and security enforcement.

Public Claims

Public claims are custom claims that can be defined by anyone but should be registered to avoid collisions. Examples include "name", "email", "role", and "permissions". These claims are typically used to store user-specific information that applications need for authorization and personalization.

Private Claims

Private claims are custom claims that are agreed upon between parties and are not registered. These are application-specific and can include any data that the parties agree to include. Examples might include "user_id", "company_id", "preferences", or any other application-specific information.

Claim Validation

Proper claim validation is crucial for JWT security. Applications should validate expiration times, issuer claims, audience claims, and other relevant fields to ensure tokens are valid and appropriate for the current context. Missing or invalid claims can lead to security vulnerabilities.

JWT Signing Algorithms and Security

HMAC Algorithms (Symmetric)
  • • HS256, HS384, HS512
  • • Uses shared secret key
  • • Faster verification
  • • Secret must be kept secure
  • • Best for: Single server applications
RSA Algorithms (Asymmetric)
  • • RS256, RS384, RS512
  • • Uses public/private key pair
  • • More secure key distribution
  • • Slower than HMAC
  • • Best for: Distributed systems
ECDSA Algorithms (Elliptic Curve)
  • • ES256, ES384, ES512
  • • Uses elliptic curve cryptography
  • • Smaller key sizes
  • • Good performance
  • • Best for: Resource-constrained environments
None Algorithm (Unsecured)
  • • No signature verification
  • • Extremely insecure
  • • Should never be used in production
  • • Only for testing/debugging
  • • Warning: Major security risk

JWT Security Best Practices

Implementing JWTs securely requires careful attention to various security considerations. Following established best practices helps prevent common vulnerabilities and ensures robust token-based authentication systems.

Token Security
  • • Use strong, cryptographically secure algorithms
  • • Keep signing keys secure and rotate regularly
  • • Set appropriate expiration times
  • • Validate all claims thoroughly
  • • Use HTTPS for token transmission
Storage Security
  • • Store tokens securely (HttpOnly cookies)
  • • Avoid localStorage for sensitive tokens
  • • Implement token refresh mechanisms
  • • Clear tokens on logout
  • • Monitor for token leakage

Common JWT Vulnerabilities and Mitigation

Understanding common JWT vulnerabilities helps developers implement proper security measures and avoid potential security breaches in their applications.

Algorithm Confusion Attacks
  • • Attacker changes algorithm in header
  • • Exploits different verification methods
  • • Mitigation: Explicitly verify algorithm
  • • Never trust algorithm from token
  • • Use whitelist of allowed algorithms
Token Replay Attacks
  • • Reusing captured valid tokens
  • • Mitigation: Short expiration times
  • • Implement token blacklisting
  • • Use nonce or jti claims
  • • Monitor for suspicious patterns

JWT Debugging and Troubleshooting

Debugging JWT issues requires understanding common problems and having the right tools to analyze token structure, validity, and content. Our JWT debugger tool provides comprehensive analysis capabilities.

Common Debugging Issues
  • • Token expiration (exp claim)
  • • Invalid signature verification
  • • Incorrect algorithm specification
  • • Missing required claims
  • • Malformed token structure
Debugging Tools
  • • Token structure analysis
  • • Claim validation checking
  • • Signature verification
  • • Expiration time analysis
  • • Algorithm compatibility testing

JWT Implementation Guidelines

Proper JWT implementation requires careful consideration of various factors including token size, performance, security, and maintainability. Following established guidelines ensures robust and secure JWT-based systems.

Token Design
  • • Keep payload size minimal
  • • Include only necessary claims
  • • Use appropriate expiration times
  • • Consider token refresh strategies
  • • Plan for token revocation
Performance Considerations
  • • Choose appropriate signing algorithms
  • • Optimize token size for network efficiency
  • • Implement caching strategies
  • • Monitor token processing overhead
  • • Consider token compression for large payloads

Using Our JWT Debugger Tool

Our comprehensive JWT debugger provides powerful tools for analyzing, validating, and troubleshooting JSON Web Tokens. Whether you're developing JWT-based authentication systems, debugging token issues, or learning about JWT structure, our tool offers the functionality you need to work with JWTs effectively.

The debugger can decode JWT tokens to reveal their header, payload, and signature components in a human-readable format. It validates token structure, checks claim values, verifies signatures using provided keys, and identifies potential security issues. The tool supports all major JWT signing algorithms and provides detailed error messages to help you understand and resolve token-related problems.

Whether you're a developer implementing JWT authentication, a security professional auditing token-based systems, or a student learning about web security, our JWT debugger provides the insights and tools needed to work confidently with JSON Web Tokens. The comprehensive analysis capabilities help ensure your JWT implementations are secure, properly configured, and functioning as expected.

Frequently Asked Questions

What is a JWT token and how does it work?
A JWT (JSON Web Token) is a compact, URL-safe token used for securely transmitting information between parties as a JSON object. It consists of three parts: Header, Payload, and Signature, separated by dots. The process involves: 1) Client logs in and sends credentials, 2) Server validates credentials and generates a JWT, 3) Client stores the JWT and sends it with each request, 4) Server verifies the JWT's signature and grants access if valid.
Is JWT secure for authentication?
Yes, JWT is secure if implemented correctly. Use HTTPS to prevent interception, sign tokens with a strong algorithm (e.g., HS256, RS256), set short expiration times for tokens, and store tokens securely (avoid localStorage if vulnerable to XSS).
How to decode a JWT token?
A JWT can be decoded (not decrypted) since it's Base64Url-encoded. You can split the token by dots and decode the payload part using base64 decoding. However, decoding does not verify the signature - it only shows the contents.
What is the structure of a JWT token?
A JWT has 3 parts: 1) Header – Algorithm & token type (e.g., {"alg": "HS256", "typ": "JWT"}), 2) Payload – Claims (e.g., {"sub": "123", "name": "Alice", "exp": 1735689600}), 3) Signature – Ensures integrity using HMACSHA256.
What is the difference between JWT and OAuth?
JWT is a token format (compact, self-contained), while OAuth 2.0 is an authorization framework (defines how tokens are issued & used). JWT can be used within OAuth as an access token.
How to validate a JWT token in backend?
To validate a JWT: 1) Check token structure (header.payload.signature), 2) Verify signature using the secret key, 3) Validate claims (exp, iss, aud). Use libraries like jsonwebtoken in Node.js for proper validation.
What does "HS256" mean in a JWT token?
HS256 = HMAC + SHA-256 (symmetric signing). A secret key is used for both signing & verification. Alternative: RS256 (asymmetric, uses private/public keys).
Is it safe to store JWT in localStorage or cookies?
localStorage is vulnerable to XSS (JavaScript can steal it). Cookies with HttpOnly flag prevent XSS but need CSRF protection. Best practice: Use HttpOnly cookies for better security.
How to refresh JWT tokens securely?
Issue a short-lived access token (e.g., 15 min) and a long-lived refresh token (stored securely in HTTP-only cookie). When access token expires, client sends refresh token to get a new one.
What are the best practices for using JWT?
Use HTTPS, set short expiry (e.g., 15-30 mins), use strong algorithms (avoid none), store tokens securely (HttpOnly cookies), and validate all claims (iss, aud, exp).
What are JWT claims?
Claims are statements about an entity and additional data. Registered claims include: iss (issuer), exp (expiration), sub (subject), aud (audience), iat (issued at), nbf (not before). Custom claims can be added for application-specific data.
How do I create a JWT token?
Use a JWT library in your preferred language. In Node.js: const jwt = require('jsonwebtoken'); const token = jwt.sign({userId: 123}, 'secret_key', {expiresIn: '1h'});. Include payload data and sign with a secret key.
What is the difference between HS256 and RS256?
HS256 uses symmetric encryption (same key for signing/verification), while RS256 uses asymmetric encryption (private key for signing, public key for verification). RS256 is more secure for distributed systems.
How do I verify a JWT signature?
Use your JWT library's verify function. In Node.js: const decoded = jwt.verify(token, 'secret_key');. The library will check the signature and return the payload if valid, or throw an error if invalid.
What is JWT token expiration?
JWT tokens have an expiration time (exp claim) after which they become invalid. Set short expiration times (15-60 minutes) for access tokens to limit the damage if a token is compromised.
How do I handle JWT token errors?
Common JWT errors include: TokenExpiredError (token expired), JsonWebTokenError (invalid signature), NotBeforeError (token not yet valid). Handle these gracefully and redirect to login when appropriate.
What is the JWT secret key?
The secret key is used to sign and verify JWT tokens. It should be kept secure and not exposed in client-side code. Use environment variables to store the secret key securely.
How do I decode a JWT without verification?
Use the decode function (not verify) to see the payload without checking the signature. In Node.js: const decoded = jwt.decode(token);. This is useful for debugging but doesn't verify authenticity.
What is the difference between JWT and session tokens?
JWT tokens are stateless (server doesn't store them), while session tokens require server-side storage. JWT tokens are self-contained and can be validated without database lookups.
How do I implement JWT authentication in React?
Store JWT in HttpOnly cookies or secure storage, send with requests in Authorization header, handle token expiration with refresh tokens, and implement logout by clearing tokens.
What is JWT token blacklisting?
Blacklisting invalidates tokens before expiration by storing them in a database or cache. This is useful for logout functionality but adds complexity to the stateless nature of JWTs.
How do I handle JWT in microservices?
Pass JWT tokens between services in headers, verify tokens at API gateways, use shared secrets or public keys for verification, and implement proper error handling for invalid tokens.
What is the JWT header?
The JWT header contains metadata about the token: alg (algorithm used), typ (token type, usually 'JWT'), and optionally kid (key ID) for key rotation. It's Base64Url encoded.
How do I implement JWT refresh tokens?
Issue short-lived access tokens (15-60 min) and long-lived refresh tokens (days/weeks). Store refresh tokens securely (HttpOnly cookies), validate them server-side, and issue new access tokens when needed.
What is JWT token rotation?
Token rotation involves issuing new tokens before the current ones expire. This limits the window of opportunity if a token is compromised and improves security.
How do I debug JWT token issues?
Use JWT debugger tools to decode tokens, check expiration times, verify signatures, examine claims, and identify common issues like incorrect algorithms or malformed tokens.
What is the difference between JWT and API keys?
JWT tokens contain user information and can expire, while API keys are typically long-lived and don't contain user data. JWT tokens are more suitable for user authentication.
How do I implement JWT in Python?
Use PyJWT library: import jwt; token = jwt.encode({'user_id': 123}, 'secret_key', algorithm='HS256'). For verification: payload = jwt.decode(token, 'secret_key', algorithms=['HS256']).
What is JWT token size limit?
JWT tokens should be kept small (under 8KB) to avoid HTTP header size limits and performance issues. Keep payload data minimal and consider using references for large data.
How do I handle JWT in mobile apps?
Store JWT tokens securely using platform-specific secure storage (Keychain for iOS, Keystore for Android), implement token refresh logic, and handle network errors gracefully.
What is the JWT payload?
The JWT payload contains claims (statements about the entity). It includes registered claims (iss, exp, sub, aud, iat, nbf) and custom claims for application-specific data. It's Base64Url encoded.
How do I implement JWT in Java?
Use jjwt library: String token = Jwts.builder().setSubject('user123').setExpiration(new Date()).signWith(SignatureAlgorithm.HS256, 'secret_key').compact();. For verification: Claims claims = Jwts.parser().setSigningKey('secret_key').parseClaimsJws(token).getBody();
What is JWT token replay attack?
A replay attack occurs when an attacker captures a valid JWT and reuses it. Prevent this by using short expiration times, implementing token blacklisting, and using nonces or timestamps.
How do I handle JWT in PHP?
Use firebase/php-jwt library: $token = JWT::encode($payload, $key, 'HS256');. For verification: $decoded = JWT::decode($token, $key, array('HS256'));
What is JWT token signature?
The JWT signature is created by signing the header and payload with a secret key using the specified algorithm. It ensures the token hasn't been tampered with and verifies authenticity.
How do I implement JWT in C#?
Use System.IdentityModel.Tokens.Jwt: var token = new JwtSecurityToken(claims: claims, signingCredentials: new SigningCredentials(key, SecurityAlgorithms.HmacSha256)); var tokenString = new JwtSecurityTokenHandler().WriteToken(token);
What is JWT token validation?
JWT validation involves checking the token structure, verifying the signature, validating claims (exp, iss, aud), and ensuring the token hasn't been tampered with or expired.
How do I handle JWT in Go?
Use golang-jwt/jwt library: token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims); tokenString, err := token.SignedString([]byte('secret_key')). For verification: token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) { return []byte('secret_key'), nil });
What is JWT token security?
JWT security involves using HTTPS, strong algorithms (avoid 'none'), secure secret keys, short expiration times, proper token storage, and implementing refresh token rotation.
How do I implement JWT in Ruby?
Use jwt gem: token = JWT.encode(payload, 'secret_key', 'HS256'). For verification: decoded = JWT.decode(token, 'secret_key', true, { algorithm: 'HS256' });
What is JWT token expiration handling?
Handle token expiration by implementing refresh token logic, catching TokenExpiredError exceptions, redirecting users to login, and providing clear error messages about expired sessions.
How do I handle JWT in Swift?
Use JWTDecode library or implement manually: let token = try JWTDecode.decode(jwt: tokenString); let claims = token.body; let expiration = token.expiresAt;
What is JWT token debugging?
JWT debugging involves decoding tokens to examine their structure, checking claims for validity, verifying signatures, identifying common issues, and using tools like jwt.io for analysis.
How do I implement JWT in Kotlin?
Use jjwt library: val token = Jwts.builder().setSubject('user123').setExpiration(Date()).signWith(SignatureAlgorithm.HS256, 'secret_key'.toByteArray()).compact();
What is JWT token storage?
Store JWT tokens securely using HttpOnly cookies (preferred), secure HTTP-only cookies, or platform-specific secure storage. Avoid localStorage due to XSS vulnerabilities.
How do I handle JWT in Rust?
Use jsonwebtoken crate: let token = encode(&Header::default(), &claims, &EncodingKey::from_secret('secret_key'.as_ref()))?;. For verification: let token_data = decode::<Claims>(&token, &DecodingKey::from_secret('secret_key'.as_ref()), &Validation::default())?;
What is JWT token format?
JWT format is: header.payload.signature (three parts separated by dots). Each part is Base64Url encoded. The header contains algorithm info, payload contains claims, and signature ensures integrity.
How do I implement JWT in Scala?
Use pdi/jwt library: val token = Jwt.encode('{"user":"123"}', 'secret_key', JwtAlgorithm.HS256); val decoded = Jwt.decode(token, 'secret_key', Seq(JwtAlgorithm.HS256));
What is JWT token verification?
JWT verification involves checking the token's signature using the secret key or public key, validating the algorithm, and ensuring the token hasn't been tampered with or expired.
How do I handle JWT in Perl?
Use JSON::WebToken module: my $token = encode_jwt(payload => $payload, key => $secret_key, alg => 'HS256'); my $decoded = decode_jwt(token => $token, key => $secret_key);
What is JWT token algorithm?
JWT algorithms include HMAC (HS256, HS384, HS512), RSA (RS256, RS384, RS512), and ECDSA (ES256, ES384, ES512). Choose based on security requirements and key management capabilities.
How do I implement JWT in Haskell?
Use jwt library: let token = encodeSigned (hmacSecret 'secret_key') mempty claims; let result = decodeAndVerifySignature (hmacSecret 'secret_key') token;
What is JWT token claims validation?
Claims validation involves checking registered claims (exp, iss, aud, iat, nbf) and custom claims for validity, ensuring they meet application requirements and haven't been tampered with.
How do I handle JWT in Lua?
Use lua-resty-jwt library or implement manually using crypto libraries for HMAC/SHA256 and base64 encoding/decoding functions.
What is JWT token header validation?
Header validation involves checking the algorithm (alg) claim matches the expected algorithm, verifying the token type (typ) is 'JWT', and ensuring no unexpected header claims are present.
How do I implement JWT in Elixir?
Use joken library: token = Joken.encode_and_sign(%{user_id: 123}, Joken.Signer.create('HS256', 'secret_key')); claims = Joken.verify_and_verify(token, Joken.Signer.create('HS256', 'secret_key'));
What is JWT token signature verification?
Signature verification involves recreating the signature using the header, payload, and secret key, then comparing it with the provided signature to ensure the token hasn't been tampered with.
How do I handle JWT in Clojure?
Use buddy-sign library: (jwt/sign {:user-id 123} {:alg :hs256 :secret 'secret_key'}) for signing and (jwt/unsign token {:alg :hs256 :secret 'secret_key'}) for verification.
What is JWT token payload validation?
Payload validation involves checking that required claims are present, validating claim values (e.g., exp is in the future), ensuring custom claims meet application requirements, and handling missing or invalid claims appropriately.

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