JWT Decoder
The JWT Decoder examines JSON Web Tokens to extract, verify, and analyze token contents including headers, payload claims, and signature information for authentication and authorization debugging. This specialized security tool processes JWT strings to decode base64url-encoded sections, display human-readable token contents, and provide insights into token structure, claim validity, and potential security issues in JWT implementation. Essential for developers, security engineers, and API consumers, it provides reliable JWT analysis for authentication troubleshooting, authorization verification, and security auditing across various JWT implementations and use cases. The tool offers comprehensive decoding options including signature verification (when possible), claim validation, expiration checking, and detailed structural analysis with security recommendations. Features include bulk decoding capabilities, different JWT format support, and integration with various authentication workflows and security standards. Whether debugging authentication issues, verifying token contents, or conducting security reviews, this decoder provides comprehensive JWT analysis with structural insights and security considerations.
JWT Decoder
Paste your JWT token here to decode and analyze it
Quick Examples:
JWT Structure
Header (Algorithm & Token Type)
Payload (Claims)
Signature
Token Analysis
Standard Claims
Custom Claims
JWT Tools
Optional: Used to verify HS256 signatures
About JWT (JSON Web Tokens)
JWT Structure:
- • Header: Algorithm & token type
- • Payload: Claims & data
- • Signature: Cryptographic verification
Common Uses:
- • API authentication
- • Session management
- • Information exchange
- • Authorization tokens
- • Single sign-on (SSO)
Security Considerations
🔐 Never expose secret keys: JWT secrets should never be shared or exposed in client-side code.
⏰ Check expiration: Always validate the 'exp' claim to ensure tokens haven't expired.
🎯 Verify signatures: Always verify JWT signatures on the server side before trusting the contents.
📏 Keep payloads small: JWTs are sent with every request, so avoid large payloads.
🔄 Use HTTPS: Always transmit JWTs over secure connections to prevent interception.
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What is JWT Decoder?
The JWT Decoder parses JSON Web Tokens (JWTs) into their three component parts—header, payload, and signature—providing comprehensive token analysis and inspection capabilities. It automatically highlights standard JWT claims (iss for issuer, sub for subject, aud for audience, exp for expiration, nbf for not-before, iat for issued-at time), checks token expiration status and validity, and optionally verifies HS256 signatures with a provided secret key—all processing happens entirely in your browser for complete privacy and security. The tool formats decoded claims in readable JSON, surfaces custom claim keys beyond standard fields, and offers quick example tokens (basic and expired) for learning and testing purposes. Use it to understand token structure during development, debug authentication issues, troubleshoot API integration problems, and provide customer support without sending sensitive tokens or secrets to external services, ensuring JWT inspection happens securely on your local device.
How to Use
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1
Paste a JWT token string into the input field; the tool automatically splits and decodes the three parts: header (algorithm and token type), payload (claims and data), and signature (verification hash).
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2
Review the decoded JSON sections showing the header with algorithm information (alg, typ) and the payload containing all claims, custom fields, and token metadata in human-readable format.
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3
Check the token validity status panel which indicates whether the token is expired (checking exp claim), not yet valid (checking nbf claim), or currently valid, helping identify timing-related authentication issues.
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4
Optionally enter a secret key in the verification field to attempt HS256 signature verification, which confirms whether the token was signed with the provided secret and hasn't been tampered with.
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5
Use the Format JSON button to pretty-print decoded payloads with proper indentation and formatting for easier reading and analysis of complex claim structures.
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6
Examine highlighted standard claims (iss, sub, aud, exp, nbf, iat) and review any custom claim keys that appear in the payload, understanding all data contained in the token.
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7
Load a sample token (basic valid token or expired token example) from the quick examples to quickly learn JWT structure, claim behavior, and how different token states appear when decoded.
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8
Copy decoded information or the original token when needed for documentation, troubleshooting, or sharing with team members (while being careful not to expose sensitive production tokens).
Use Cases & Examples
Authentication debugging and troubleshooting
Inspect JWT claims, expiration times, and token structure after login failures, authentication errors, or access denied issues to identify problems with token generation, expiration, or claim validation in authentication workflows.
API integration and service communication
Verify token audience (aud) and issuer (iss) claims for backend services, microservices, or third-party API integrations to ensure tokens are properly scoped and issued by expected authorities before making authenticated API requests.
Support ticket triage and customer assistance
Confirm clock-skew issues, expired tokens, or missing claim problems in customer support tickets by decoding JWTs to identify authentication failures, helping resolve access issues without exposing sensitive production systems or secrets.
Developer education and training
Demonstrate JWT structure, header/payload/signature roles, and claim usage to new developers learning JWT authentication, providing hands-on examples that help teams understand token-based authentication mechanisms and best practices.
Security policy verification and compliance
Ensure JWTs omit sensitive data like passwords or personal information, verify payload sizes are minimal to reduce exposure, and check that tokens follow organizational security policies regarding claim content and token lifetime for compliance audits.
Token validation and signature verification
Verify HS256 signatures with known secrets to confirm token authenticity, detect tampering or forgery attempts, and validate that tokens were issued by trusted authorities before processing authenticated requests in applications or APIs.
Development and testing workflows
Inspect test tokens, mock JWTs, or development tokens to verify claim structure, expiration settings, and token configuration during application development, API testing, or integration testing that requires JWT authentication.
Security auditing and token analysis
Analyze JWT structure and claims for security audits, penetration testing, or vulnerability assessments to identify insecure token configurations, overly permissive claims, or security weaknesses in JWT implementation and usage patterns.
Tips & Best Practices
Never paste production JWT secrets or sensitive tokens unless you control and trust the device/environment completely, as decoded tokens and secrets could be logged, cached, or exposed if the device is compromised or monitored.
Prefer short token expiration times (exp claims) combined with refresh tokens to limit risk exposure—if a token is compromised, shorter expiration windows reduce the time attackers can misuse stolen tokens before they expire naturally.
Always validate audience (aud) and issuer (iss) claims server-side for every protected route, as these claims ensure tokens are intended for your service and issued by trusted authorities, preventing token misuse or replay attacks.
Use HTTPS everywhere for JWT transmission; never transmit JWTs over insecure HTTP channels, as unencrypted transmission allows token interception and replay attacks that compromise authentication security.
Keep JWT payloads small to reduce request overhead, improve performance, and minimize exposure—large payloads increase token size, slow transmission, and potentially expose unnecessary information if tokens are intercepted or logged.
Understand that decoded payloads are not automatically trusted—always verify signatures server-side before processing claims, as anyone can decode JWTs but signature verification confirms authenticity and prevents tampering.
Check all relevant claims (not just exp) including nbf (not-before), aud (audience), iss (issuer), and custom scopes/roles to ensure comprehensive token validation beyond simple expiration checking for robust security.
Use this tool for development and debugging only—never rely on client-side verification for production security, as client-side code can be bypassed or manipulated; always perform signature verification and claim validation server-side.
Common Mistakes to Avoid
Treating decoded JWT payloads as trusted without verifying signatures server-side, when decoded payloads can be easily modified or forged—always verify signatures before trusting any claims in production applications, as decoding doesn't confirm authenticity.
Relying solely on expiration (exp) claim checking without validating other critical claims like nbf (not-before), aud (audience), iss (issuer), and scopes/roles, potentially allowing tokens that are expired, not yet valid, or intended for different services to be accepted.
Leaking JWT secrets, tokens, or decoded payloads in screenshots, logs, code repositories, or support communications, exposing sensitive authentication information that could be used by attackers to forge tokens or compromise authentication systems.
Using HS256 symmetric signatures with widely shared secrets across many services or applications, as compromised secrets allow token forgery for all services using that secret—prefer RS256 asymmetric signatures with private/public key pairs for better security.
Embedding personally identifiable information (PII), passwords, API keys, or other secrets directly in JWT payloads, which are base64-encoded (not encrypted) and readable by anyone who decodes the token, potentially exposing sensitive data to unauthorized parties.
Not validating token signature algorithm (alg claim) before processing, potentially allowing algorithm confusion attacks where attackers modify the alg claim to "none" or weaker algorithms, bypassing signature verification and creating forged tokens that appear valid.
Storing JWTs in localStorage or sessionStorage without proper security considerations, as these storage mechanisms are vulnerable to XSS attacks—prefer httpOnly secure cookies for high-risk applications to prevent JavaScript access to tokens.
Assuming JWT decoding provides security validation, when decoding only reveals token contents—signature verification and claim validation must be performed server-side, as client-side verification can be bypassed, manipulated, or disabled by attackers.
Using extremely long token expiration times or omitting expiration claims entirely, increasing risk exposure if tokens are compromised, as long-lived tokens provide extended attack windows for malicious actors who obtain stolen or leaked tokens.
Not implementing proper token revocation mechanisms, assuming expiration is sufficient for security, when compromised tokens remain valid until expiration—implement token blacklisting, refresh token rotation, or revocation mechanisms for robust security.
Sharing production JWTs in support tickets, logs, or communications without redaction, potentially exposing sensitive authentication tokens that could be reused by unauthorized parties or used to understand token structure for attack purposes.
Modifying JWT payloads for testing without understanding that signature must be regenerated by the issuer, as modified tokens will fail signature verification and be rejected by applications that properly validate token signatures and authenticity.
