A three-part, signed bearer token
A JWT (JSON Web Token, RFC 7519) consists of three Base64URL segments separated by dots: header.payload.signature. The header declares the signing algorithm (alg) and type (typ); the payload carries application-specific claims such as sub, exp, iat, iss, aud; the signature is produced over header + payload using HMAC (HS256) or an asymmetric algorithm (RS256, ES256).
Security notes: a verifier that accepts alg: none is catastrophic — always pin the expected algorithm. Keep token lifetime (exp) short, use a separate refresh token, and rotate keys periodically. See our REST API security guide for more.
About JWTs
Yes. Standard JWTs are signed (JWS) but not encrypted. Header and payload are plain Base64URL, readable by anyone. The secret is only needed to verify the signature. Use JWE if you need confidentiality, or simply do not put sensitive data in a token.
localStorage is accessible from JavaScript — an XSS lets the attacker steal the token. An HttpOnly + Secure + SameSite=Lax cookie is not accessible from JS and, combined with CSRF protection, is safer. Prefer HttpOnly cookies in production and use the Authorization header for non-SPA integrations.
Some old libraries trust the `alg` value from the header. An attacker can send a token with `alg: none` and no signature, bypassing verification. Mitigation: pin the expected algorithm in your verifier (allow-list) and always reject `none`.
JWS (JSON Web Signature) provides integrity and authenticity — the payload is readable. JWE (JSON Web Encryption) encrypts the payload so only the holder of the key can read it. A plain "JWT" is typically JWS; use JWE when you need confidentiality.
API security consulting
JWT configuration, OAuth2 flows, rate limiting and penetration testing — the KEYDAL team supports you end-to-end.
When should you use JWT Decoder?
The KEYDAL JWT Decoder tool is a browser-based utility that developers, system administrators, SEO specialists and enterprise technology teams use in their daily operations. It requires no installation, is free, and produces results instantly. It is designed so local teams can run audits without connecting to server environments and run analyses without touching production.
Typical scenarios include: post-migration verification, comparing domain or hosting providers, diagnosing customer issues, security auditing (pre-pentest reconnaissance), root-cause analysis of email deliverability problems, validating CDN or proxy configuration, surfacing technical audit data for SEO teams, and rapid information gathering during incident response. You can copy results as text and share them or paste them into internal documentation.
The KEYDAL infrastructure team provides web hosting, VPS, dedicated server management, server hardening, DNS configuration and SSL/TLS deployment services from Türkiye. Beyond these tools, we deliver server setup and operations support across Hetzner, OVHcloud, Contabo, DigitalOcean and Turkish providers.
Your queries are never stored on our servers
KEYDAL tools run stateless: domain names, IPs, URLs or other inputs are not persisted to any database. Logs are kept only for security purposes (rate limiting, abuse detection) and deleted within 30 days. For tools that handle sensitive data (tokens, API keys, JWTs), processing happens entirely in your browser — nothing is sent over the network. See our Privacy Policy for details.
All tools run over HTTPS with TLS 1.3 support. KEYDAL is a Türkiye-based technology company and complies fully with local data-protection regulations (KVKK) and GDPR principles.
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The KEYDAL free tools collection includes DNS lookup, WHOIS lookup, SSL certificate checker, HTTP headers analyzer, IP geolocation, uptime checker, JSON formatter, JWT decoder, Base64 encode/decode, QR code generator, meta tag analyzer and robots.txt tester. All browser-based, free, no installation.
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