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JWT Debugger & Security Inspector

Decode, verify, and security-audit JSON Web Tokens (JWT) in real time. 100% client-side privacy with zero server logging.

JWT Debugger & Security Inspector: the tool

Load Sample Token:
Encoded JWT String

Tested in-browser using Web Crypto API. Secret keys never leave your device.

Header: Algorithm & Token Type
{
  "alg": "HS256",
  "typ": "JWT"
}
Payload: Decoded Claims
{
  "sub": "usr_0a1b2c3d",
  "name": "Alex Vance",
  "email": "alex@example.com",
  "role": "engineer",
  "iat": 1761830400,
  "exp": 1793394400,
  "iss": "https://auth.deesu.org"
}
Signature Component43 chars
vN37wG_vLqD7jVz7hQW6J3bFm7uT8kX0yZ1wE2rP4mA

Automated JWT Security & Vulnerability Audit

Checked 3 security vectors
Symmetric HS256 Algorithm In Use

Ensure your secret key has at least 256 bits of entropy and your verification logic explicitly prevents algorithm confusion attacks against RS256 public keys.

Token Is Expired

Expired 101h 43m ago (10/30/2026, 9:06:40 PM). Server will reject this token.

No Plaintext Secrets Detected

Payload contains standard claims without obvious credentials or high-risk field names.

Need enterprise authentication, OAuth 2.0, or security hardening?

DEESU audits and architects zero-trust identity pipelines, token revocation state machines, and API security firewalls across mobile and web platforms.

Consult With Our Engineers

How it works

How JSON Web Tokens (JWT) Work in Modern Web and Mobile Architecture

JSON Web Tokens (RFC 7519) are a compact, URL-safe standard for cryptographically transmitting claims between an identity provider (such as Auth0, Firebase, Supabase, or a custom OAuth 2.0 backend) and client applications. A standard JWT comprises three base64url-encoded parts separated by periods: Header, Payload, and Signature.

The Header specifies the cryptographic signing algorithm (such as HMAC SHA-256 or RSA PKCS#1 v1.5) and token type. The Payload contains identity claims such as the subject (sub), issuance timestamp (iat), and expiration timestamp (exp). The Signature is generated by hashing the header and payload with a secret or private key, allowing the receiving API to verify authenticity without querying a database.

Critical Security Flaws Found in Production JWT Implementations

Because JWTs are self-contained and widely supported, developer oversights can lead to severe privilege escalation and authentication bypasses:

  • The 'alg: none' Vulnerability (CVE-2015-9235): When JWT verification libraries accept tokens with alg: 'none', attackers strip the signature and forge arbitrary administrative claims.
  • Key Confusion / Algorithm Switching (HS256 vs RS256): If an API expects an RS256 token (signed with an asymmetric private key and verified with a public key) but accepts HS256, an attacker can sign a token using the server's publicly accessible RSA public key as the HMAC symmetric secret.
  • Missing Expiration (exp) and Long-Lived Sessions: Tokens issued without an exp claim never expire. If leaked via client-side XSS or proxy logs, attackers retain permanent account access.
  • Sensitive Data Leaks: Storing passwords, credit cards, or internal API keys in the payload. Remember: base64url encoding is NOT encryption; anyone with access to the token can read the payload.

Best Practices for Token Lifecycle and Replay Prevention

Production microservices should never rely on raw JWTs for long-term session storage. Best practice pairs short-lived access tokens (5 to 15 minutes) with rotating refresh tokens stored in secure, HttpOnly, SameSite=Strict cookies or native OS keychain storage (EncryptedSharedPreferences on Android and Keychain on iOS).

For financial and high-security endpoints, verify the JWT ID (jti) claim against a high-speed Redis revocation list to immediately neutralize compromised sessions upon logout or password reset.

FAQ

Questions, answered

No. Base64url is simply an encoding scheme that translates binary data into safe ASCII characters for HTTP headers and URLs. The payload of a standard JWT is completely readable by anyone who inspects the network traffic or local storage. Never store passwords, private keys, or sensitive PII inside a JWT unless using JWE (JSON Web Encryption).

This vulnerability occurs when a server configures an asymmetric algorithm (like RS256) where the public key is known, but the verification library allows the client to dictate the algorithm. If an attacker changes the header to HS256 and signs the token using the server's public key (PEM text) as the symmetric secret, the vulnerable server verifies the token as valid.

Security and privacy. Sending authentication tokens containing corporate user data, email addresses, or API secrets across the internet to a third-party server creates an unnecessary security liability. Our tool decodes and audits tokens entirely in your browser memory using the native Web Cryptography API. Zero data leaves your machine.

Set your access token lifetime between 5 and 15 minutes. Use refresh tokens with token family rotation to grant new access tokens. If you must support immediate revocation (e.g. user logout, role change), maintain a short-lived denylist in memory (Redis) keyed by the token's unique jti claim.

JWT is the general standard for JSON claims. JWS (JSON Web Signature) represents signed, unencrypted tokens where anyone can view the payload but cannot modify it without breaking the signature. JWE (JSON Web Encryption) encrypts the payload so that only holders of the private decryption key can read the claims.

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