May 10, 2026·auth

JWT Decoder + Verifier

Paste a JSON Web Token, split into header / payload / signature, verify with an HMAC secret. Auto-flags expired, not-yet-valid, and dangerous "alg none" tokens.

jwt · jose · hmac · hs256

A JWT (JSON Web Token) is header.payload.signature joined by dots, base64url-encoded, optionally signed. Understanding what each part holds and what the signature buys you covers ~80% of modern auth.

This lab takes a pasted token, splits it into three parts, base64url-decodes them, parses the JSON. Paste an HMAC-SHA256 secret and it verifies the signature client-side. exp, nbf, iat claims become human-readable timestamps; expired tokens are flagged red. The dangerous alg: "none" case is highlighted with a security banner.

JwtDecoder — decode + verify, all in browser

jwt token

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IllpxJ9pdCBZYWzEsW0iLCJpYXQiOjE3Nzg2OTk1NjgsImV4cCI6MTc3ODcwMzE2OH0.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

header

36 chars

{
  "alg": "HS256",
  "typ": "JWT"
}

payload

103 chars

{
  "sub": "1234567890",
  "name": "Yiğit Yalım",
  "iat": 1778699568,
  "exp": 1778703168
}

time-based claims

valid · expires in 1h

exp: 17787031682026-05-13 20:12:48 UTC+00:00
iat: 17786995682026-05-13 19:12:48 UTC+00:00

signature verify · HS256

The three parts

A JWT is the dot-joined concatenation of:

Header (red) — algorithm + token type:
```
{ "alg": "HS256", "typ": "JWT" }
```
Payload (violet) — claims. sub (subject), iat (issued at), exp (expires at) are the most common:
```
{ "sub": "user-123", "iat": 1745731600, "exp": 1745735200 }
```
Signature (sky) — HMAC(secret, base64url(header) + "." + base64url(payload)). Or RSA/ECDSA. To verify it you need the matching secret or public key.

Never trust without verifying

Three places where it goes wrong:

Accepting alg: "none" — some libraries (including old jsonwebtoken v0.x) accepted this algorithm; an attacker could forge unsigned tokens. Modern libraries reject it but you should still pass verify({ algorithms: ["HS256"] }) as an explicit allowlist.
Algorithm confusion — server expects RS256 (asymmetric) but an attacker submits a token signed with the public key as if it were an HMAC secret, switching to HS256. Again: explicit algorithms allowlist defeats this.
Decoding without verifying — jwt.decode() (not verify) only base64url-decodes, never checks the signature. Never use decode-only in production; always jwt.verify(token, secret, options).

`exp` vs `nbf` vs `iat`

Claim	Meaning	Server check
`iat`	Issued at	Logging only — must not be in the future
`nbf`	Not before	Reject if `now < nbf`
`exp`	Expires at	Reject if `now >= exp`

Allowing ±60-300 seconds of clock skew is common practice. More than an hour of tolerance opens auth-bypass risk.

Token storage

HttpOnly + Secure + SameSite=Strict cookie — the secure default
Not localStorage / sessionStorage — XSS exfiltrates them
Web Worker memory works too, but it's complex