jwt

name	jwt
description	JWT — structure, signing algorithms, refresh, security pitfalls.

JWT (JSON Web Token)

Structure

A JWT is three Base64URL-encoded segments separated by dots:

header.payload.signature

Header

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

Payload (claims)

{
  "sub": "user-uuid",
  "iss": "https://auth.example.com",
  "aud": "api.example.com",
  "exp": 1700000000,
  "iat": 1699996400,
  "jti": "unique-token-id",
  "email": "[e-mail]",
  "role": "admin"
}

Signature — HMAC or RSA/ECDSA over header.payload using the signing key.

---

Standard Claims (RFC 7519)

Claim	Name	Required	Description
iss	Issuer	Recommended	Who issued the token
sub	Subject	Recommended	Who the token represents (user ID)
aud	Audience	Recommended	Who the token is intended for
exp	Expiration	Required	Unix timestamp — reject after this
iat	Issued At	Recommended	When it was issued
nbf	Not Before	Optional	Not valid before this timestamp
jti	JWT ID	Optional	Unique ID for this token (for revocation)

Always validate exp. Validate iss and aud when the system has multiple token issuers or audiences.

---

Signing Algorithms

Algorithm	Type	When to use
HS256 / HS384 / HS512	Symmetric (shared secret)	Single service; secret never leaves the server
RS256 / RS384 / RS512	Asymmetric (RSA)	Multiple services; public key distributed for verification
ES256 / ES384 / ES512	Asymmetric (ECDSA)	Same as RS but smaller keys, faster

Default recommendation: HS256 for single-service systems, RS256 or ES256 when multiple services need to verify tokens independently.

Never use none algorithm — it disables signature verification entirely.

---

Token Expiry & Refresh Strategy

Short-lived access tokens + long-lived refresh tokens:

Token	Typical TTL	Where to store
Access token	15 min – 1 hour	Memory (JS), Authorization header
Refresh token	7–30 days	httpOnly secure cookie or secure storage

Refresh flow:

1. Client sends expired access token (or proactively refreshes before expiry)
2. Server validates refresh token (check DB — is it still valid, not revoked?)
3. Server issues new access token (and optionally rotates the refresh token)
4. Client uses the new access token

Refresh token rotation (recommended): each use of a refresh token invalidates the old one and issues a new one. Detects stolen tokens — if a rotated token is used, revoke the entire session.

---

Validation Checklist

Every JWT validation must check all of the following:

• Signature is valid (using the correct key and algorithm)
• exp is in the future
• iat is in the past (clock skew tolerance: ≤ 60 seconds)
• iss matches the expected issuer
• aud matches the expected audience (if used)
• Algorithm header matches what the server expects — never accept none
• Token is not on the revocation list (if implementing revocation)

---

Implementation by Language

Node.js / TypeScript

import jwt from 'jsonwebtoken'

// Sign
const token = jwt.sign(
  { sub: userId, role: 'user' },
  process.env.JWT_SECRET!,
  { expiresIn: '1h', issuer: 'api.example.com' }
)

// Verify — throws on invalid/expired
const payload = jwt.verify(token, process.env.JWT_SECRET!, {
  issuer: 'api.example.com',
  algorithms: ['HS256']
})

Python

import jwt  # PyJWT

token = jwt.encode(
    {"sub": user_id, "exp": datetime.utcnow() + timedelta(hours=1)},
    settings.JWT_SECRET,
    algorithm="HS256"
)

payload = jwt.decode(token, settings.JWT_SECRET, algorithms=["HS256"])

Go

import "github.com/golang-jwt/jwt/v5"

token := jwt.NewWithClaims(jwt.SigningMethodHS256, jwt.MapClaims{
    "sub": userID,
    "exp": time.Now().Add(time.Hour).Unix(),
})
signed, _ := token.SignedString([]byte(os.Getenv("JWT_SECRET")))

// Verify
parsed, err := jwt.Parse(signed, func(t *jwt.Token) (interface{}, error) {
    if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
        return nil, fmt.Errorf("unexpected signing method")
    }
    return []byte(os.Getenv("JWT_SECRET")), nil
})

PHP

use Firebase\JWT\JWT;
use Firebase\JWT\Key;

$token = JWT::encode(['sub' => $userId, 'exp' => time() + 3600], $_ENV['JWT_SECRET'], 'HS256');
$decoded = JWT::decode($token, new Key($_ENV['JWT_SECRET'], 'HS256'));

---

Custom Claims

Add application-specific claims to the payload. Prefix custom claims to avoid collisions:

{
  "sub": "user-uuid",
  "exp": 1700000000,
  "https://example.com/roles": ["admin", "editor"],
  "tenant_id": "acme-corp"
}

Authorization decisions must be based on claims set by the server, not user-supplied data. If claims are read from a user-controlled source, an attacker can elevate privileges.

---

Revocation

JWTs are stateless — they're valid until exp. To revoke before expiry:

Strategy	How
Blocklist	Store revoked jti values in Redis; check on each request
Refresh token revocation	Don't revoke access tokens — make them short-lived (≤15 min). Revoke refresh tokens in DB
Secret rotation	Rotate the signing secret — invalidates all tokens at once; use only for emergency

Short-lived access tokens (15 min) make individual revocation largely unnecessary — prefer this over a blocklist for most use cases.

---

Security Pitfalls

Pitfall	Risk	Fix
Long-lived access tokens (days/weeks)	Stolen token stays valid forever	Keep to ≤ 1 hour
Storing JWT in localStorage	XSS can steal it	Use memory or httpOnly cookie
Accepting none algorithm	Auth bypass — no signature checked	Allowlist algorithms explicitly
Not validating iss / aud	Token from another service accepted	Always validate when multi-service
Sensitive data in payload	JWTs are not encrypted — anyone can decode	Never put passwords, PII, or secrets in payload
Weak secret (secret123)	Brute-forceable signature	Use ≥ 256-bit cryptographically random secret
Trusting user-supplied claims	Privilege escalation	Only trust claims from verified token, issued by known issuer