What is JWT? JSON Web Token Explained — Structure, Security & Examples

The structure of a JWT (JSON Web Token) is made up of three main parts, separated by dots (.):

1. Header

   • Contains metadata about the token, such as the type of token (JWT) and the signing algorithm used (e.g., HS256, RS256).

   • Example:

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

   
   

2. Payload

   • Contains the actual data (called claims) that the token carries.

   • Claims can be about the user (like user_id, role) or token metadata (like expiration time).

   • Example:

     {
       "sub": "1234567890",
       "name": "John Doe",
       "admin": true,
       "exp": 1694102400
     }

   
   

3. Signature

• Created by taking the encoded header + encoded payload, then applying the secret key with the specified algorithm.

• Ensures that the token hasn’t been tampered with.

• Formula:

  Signature = HMACSHA256(base64UrlEncode(header) + "." + base64UrlEncode(payload), secret)

Final JWT looks like this:

xxxxx.yyyyy.zzzzz

• xxxxx → Encoded Header

• yyyyy → Encoded Payload

• zzzzz → Signature
  
  

JWT example step by step:

1. Header (before encoding)

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

After Base64Url encoding →

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9

2. Payload (before encoding)

{
  "sub": "1234567890",
  "name": "John Doe",
  "admin": true,
  "exp": 1716000000
}

After Base64Url encoding →

eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImV4cCI6MTcxNjAwMDAwMH0

3. Signature

We combine:

Base64UrlEncode(Header) + "." + Base64UrlEncode(Payload)

Then hash it with HMACSHA256 and a secret key (e.g., mysecretkey).

Example result:

TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ

Final JWT looks like this:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImV4cCI6MTcxNjAwMDAwMH0.TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ

{The first two parts can be decoded back to JSON (header + payload), but the signature can only be verified with the secret key. That’s how JWT ensures integrity and trust.}

Common Attack Scenarios & How to Mitigate Them

Here are common JWT attack vectors and actionable mitigation steps:

Refresh Tokens, Rotation & Revocation Strategies

JWTs do not inherently support revocation, making token renewal and invalidation critical. Here are strategies to implement safe token refresh and revocation workflows:

• Refresh token rotation: Issue a new refresh token per request and invalidate the previous one to reduce replay risks.

• Short-lived access tokens: Use brief expiry (e.g. 5–15 min) and require refresh for continued access.

• Blacklist / token store: Maintain a lightweight store of revoked jti identifiers to reject compromised tokens.

• Grace window & reuse detection: Allow a narrow overlap window for refresh but block reuse of old refresh tokens.

This gives developers a clear blueprint for safe token lifecycle management.

Example: Creating & Validating JWT in Python / Go / Java

# Python (PyJWT)
import jwt, time
secret = "s3cr3t"
payload = {"sub": "user123", "exp": int(time.time()) + 300}
token = jwt.encode(payload, secret, algorithm="HS256")
decoded = jwt.decode(token, secret, algorithms=["HS256"])

// Go (github.com/golang-jwt/jwt)
token := jwt.NewWithClaims(jwt.SigningMethodHS256, jwt.MapClaims{
  "sub": "user123", "exp": time.Now().Add(5 * time.Minute).Unix(),
})
signed, err := token.SignedString([]byte("s3cr3t"))

// Java (io.jsonwebtoken.Jwts)
String token = Jwts.builder()
    .setSubject("user123")
    .setExpiration(Date.from(Instant.now().plus(5, ChronoUnit.MINUTES)))
    .signWith(SignatureAlgorithm.HS256, "s3cr3t")
    .compact();
Jws<Claims> parsed = Jwts.parser().setSigningKey("s3cr3t").parseClaimsJws(token);