# JSON Web Token

## **What are JWTs?**

* Standardized format for sending **cryptographically signed JSON data** between systems.
* Commonly used for **authentication, session handling, and access control**.
* **Stores all necessary data client-side**, making it ideal for **distributed systems** (multiple back-end servers).

## **JWT Format**

A JWT has **3 parts** separated by dots (`.`):

{% code overflow="wrap" %}

```
eyJraWQiOiI5MTM2ZGRiMy1jYjBhLTRhMTktYTA3ZS1lYWRmNWE0NGM4YjUiLCJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTY0ODAzNzE2NCwibmFtZSI6IkNhcmxvcyBNb250b3lhIiwic3ViIjoiY2FybG9zIiwicm9sZSI6ImJsb2dfYXV0aG9yIiwiZW1haWwiOiJjYXJsb3NAY2FybG9zLW1vbnRveWEubmV0IiwiaWF0IjoxNTE2MjM5MDIyfQ.SYZBPIBg2CRjXAJ8vCER0LA_ENjII1JakvNQoP-Hw6GG1zfl4JyngsZReIfqRvIAEi5L4HV0q7_9qGhQZvy9ZdxEJbwTxRs_6Lb-fZTDpW6lKYNdMyjw45_alSCZ1fypsMWz_2mTpQzil0lOtps5Ei_z7mM7M8gCwe_AGpI53JxduQOaB5HkT5gVrv9cKu9CsW5MS6ZbqYXpGyOG5ehoxqm8DL5tFYaW3lB50ELxi0KsuTKEbD0t5BCl0aCR2MBJWAbN-xeLwEenaqBiwPVvKixYleeDQiBEIylFdNNIMviKRgXiYuAvMziVPbwSgkZVHeEdF5MQP1Oe2Spac-6IfA
```

{% endcode %}

1. **Header** – Contains metadata (e.g., algorithm used for signing).
   * Example: `{"alg": "HS256", "typ": "JWT"}`
2. **Payload** – Contains **claims** (user data and token info).
   * Example: `{"sub": "user123", "name": "Alice", "exp": 1735689600}`
3. **Signature** – Ensures data integrity (prevents tampering).

## **Key Features**

* **Base64URL-encoded** (not encrypted by default).
* **Stateless** (server doesn’t store session data).
* **Security relies on the signature** (if signature is weak/absent, tokens can be forged).

## How do vulnerabilities to JWT attacks arise?

### 1. Exploiting flawed JWT signature verification <a href="#exploiting-flawed-jwt-signature-verification" id="exploiting-flawed-jwt-signature-verification"></a>

### 2. Brute-forcing secret keys <a href="#exploiting-flawed-jwt-signature-verification" id="exploiting-flawed-jwt-signature-verification"></a>

### 3. JWT header parameter injections (jwk, jku, kid, cty, x5c) <a href="#exploiting-flawed-jwt-signature-verification" id="exploiting-flawed-jwt-signature-verification"></a>

### 4. JWT algorithm confusion <a href="#exploiting-flawed-jwt-signature-verification" id="exploiting-flawed-jwt-signature-verification"></a>


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