# Insecure Randomness via random.choice() for Security Token Generation

Language: Python
Severity: High
CWE: CWE-330

## Source
10

## Flow
10

## Sink
10

## Vulnerable Code
```python
import random
import string
from flask import Flask, request, jsonify

app = Flask(__name__)
iot_device_sessions = {}

@app.route('/api/iot/provision', methods=['POST'])
def provision_device():
    device_id = request.json.get('device_id')
    auth_token = ''.join(random.choice(string.ascii_letters + string.digits) for _ in range(32))
    iot_device_sessions[device_id] = {'token': auth_token, 'provisioned': True}
    return jsonify({'device_id': device_id, 'auth_token': auth_token, 'status': 'provisioned'})
```

## Explanation

The code uses random.choice() from Python's random module to generate authentication tokens for IoT devices. The random module uses a Mersenne Twister PRNG which is cryptographically weak and predictable, making the generated tokens susceptible to brute-force and prediction attacks by attackers who can observe previous tokens.

## Remediation

The fix replaces the insecure random.choice() with secrets.token_urlsafe(32), which uses a cryptographically secure random number generator (CSPRNG) to produce unpredictable tokens. The secrets module is specifically designed for generating security-sensitive tokens and is resistant to prediction attacks, unlike the Mersenne Twister PRNG used by the random module.

## Secure Code
```python
import secrets
import string
from flask import Flask, request, jsonify

app = Flask(__name__)
iot_device_sessions = {}

@app.route('/api/iot/provision', methods=['POST'])
def provision_device():
    device_id = request.json.get('device_id')
    auth_token = secrets.token_urlsafe(32)
    iot_device_sessions[device_id] = {'token': auth_token, 'provisioned': True}
    return jsonify({'device_id': device_id, 'auth_token': auth_token, 'status': 'provisioned'})
```