# YAML Deserialization via yaml.load with UnsafeLoader

Language: Python
Severity: Critical
CWE: CWE-502

## Source
8

## Flow
8-9

## Sink
9

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

app = Flask(__name__)

@app.route('/iot/device/provision', methods=['POST'])
def provision_iot_device():
    device_config = request.data.decode('utf-8')
    parsed_config = yaml.load(device_config, Loader=yaml.UnsafeLoader)
    device_id = parsed_config.get('device_id', 'unknown')
    firmware_ver = parsed_config.get('firmware_version', '1.0.0')
    telemetry_interval = parsed_config.get('telemetry_interval', 60)
    return jsonify({'status': 'provisioned', 'device': device_id, 'firmware': firmware_ver, 'interval': telemetry_interval})
```

## Explanation

The application uses yaml.load() with UnsafeLoader to deserialize untrusted YAML data from an HTTP POST request. UnsafeLoader allows arbitrary Python object instantiation, enabling attackers to execute malicious code by crafting specially-formed YAML payloads that instantiate dangerous Python objects during deserialization.

## Remediation

The fix replaces yaml.load() with yaml.UnsafeLoader with yaml.safe_load(), which only deserializes standard YAML tags (strings, numbers, lists, dicts) and prevents instantiation of arbitrary Python objects. An additional type check ensures the parsed result is a dictionary before accessing keys, providing defense against unexpected input formats.

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

app = Flask(__name__)

@app.route('/iot/device/provision', methods=['POST'])
def provision_iot_device():
    device_config = request.data.decode('utf-8')
    parsed_config = yaml.safe_load(device_config)
    if not isinstance(parsed_config, dict):
        return jsonify({'status': 'error', 'message': 'Invalid configuration format'}), 400
    device_id = parsed_config.get('device_id', 'unknown')
    firmware_ver = parsed_config.get('firmware_version', '1.0.0')
    telemetry_interval = parsed_config.get('telemetry_interval', 60)
    return jsonify({'status': 'provisioned', 'device': device_id, 'firmware': firmware_ver, 'interval': telemetry_interval})
```