# Unsafe YAML Deserialization via yaml.load() on Untrusted Input

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_manifest = request.data.decode('utf-8')
    device_config = yaml.load(device_manifest, Loader=yaml.Loader)
    device_id = device_config.get('device_id', 'unknown')
    firmware_version = device_config.get('firmware', '1.0.0')
    security_profile = device_config.get('security_profile', {})
    print(f"Provisioning device {device_id} with firmware {firmware_version}")
    return jsonify({"status": "provisioned", "device_id": device_id, "profile": security_profile})
```

## Explanation

The application uses yaml.load() with yaml.Loader on untrusted user input from request.data without validation. This allows arbitrary Python object instantiation, enabling remote code execution through malicious YAML payloads containing Python object constructors.

## Remediation

The fix replaces yaml.load() with yaml.safe_load(), which only deserializes standard YAML tags (strings, numbers, lists, dicts) and refuses to instantiate arbitrary Python objects. An additional type check ensures the parsed result is a dictionary, preventing unexpected input types from causing downstream errors.

## 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_manifest = request.data.decode('utf-8')
    device_config = yaml.safe_load(device_manifest)
    if not isinstance(device_config, dict):
        return jsonify({"error": "Invalid device manifest format"}), 400
    device_id = device_config.get('device_id', 'unknown')
    firmware_version = device_config.get('firmware', '1.0.0')
    security_profile = device_config.get('security_profile', {})
    print(f"Provisioning device {device_id} with firmware {firmware_version}")
    return jsonify({"status": "provisioned", "device_id": device_id, "profile": security_profile})
```