CWE-441: Unintended Proxy or Intermediary ('Confused Deputy')

What is Unintended Proxy or Intermediary ('Confused Deputy')?

• Overview: This vulnerability occurs when a software product unintentionally forwards requests to an external system without preserving the original source information, causing it to act as a proxy. This can allow attackers to bypass access controls or disguise the origin of malicious requests.

• Exploitation Methods:

• Attackers can exploit this vulnerability by sending requests to the product, which then forwards them to a target system as if coming from the product itself.
• Common attack patterns include using the product to bypass firewalls, access restricted systems, or execute unauthorized commands by exploiting the trust relationships.

• Security Impact:

• Direct consequences include unauthorized access to systems, execution of commands, or retrieval of sensitive data.
• Potential cascading effects involve compromising multiple systems or networks when trust boundaries are breached.
• Business impact can be severe, including data breaches, loss of customer trust, legal penalties, and financial loss.

• Prevention Guidelines:

• Specific code-level fixes include validating and sanitizing all requests before forwarding and ensuring original source information is preserved.
• Security best practices involve implementing strict access control policies, logging and monitoring all forwarded requests, and using secure channels for communication.
• Recommended tools and frameworks include proxy-aware libraries that maintain source integrity and intrusion detection systems that can identify and block unauthorized proxy behavior.

Corgea can automatically detect and fix Unintended Proxy or Intermediary ('Confused Deputy') in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not Technology-Specific

Vulnerable Code Example

Python Example

from flask import Flask, request, jsonify
import requests

app = Flask(__name__)

# Vulnerable code: The application acts as an intermediary without authenticating the original sender
@app.route('/fetch-data', methods=['POST'])
def fetch_data():
    # The application forwards the request to an external API without verifying the original requester
    # This can lead to a confused deputy problem where an attacker can misuse this service to make unauthorized requests
    external_url = request.json.get('url')  # The URL is taken directly from the request without validation
    response = requests.get(external_url)  # The request is forwarded without checking the sender's authenticity
    return jsonify(response.json())  # The response is returned directly, potentially leaking sensitive information

How to fix Unintended Proxy or Intermediary ('Confused Deputy')?

To fix this vulnerability, it is crucial to authenticate and authorize the original source of the request before forwarding any requests to external resources. This ensures that only trusted entities can initiate requests through your service, preventing it from becoming an unintended proxy or intermediary.

Steps to fix:

1. Authenticate: Ensure the incoming request is from a legitimate and authorized source.
2. Authorize: Check if the requester has permission to perform the requested operation.
3. Validate Input: Validate and sanitize the input data to prevent injection attacks.
4. Log Requests: Log all requests for auditing and monitoring purposes.
5. Restrict Access: Use allowlists for URLs that can be accessed via your proxy.
6. Rate Limiting: Implement rate limiting to prevent abuse.

Fixed Code Example

from flask import Flask, request, jsonify, abort
import requests
import logging

app = Flask(__name__)

# Secure code: The application authenticates and authorizes the request before acting as an intermediary
@app.route('/fetch-data', methods=['POST'])
def fetch_data():
    # Step 1: Authenticate the requester using an API key
    api_key = request.headers.get('API-Key')
    if not api_key or not is_valid_api_key(api_key):
        abort(403, description="Unauthorized access")

    # Step 2: Validate the URL against an allowlist
    external_url = request.json.get('url')
    if not is_valid_url(external_url):
        abort(400, description="Invalid URL")

    # Step 3: Log the request details for auditing purposes
    logging.info(f"Authorized request from API-Key: {api_key} to URL: {external_url}")

    # Step 4: Forward the request and return the response
    response = requests.get(external_url)
    return jsonify(response.json())

def is_valid_api_key(api_key):
    # Validate the API key (placeholder function)
    return api_key == "trusted-api-key"

def is_valid_url(url):
    # Validate the URL against an allowlist (placeholder function)
    allowed_urls = ["https://api.trusted.com/data"]
    return url in allowed_urls

In the fixed code, we added checks to authenticate the requester using an API key and validate the destination URL against an allowlist. This ensures that only authorized requests are forwarded, mitigating the risk of the service being used as an unintended proxy. Additionally, we log each request for monitoring and auditing purposes, which helps in detecting misuse and maintaining accountability.