Arp Spoofing in Flask

ARP spoofing is a layer-2 network attack where an attacker sends falsified ARP messages onto a local area network, linking the attacker's MAC address with the IP address of a legitimate host (like a gateway or another server). This enables man-in-the-middle (MITM) or denial-of-service attacks. While ARP spoofing targets the network infrastructure, Flask applications become vulnerable when they make outbound HTTP requests to internal services or when they are deployed in environments where the underlying network can be compromised.

In Flask, this manifests primarily through two patterns:

• Server-Side Request Forgery (SSRF): A Flask route that accepts a user-supplied URL and forwards requests to internal services (e.g., http://internal-api:8080/admin) can be exploited. If an attacker controls the URL parameter and the network is ARP-spoofed, they can intercept or manipulate the traffic between the Flask app and the internal service. Example vulnerable code:

from flask import Flask, request
import requests

app = Flask(__name__)

@app.route('/fetch')
def fetch_data():
    url = request.args.get('url')  # User-controlled input
    resp = requests.get(url)  # No validation
    return resp.text

# Attack: /fetch?url=http://169.254.169.254/latest/meta-data/ (AWS metadata) or http://internal-db:5432

Here, if the Flask host's ARP cache is poisoned for the internal-db IP, the attacker receives the database connection attempt.

• Misconfigured Proxies and Internal API Calls: Flask apps often sit behind reverse proxies (Nginx, HAProxy) or call internal microservices. If the Flask application or its proxy relies on untrusted network paths (e.g., communicating with a payment service over HTTP on a shared LAN), ARP spoofing can intercept those calls. For instance, a Flask app using requests.post('http://payment-gateway.local/charge', ...) on a corporate network exposes transaction data if the gateway's MAC is spoofed.

Flask's development server (app.run()) is particularly vulnerable in local testing on untrusted networks, as it lacks encryption and runs on HTTP by default. Production deployments with werkzeug.middleware.proxy_fix.ProxyFix may also inadvertently trust client IP headers (X-Forwarded-For) that could be spoofed in a MITM scenario, though this is a separate IP spoofing issue.

ARP spoofing itself is a network-layer attack and cannot be detected via HTTP scanning alone. However, the vulnerabilities that make Flask apps susceptible to ARP-mediated MITM—primarily SSRF and unencrypted internal communications—can be identified through security scanning. middleBrick's SSRF check specifically probes for endpoints that accept URLs and attempt outbound requests, flagging them as high-risk if they can reach internal IP ranges (e.g., 127.0.0.1, 10.0.0.0/8, 169.254.169.254).

To detect these issues in a Flask codebase:

• Static Code Analysis: Search for Flask routes that use request.args, request.form, or request.json to build URLs passed to requests.get(), httpx, or urllib. Tools like Bandit can flag unsafe URL construction.
• Dynamic Scanning with middleBrick: Submit your Flask API endpoint to middleBrick. The SSRF test will attempt to fetch from http://127.0.0.1 and other internal ranges via any URL parameters or request bodies. A finding indicates an SSRF vulnerability that could be exploited over a compromised network. Example CLI usage:

middlebrick scan https://your-flask-api.com/fetch?url=http://127.0.0.1

middleBrick will report if the endpoint responds differently to internal vs. external URLs, confirming SSRF. This is critical because an SSRF flaw allows an attacker, once on the same network (via ARP spoofing or Wi-Fi compromise), to pivot to internal services.

• Network Monitoring: Use tools like arp-scan or Wireshark to detect duplicate MAC addresses or unsolicited ARP replies on the network segment hosting the Flask server. This is outside the scope of middleBrick but complements it.

Note: middleBrick does not simulate ARP spoofing; it identifies application-layer flaws that network-layer attacks could exploit. The combination of SSRF + untrusted network = high risk.

Remediation focuses on eliminating SSRF vectors and securing internal communications. Flask provides several native patterns to mitigate these risks:

• Input Validation and Allowlisting: Never trust user-supplied URLs. Validate against a strict allowlist of permitted domains/IPs. Use urllib.parse to parse and compare the hostname:

from flask import Flask, request, abort
import requests
from urllib.parse import urlparse

ALLOWED_HOSTS = {'internal-api.example.com', '10.0.1.5'}

@app.route('/fetch')
def fetch_data():
    url = request.args.get('url')
    parsed = urlparse(url)
    if parsed.hostname not in ALLOWED_HOSTS:
        abort(403, 'Host not allowed')
    # Optional: enforce HTTPS
    if parsed.scheme != 'https':
        abort(403, 'Only HTTPS permitted')
    resp = requests.get(url, timeout=5)
    return resp.text

This blocks requests to 127.0.0.1, 169.254.169.254, and any unexpected internal IPs.

• Use Environment-Specific Service Discovery: Instead of accepting URLs, have the Flask app select internal services via configuration. Example using environment variables:

import os
PAYMENT_SERVICE_URL = os.getenv('PAYMENT_SERVICE_URL', 'https://payment-gateway.prod:443')

@app.route('/charge', methods=['POST'])
def charge():
    # Use fixed URL, no user input
    resp = requests.post(PAYMENT_SERVICE_URL, json=request.json)
    return resp.json()

This removes the SSRF vector entirely.

• Encrypt Internal Traffic: Even on private networks, use TLS for internal API calls to prevent MITM. In Flask, when using requests, verify certificates and consider certificate pinning for critical services:

import requests
from requests.adapters import HTTPAdapter
from urllib3.util.ssl_ import create_urllib3_context

# Custom adapter with pinned certificate
class PinnedAdapter(HTTPAdapter):
    def init_poolmanager(self, *args, **kwargs):
        context = create_urllib3_context()
        context.load_verify_locations(cafile='/path/to/internal-ca.pem')
        kwargs['ssl_context'] = context
        return super().init_poolmanager(*args, **kwargs)

session = requests.Session()
session.mount('https://internal-api.example.com', PinnedAdapter())

@app.route('/secure-fetch')
def secure_fetch():
    resp = session.get('https://internal-api.example.com/data')
    return resp.text

• Network Segmentation: Deploy Flask apps in a DMZ or separate subnet from critical internal services. Use firewall rules to block the Flask host from directly accessing sensitive internal IPs unless explicitly required. This is infrastructure-level but essential.

Additionally, ensure Flask is not running in debug mode (app.run(debug=False)) in production, as the debugger can expose code paths. Use werkzeug.middleware.proxy_fix.ProxyFix only if you control the proxy and have validated its headers.