Arp Spoofing in Flask with Basic Auth

Arp Spoofing is a Layer 2 network attack where an attacker sends falsified Address Resolution Protocol messages to associate their MAC address with the IP address of a legitimate host, typically the gateway. In a Flask application that uses HTTP Basic Authentication, this attack becomes especially relevant because credentials are transmitted in an easily recoverable format if the traffic is intercepted. When an attacker successfully spoofs the ARP table of a client or the local network segment, they can position themselves as a man-in-the-middle on the same broadcast domain. Even though Basic Auth encodes credentials using Base64, it does not encrypt them; an attacker performing ARP spoofing can capture these credentials in transit if the traffic is not additionally protected by transport-layer encryption.

The combination of Flask, Basic Auth, and an unsegmented or poorly controlled network increases the practical risk. Flask’s development server is not designed for production use and often runs on a single-threaded, single-process model that does not inherently defend against low-level network manipulation. If an attacker maintains a spoofed ARP association, they can observe or alter unencrypted HTTP requests, including the Authorization header. This exposure is not a flaw in Flask itself but a contextual risk introduced when sensitive authentication is handled without transport protection. The API security scanner performs unauthenticated black-box checks, including protocol-layer analysis where relevant, and would flag missing encryption as a high-severity finding in such a scenario.

ARP spoofing does not exploit a Flask-specific bug but leverages the weak security context created by using Basic Auth over plaintext HTTP. The scanner’s checks for encryption and data exposure are designed to detect the absence of TLS, which is the primary control needed to mitigate this class of attack. Without TLS, credentials intercepted via ARP spoofing can be reused immediately. The presence of an OpenAPI specification does not mitigate this risk; runtime analysis must confirm that endpoints requiring authentication are served exclusively over encrypted channels to prevent credential recovery through network-level attacks.

Remediation centers on replacing HTTP Basic Auth over plaintext HTTP with TLS-protected transport and, where possible, migrating to more secure authentication mechanisms. At minimum, enforce HTTPS for all routes, especially those handling authentication. Below is a minimal Flask example that enforces TLS and uses HTTP Basic Auth in a secure context. This code assumes a valid certificate and private key are available at the specified paths.

from flask import Flask, request, Response
from functools import wraps
import ssl

app = Flask(__name__)

def check_auth(username, password):
    # Replace with secure credential verification, e.g., constant-time compare
    return username == 'admin' and password == 'secure_password'

def authenticate():
    return Response(
        'Could not verify your access level.',
        401,
        {'WWW-Authenticate': 'Basic realm="Login Required"'}
    )

def requires_auth(f):
    @wraps(f)
    def decorated(*args, **kwargs):
        auth = request.authorization
        if not auth or not check_auth(auth.username, auth.password):
            return authenticate()
        return f(*args, **kwargs)
    return decorated

@app.route('/secure')
@requires_auth
def secure_endpoint():
    return 'Authenticated access granted'

if __name__ == '__main__':
    context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
    context.load_cert_chain(certfile='server.crt', keyfile='server.key')
    app.run(host='0.0.0.0', port=443, ssl_context=context)

For production, use a robust WSGI server behind a reverse proxy that terminates TLS, and store credentials using secure hashing rather than plain-text comparison. The scanner’s encryption and authentication checks will validate that endpoints using Basic Auth are served over encrypted channels and that authentication is not bypassed. If you prefer not to manage certificates directly, the middleBrick CLI allows you to run middlebrick scan <url> to confirm whether your Flask service exposes authenticated endpoints without TLS. Teams using the Pro plan can enable continuous monitoring to detect regressions where HTTP is allowed, and the GitHub Action can fail builds if risk scores drop below the configured threshold due to missing encryption.