Need

Implementation of proper input validation and sanitization to prevent host header injection

Context

• Usage of Python 3 for building and running Python applications
• Usage of Django for building web applications in Python

Description

Non compliant code

        from django.http import HttpResponseRedirect
from django.shortcuts import render

def redirect(request):
    # Get the next URL from the host header
    next_url = request.META.get('HTTP_HOST')

    # Redirect to the next URL
    return HttpResponseRedirect(next_url)

def home(request):
    return render(request, 'home.html')
        
        

The above code snippet is an example of a Django view that redirects the user to a URL obtained from the HTTP_HOST header of the incoming request. The function redirect(request) reads the host header from the request's meta data using request.META.get('HTTP_HOST') and then redirects to the URL obtained from the host header.

The problem with this code is that it doesn't validate the URL obtained from the host header before using it in a redirect. This means an attacker could manipulate the host header to point to a malicious website, causing the application to redirect users to that website.

This is known as Host Header Injection, a type of vulnerability that can be exploited to perform web-cache poisoning, password reset poisoning, or malicious redirects, among other things. This is a critical security issue that can have serious implications, including the potential for remote code execution.

Steps

• Validate the 'next_url' variable to ensure it's a safe and expected value before redirecting.
• Use a whitelist of allowed hosts and check if the 'next_url' is in this list.
• If the 'next_url' is not in the whitelist, redirect to a default or error page.
• Consider using Django's built-in 'is_safe_url()' function to check if the URL is safe for redirection.
• Never trust user input blindly, always sanitize and validate before using it.

Compliant code

        from django.http import HttpResponseRedirect
from django.shortcuts import render
from django.utils.http import is_safe_url

ALLOWED_HOSTS = ['www.example.com', 'example.com']  # Add your allowed hosts here

def redirect(request):
    # Get the next URL from the host header
    next_url = request.META.get('HTTP_HOST')

    # Validate the next_url
    if next_url not in ALLOWED_HOSTS or not is_safe_url(url=next_url, allowed_hosts=ALLOWED_HOSTS):
        # If the next_url is not valid, redirect to the home page
        next_url = 'home'

    # Redirect to the next URL
    return HttpResponseRedirect(next_url)

def home(request):
    return render(request, 'home.html')
        
        

The updated code now includes a validation step before redirecting to the next_url. This is done by checking if the next_url is in the ALLOWED_HOSTS list and by using Django's built-in is_safe_url() function.

The ALLOWED_HOSTS list is a whitelist of hosts that the application is allowed to redirect to. This should be updated to include all the hosts that your application is expected to redirect to.

The is_safe_url() function checks if the provided URL is safe for redirection. It does this by checking if the URL is a relative URL or if it is an absolute URL that matches one of the allowed hosts.

If the next_url is not in the ALLOWED_HOSTS list or if it is not a safe URL, the code redirects to the home page instead. This prevents the application from redirecting to potentially malicious websites.

Remember, never trust user input blindly. Always sanitize and validate it before using it.

References

• 193. Lack of data validation - Host Header Injection