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Server side cross-site scripting

Need​

Mitigation of server-side cross-site scripting attacks

Context​

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

Description​

Non compliant code​

from django.http import HttpResponse
from django.views.decorators.http import require_http_methods

@require_http_methods(["POST"])
def post_comment(request):
    comment = request.POST['comment']
    return HttpResponse(f'Comment: {comment}')

In the above code, the application is using Django, a Python-based web framework. The function post_comment is designed to handle POST requests and display user comments. The comment is retrieved from the POST data using request.POST['comment'].

The vulnerability lies in the fact that the code directly uses the user's input (comment) without any form of sanitization or encoding. This means that if a user inputs a malicious script as a comment, the server will execute that script. This is known as server-side cross-site scripting (XSS).

For example, an attacker could post a comment like <script>malicious_code_here</script>. The server will then execute this script, potentially leading to unauthorized access, data theft, or other harmful actions.

Steps​

  • Sanitize the user input by using Django's built-in escaping functions to prevent any potentially harmful code from being executed.
  • Use Django's form and field validation to ensure that the input matches the expected format.
  • Implement Content Security Policy (CSP) headers to restrict the types of content that can be executed by the browser.
  • Use HttpOnly cookies to prevent cross-site scripting attacks.
  • Regularly update and patch your server and its software to protect against known vulnerabilities.

Compliant code​

from django.http import HttpResponse
from django.views.decorators.http import require_http_methods
from django.utils.html import escape

@require_http_methods(["POST"])
def post_comment(request):
    comment = escape(request.POST['comment'])
    return HttpResponse(f'Comment: {comment}')

The updated code now includes Django's built-in escape function, which is used to sanitize the user input. This function works by escaping any characters that have special meaning in HTML. This means that if a user tries to input something like &lt;script>malicious_code_here&lt;/script>, it will be turned into &lt;script&gt;malicious_code_here&lt;/script&gt;, which will be displayed as plain text instead of being executed as JavaScript.

This is a simple yet effective way to prevent server-side cross-site scripting (XSS) attacks. However, it's important to note that this is just one layer of defense and should be used in conjunction with other security measures such as form and field validation, Content Security Policy (CSP) headers, HttpOnly cookies, and regular server updates and patches.

References​

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