Modern XSS Challenges: Beyond the Basics

Modern XSS Challenge By Dhanush Nehru

The landscape of web security has evolved dramatically, and the days of simple exploits like <script>alert(1)</script> or spinning up a quick python -m http.server to exfiltrate data are fading into obscurity.

Modern browser protections and heightened developer awareness have rendered many classic Cross-Site Scripting (XSS) techniques impractical outside of localhost environments.

We often see Proof of Concepts (PoCs) that ignore these contemporary hurdles, leaving us restless about their real-world applicability.

So, let’s dive into some frequent obstacles encountered when crafting full-fledged XSS exploits against today’s applications. This isn’t about reinventing the wheel or dissecting every browser’s XSS filter bypasses — rather, it’s about creatively adapting what we know to overcome application-specific barriers.

Photo by Luther.M.E. Bottrill on Unsplash

Here’s a rundown of common pitfalls and practical workarounds, illustrated with fresh examples.

The InnerHTML Trap: A Silent XSS Blocker

Dynamic web pages are the norm now, with the Document Object Model (DOM) constantly reshaped by API responses. A frequent (and risky) practice is injecting API-fetched content directly into an element’s innerHTML. Consider this scenario:

Example API Interaction

$ curl -X POST -H "Content-Type: application/json" --cookie "SESSION=abc123" \
  -d '{"user_id":1234, "display_name":"<script>alert(1)</script>", "bio":"Hello"}' \
  http://example.app/api/profile

{"status":"Profile updated"}

$ curl --cookie "SESSION=abc123" http://example.app/api/profile/name

{"display_name":"<script>alert(1)</script>"}

Client-Side Code

function fetchDisplayName() {
  const xhr = new XMLHttpRequest();
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
      const response = JSON.parse(xhr.responseText);
      document.getElementById('profile-name').innerHTML = response.display_name;
    }
  };
  xhr.open("GET", "/api/profile/name", true);
  xhr.send();
}