By Porter Throckmorton, CRTO, OSCP, CBBH, PNPT, eWPT
Senior Consultant, Technical Testing Services
It is likely you have seen the number 1 in an alert box if you have ever had a penetration test performed on your web application. The alerted number is the most common proof-of-concept for Cross-Site Scripting.
Cross-Site Scripting (XSS) is a common and dangerous vulnerability discovered during web application penetration tests where the penetration tester injects arbitrary JavaScript into the application. In turn, the application executes the injected code in an unsuspecting victim’s browser. The injected code can drive a number of cyberattacks, from credential gathering to social engineering attacks. Due to the dynamic and interactive nature of modern web platforms, especially those that incorporate user-generated content, the risk of XSS remains persistent.
Security testers and developers alike encounter XSS across a wide spectrum of applications — from e-commerce to enterprise dashboards, and especially in healthcare systems where sensitive data, complex user roles, and third-party integrations increase the attack surface. Despite being well-documented in OWASP’s Top Ten list for years, XSS continues to plague systems due to improper handling of user input and inconsistent security hygiene across development teams.
Understanding Different XSS Types – Stored vs. Reflected XSS
Stored XSS (Persistent)
Stored XSS occurs when malicious input submitted by a user is saved on the server and then served back to other users without proper sanitization. This means the payload lives within the application and is often found in comment fields, user profiles, file metadata, or content management systems — and executes every time a victim views the affected page.
Example:
A user enters a comment on a health portal containing the following payload:
<script>alert(1)</script>
If the backend saves this without encoding or sanitizing, every user who loads the comment section triggers the script and an alert box pops up with the number 1 inside.
Reflected XSS (Non-Persistent)
Reflected XSS occurs when malicious data is sent as part of a request (usually via URL parameters, forms, or headers) and is immediately echoed back in the application’s response.
Example:
A crafted URL such as:
https://healthapp.com/search?q=<script>alert(1)</script>
If the server reflects the q parameter in its response without encoding, the payload executes in the victim’s browser once they click the link.
Reflected XSS is typically used in phishing campaigns where an attacker tricks a user into clicking a malicious URL, making it ideal for social engineering attacks.
Common Payloads: alert(1)
The most basic and universally recognized XSS payload is:
<script>alert(1)</script>
While simple, this payload is used extensively during testing to confirm the presence of a vulnerability. It doesn’t cause real damage but proves that arbitrary JavaScript execution is possible. Once that’s confirmed, attackers may pivot to more complex payloads involving data exfiltration, session hijacking, or persistent browser hooks.
Other examples include:
- <img src=x onerror=alert(1)>
- <svg onload=alert(1)>
- <iframe src=”javascript:alert(1)>
These variations demonstrate how different HTML contexts can be exploited.
Marked as High Severity
Why is XSS High Risk?
XSS vulnerabilities are almost always classified as high-severity issues in security assessments for several compelling reasons:
- Prevalence in Web Applications: As web apps become more complex, particularly in healthcare where portals allow communication, document uploads, and real-time feedback, the exposure to XSS increases dramatically.
- CSRF and Chained Attacks: XSS can be leveraged to carry out Cross-Site Request Forgery (CSRF), effectively allowing an attacker to perform unauthorized actions on behalf of a user.
- Session Stealing: With access to session cookies (if not marked HttpOnly), attackers can hijack authenticated sessions and impersonate users, which is especially critical in applications with PHI (Protected Health Information).
- Execution of Unintended Actions: Malicious scripts can modify page behavior, impersonate clicks, or redirect users to malicious sites.
- Website Defacement: Public-facing applications, such as patient portals, can be defaced using stored XSS to damage a healthcare organization’s reputation.
- BeEF Integration: The Browser Exploitation Framework (BeEF) allows attackers to hook into a victim’s browser and execute more advanced commands post-XSS, such as gathering system info or even turning on a victim’s webcam and microphone.
- XSS Enables Everything: Once JavaScript execution is achieved, the attacker essentially has the same level of access to the page as the user, enabling a wide range of abuses: from data theft to surveillance and fraud.
How XSS Affects Healthcare Applications
Healthcare organizations face unique challenges due to regulatory, technical, and operational constraints. XSS can have particularly devastating consequences in these environments:
File Upload Vectors
- File Names: Uploading a file named <script>alert(1)</script>.pdf might trigger a stored XSS if the application displays filenames without encoding them.
- HTML Files: If the application allows .html files without strict MIME type checking, an uploaded file may contain executable code.
- SVG Uploads: SVG is an XML-based format that allows embedded JavaScript, which can be exploited via onload or onclick events.
- PDF Rendering Libraries: Many document viewers or converters have known vulnerabilities that mishandle embedded scripts or improperly sanitize content. This creates attack vectors for stored XSS when viewing PDFs within the app.
User Interaction Points
- Patient Comments or Feedback Forms: These are frequently user-driven and susceptible to stored XSS if not sanitized correctly.
- Internal Notes: Sometimes, administrative features (e.g., clinician notes) are assumed to be safe but can become XSS entry points if privilege separation isn’t enforced.
- Third-party Widgets: Integrations with external tools (e.g., live chat, analytics, e-signatures) can unintentionally expose XSS vectors through less secure iframes or script injection.
In a healthcare context, successful exploitation may lead to:
- Disclosure of PHI
- Tampering with medical records
- Credential theft of doctors or nurses
Remediation Strategies
Preventing XSS requires a multi-layered approach focused on strict input handling, secure coding practices, and continuous validation:
1. Input Validation
- Implement strict allow-lists (e.g., allow only alphanumeric characters for usernames).
- Reject inputs that contain suspicious HTML or JavaScript code unless explicitly required.
- Use regular expressions to filter form data before processing.
2. Output Encoding
- Encode user input on output, especially when injecting data into HTML, JavaScript, or attribute contexts.
- Use libraries like:
- OWASP Java Encoder
- Microsoft’s AntiXSS
- DOMPurify (for client-side sanitation)
- Always HTML-encode characters like <, >, &, ‘, and ” before rendering to the DOM.
3. Content Security Policy (CSP)
- Implement a strong CSP header to limit the execution of inline scripts and restrict resource loading.
- Disallow unsafe-inline and use nonces or hashes to allow trusted scripts.
4. Keep Libraries Updated
- Ensure all JavaScript libraries and third-party plugins are updated regularly.
- Monitor CVEs for known vulnerabilities in rendering libraries (PDF.js, video players, etc.).
5. Use Security Headers
- Prevent session hijacking and other dangerous XSS attacks by setting HTTP Security Headers, such as HttpOnly, Secure, and SameSite attributes.
6. File Upload Security
- Restrict file types using allow-lists.
- Rename uploaded files to neutral identifiers.
- Scan uploaded files for embedded scripts.
- Serve uploads from a separate domain to isolate execution context (prevent DOM-based attacks).
Conclusion
Cross-Site Scripting remains a pervasive and high-severity risk in web application security, especially within the healthcare industry where the stakes are uniquely high. It allows attackers to execute arbitrary JavaScript, steal sensitive data, impersonate users, and compromise patient trust.
Healthcare providers, developers, and security professionals must prioritize XSS mitigation through proper input validation, output encoding, content security policies, and regular code auditing. By treating XSS not just as a “pentest finding” but a real-world threat, organizations can significantly reduce risk and improve patient data security.
For security teams and developers seeking to strengthen defenses against XSS and similar vulnerabilities, integrating secure development lifecycles (SDLC), routine threat modeling, and application security testing is essential to staying ahead.
The post Cross-Site Scripting (XSS): More Than a Pesky Alert appeared first on Clearwater.
