TL;DR
An attacker is changing options in your forms. This usually happens because you’re not properly validating user input before using it to build the form. We’ll show you how to fix this by escaping output, using whitelists for allowed values, and employing CSRF protection.
1. Understand the Problem
Attackers can inject malicious code or alter data if your application doesn’t carefully check what users submit. In this case, they’re likely injecting HTML or JavaScript into form options, which then gets displayed to other users. This could lead to cross-site scripting (XSS) attacks or simply broken functionality.
2. Escape Output
The most important step is to always escape user input before displaying it in your HTML. Escaping converts potentially dangerous characters into safe equivalents. The specific escaping method depends on where the data is being used (HTML, JavaScript, URL, etc.).
- HTML Escaping: Use this when displaying data within HTML tags.
<!-- Example in PHP -->
- JavaScript Escaping: Use this when embedding data within JavaScript code.
<!-- Example in PHP -->
3. Use Whitelists for Allowed Values
Instead of trying to block bad input, explicitly allow only known-good values. This is particularly effective for form options like dropdown lists or radio buttons.
- Define a list: Create an array (or similar data structure) containing the valid option values.
- Check against the list: Before using any user input to populate the form, verify that it exists in your whitelist.
<!-- Example in PHP -->
4. Implement CSRF Protection
Cross-Site Request Forgery (CSRF) attacks can allow attackers to submit malicious requests on behalf of logged-in users. While not directly related to form option manipulation, it’s a good security practice to protect your forms against CSRF.
- Generate a unique token: Create a random, unpredictable token for each user session or form submission.
- Include the token in the form: Add a hidden field containing the token to your form.
- Validate the token on submission: When the form is submitted, verify that the received token matches the one you generated.
<!-- Example HTML for CSRF Token -->
<input type="hidden" name="csrf_token" value="">5. Server-Side Validation
Never trust client-side validation alone! Always validate user input on the server side before processing it. Client-side validation is easily bypassed.
- Check data types: Ensure that numeric fields contain numbers, email fields contain valid email addresses, etc.
- Check lengths: Limit the maximum length of text fields to prevent buffer overflows or other issues.
6. Content Security Policy (CSP)
Consider implementing a Content Security Policy (CSP). CSP allows you to define which sources your browser is allowed to load resources from, mitigating XSS attacks.
7. Regular Security Audits
Regularly review your code for security vulnerabilities and keep your software up-to-date with the latest security patches.