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When a web application allows users to construct URLs freely, it opens the door to potential XSS vulnerabilities. An attacker could input javascript:*, and if the server does not properly validate this input, it could lead to script execution.
Best Practices to Mitigate Stored XSS Risks
✅ Never Trust Client Input
- If the input contains
javascript:,data:, or similar schemes, the API should return a `401 Unauthorized` or an appropriate error response.
✅ Enforce Strict Input Validation
- If user-provided URLs are necessary, apply two layers of validation:
🔹 Full URL Validation → Ensure the format follows `https://example.com`
🔹 Path Validation → Restrict input to predefined relative paths like `/any/any` - If the origin of a full URL is not in the approved whitelist, return an error or
401 Unauthorized.
✅ Use a Backend Proxy to Manage File URLs
– Instead of allowing direct client input, route file access through a secure backend proxy that enforces validation and sanitization.
You Should Know:
1. Input Sanitization with Linux Commands
To filter malicious inputs, use `sed` or `grep` to detect and block dangerous patterns:
echo "$user_input" | grep -q -E 'javascript:|data:' && echo "Malicious input detected!"
#### **2. Automating URL Validation with Python**
import re
def validate_url(url):
if re.match(r'^https?://(?:[-\w.]|(?:%[\da-fA-F]{2}))+', url):
return True
return False
#### **3. Secure Proxy Setup with Nginx**
Configure Nginx to act as a proxy and enforce URL whitelisting:
location /proxy/ {
proxy_pass https://trusted-domain.com/;
if ($args ~* "javascript:|data:") { return 403; }
}
#### **4. Browser Security Headers**
Add these HTTP headers to prevent XSS:
Content-Security-Policy: default-src 'self'; script-src 'none' X-XSS-Protection: 1; mode=block
#### **5. JavaScript Sanitization**
Use DOMPurify to clean HTML inputs:
const clean = DOMPurify.sanitize(user_input);
### **What Undercode Say:**
Stored XSS remains a critical threat due to improper input handling. Developers must enforce strict validation, sanitize inputs, and use secure proxies. Linux commands like `grep` and `sed` help in filtering malicious patterns, while tools like DOMPurify and Nginx enhance security. Always implement CSP headers and automate URL checks to minimize risks.
### **Expected Output:**
A secure web application that blocks malicious inputs, validates URLs strictly, and prevents XSS attacks through proper backend checks and security headers.
**Reference:**
References:
Reported By: Muhamad Rizki – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
