Sub-Domain Based XSS

WHAT IS SUB-DOMAIN BASED XSS?

Sub-Domain Based XSS occurs when a vulnerability exists on a subdomain of a website, allowing attackers to execute malicious scripts. Since subdomains are often treated as part of the parent domain, they can share cookies or other sensitive data, potentially putting the entire domain ecosystem at risk.

HOW DOES SUB-DOMAIN BASED XSS WORK?

Vulnerable Subdomain: A subdomain (e.g., vuln.example.com) is found to have an XSS vulnerability, such as improper input sanitization.

Cookie or Data Theft: Since many websites use *.example.com for authentication cookies or other sensitive data, the attacker can use the vulnerable subdomain to steal cookies, tokens, or perform actions on behalf of users.

Payload Execution: The attacker lures victims into visiting the vulnerable subdomain, which then executes malicious JavaScript (e.g., stealing session cookies, redirecting to phishing pages, or performing CSRF attacks).

Privilege Escalation: If the main domain (example.com) trusts the subdomain, the attacker might gain access to more critical resources.

MITIGATION FOR SUB-DOMAIN BASED XSS:

Restrict Cookie Scope – Avoid *.example.com, use HttpOnly, Secure, and SameSite=Strict.

Content Security Policy (CSP) – Block inline scripts and untrusted sources.

Input Validation & Output Encoding – Sanitize inputs and encode outputs properly.

Separate Privileges – Host sensitive services separately, sandbox risky subdomains.

Disable Unnecessary Features – Restrict JavaScript execution and enforce Subresource Integrity (SRI).

CONCLUSION

Sub-domain based XSS is a serious security risk that arises when a vulnerable subdomain allows an attacker to execute malicious scripts, potentially compromising user data and main domain security. To mitigate this threat, organizations should enforce strict cookie policies, implement a robust Content Security Policy (CSP), sanitize user inputs, and separate privileges between subdomains. Regular security audits and proactive defenses are essential to prevent exploitation. By securing subdomains, businesses can protect users and maintain overall domain integrity.