Cross-Site Scripting (XSS): Common Attacks

Definition of Cross-Site Scripting (XSS):

• XSS is a vulnerability that allows unauthorized individuals to inject client-side scripts into a web application.

• These malicious scripts can come from various sources, most commonly through HTTP parameters like form inputs, but also through HTTP headers, cookies, JSON/XML files, databases, and uploaded files.

Dangers of Cross-Site Scripting:

• Credential Harvesting: Attackers can steal user credentials.

• Session Hijacking: Attackers can take over user sessions.

• Facilitation of Cross-Site Request Forgery (CSRF): XSS can assist in carrying out CSRF attacks.

• Cookie Theft: Attackers can steal cookies or locally stored data.

• Privilege Escalation: Attackers can elevate privileges within the application.

• Redirecting to Malicious Sites: Users can be redirected to harmful sites.

Prevalence of XSS:

• XSS is one of the most common vulnerabilities found in web applications.

• Examples:

  • WordPress and Drupal often have XSS as the most common vulnerability.

  • Risk Based Security study showed XSS as the highest occurring vulnerability across many products (2007-2015 data).

• Real-World Exploits:

  • eBay was targeted with phishing attacks based on XSS.

  • Apache Foundation was hacked, with XSS being the initial point of attack.

OWASP and SANS Listings:

• OWASP Top 10: XSS is a common entry in the list of top web vulnerabilities.

• SANS Top 25: XSS ranks 4th on this list, highlighting its danger.

Mechanism of XSS:

• Example of a Simple Application:

  • An application with a simple data entry form for username, password, and role.

  • If there are no checks on the input or output, any data entered (including HTML tags or scripts) will be stored and rendered as is.

Stored vs. Reflected XSS:

• Stored XSS: More dangerous because the malicious script is stored in the database and affects multiple users. It can lead to privilege escalation.

• Reflected XSS: Less dangerous, typically sent via email or a malicious link, affecting only one user.

Example of a Malicious Attack:

• A malicious user can inject scripts that mimic legitimate prompts (like session expiration dialogs) to trick users into entering their credentials.

• These credentials can be stolen and used to impersonate the user, potentially across multiple applications.

Conclusion:

• XSS is a serious security issue that needs to be mitigated to prevent significant harm to both the application and its users.