Hack with confidence.
Page xml-validator.php is vulnerable to at least the following:
HTML Injection: It is possible to inject your own HTML into this page because the input is not encoded prior to be used as output. Determine which input field contributes output here and inject HTML, CSS, and/or Javascripts in order to alter the client-side code of this page.
System File Compromise: It is possible to access system files by injecting input parameters with the pathnames of system files. The web application will fetch the system files instead of application files. The system files may be displayed and/or included in page output. Remember web applications are usually served from a system directory like /var/www or C:XAMPP. You may need to move up directories.
XML External Entity Injection Attack: This page parses XML which the user can influence. If external entities embedded in the XML contain system file directives, it is possible to cause the page to load system files and include the contents in the XML output.
Reflected Cross-Site Scripting: This page is vulnerable to reflected cross-site scripting because the input is not encoded prior to be used as output. Determine which input field contributes output here and inject scripts. Try to redirect the user to the capture-data.php page which records cookies and other parameters. Visit the captured-data.php page to view captured data.