The vulnerability stems from insufficient output sanitization in the web service token display. Moodle's security advisory (MSA-21-0026) specifically mentions ID numbers in the token list required additional sanitizing. The token management interface (admin/webservice/tokens.php) would be the logical location for rendering this data. Moodle typically uses format_string() or other output filters for XSS protection, and the vulnerability suggests this sanitization was missing in the token list rendering function. The high confidence comes from the direct correlation between the vulnerability description and the token management interface's responsibility to display user-supplied ID numbers.