Cross-site Scripting (XSS) has emerged to one of the most prevalent type of security vulnerabilities. While the reason for the vulnerability primarily lies on the serverside, the actual exploitation is within the victim's web browser on the client-side. Therefore, an operator of a web application has only very limited evidence of XSS issues. In this paper, we propose a passive detection system to identify successful XSS attacks. Based on a prototypical implementation, we examine our approach's accuracy and verify its detection capabilities. We compiled a data-set of 500.000 individual HTTP request/response-pairs from 95 popular web applications for this, in combination with both real word and manually crafted XSS-exploits; our detection approach results in a total of zero false negatives for all tests, while maintaining an excellent false positive rate for more than 80% of the examined web applications.