CCSP: Controlled relaxation of content security policies by runtime policy composition

Content Security Policy (CSP) is a W3C standard designed to prevent and mitigate the impact of content injection vulnerabilities on websites by means of browser-enforced security policies. Though CSP is gaining a lot of popularity in the wild, previous research questioned one of its key design choices, namely the use of static white-lists to define legitimate content inclusions. In this paper we present Compositional CSP (CCSP), an extension of CSP based on runtime policy composition. CCSP is designed to overcome the limitations arising from the use of static white-lists, while avoiding a major overhaul of CSP and the logic underlying policy writing. We perform an extensive evaluation of the design of CCSP by focusing on the general security guarantees it provides, its backward compatibility and its deployment cost. We then assess the potential impact of CCSP on the web and we implement a prototype of our proposal, which we test on major websites. In the end, we conclude that the deployment of CCSP can be done with limited efforts and would lead to significant benefits for the large majority of the websites.