Why is CSP Failing? Trends and Challenges in CSP Adoption

Content Security Policy (CSP) has been proposed as a principled and robust browser security mechanism against content injection attacks such as XSS. When configured correctly, CSP renders malicious code injection and data exfiltration exceedingly difficult for attackers. However, despite the promise of these security benefits and being implemented in almost all major browsers, CSP adoption is minuscule—our measurements show that CSP is deployed in enforcement mode on only 1% of the Alexa Top 100.

In this paper, we present the results of a long-term study to determine challenges in CSP deployments that can prevent wide adoption. We performed weekly crawls of the Alexa Top 1M to measure adoption of web security headers, and find that CSP both significantly lags other security headers, and that the policies in use are often ineffective at actually preventing content injection. In addition, we evaluate the feasibility of deploying CSP from the perspective of a security-conscious website operator. We used an incremental deployment approach through CSP’s report-only mode on four websites, collecting over 10M reports. Furthermore, we used semi-automated policy generation through web application crawling on a set of popular websites. We found both that automated methods do not suffice and that significant barriers exist to producing accurate results. Finally, based on our observations, we suggest several improvements to CSP that could help to ease its adoption by the web community.