Nowadays, applications expose administrative endpoints to the Web that can be used for a plethora of security sensitive actions. Typical use cases range from running small snippets of user-provided code for rapid prototyping, administering databases, and running CI/CD pipelines, to managing job scheduling on whole clusters of computing devices. While accessing these applications over the Web make the lives of their users easier, they can be leveraged by attackers to compromise the underlying infrastructure if not properly configured.

Cross-Site Scripting (XSS) is an unremitting problem for the Web. Since its initial public documentation in 2000 until now, XSS has been continuously on top of the vulnerability statistics. Even though there has been a considerable amount of research and developer education to address XSS on the source code level, the overall number of discovered XSS problems remains high. Because of this, various approaches to mitigate XSS have been proposed as a second line of defense, with HTML sanitizers, Web Application Firewalls, browser-based XSS filters, and the Content Security Policy being some prominent examples. Most of these mechanisms focus on script tags and event handlers, either by removing them from user-provided content or by preventing their script code from executing.

Content Security Policy is a web platform mechanism designed to mitigate cross-site scripting (XSS), the top security vulnerability in modern web applications. In this paper, we take a closer look at the practical benefits of adopting CSP and identify significant flaws in real-world deployments that result in bypasses in 94.72% of all distinct policies. We base our Internet-wide analysis on a search engine corpus of approximately 100 billion pages from over 1 billion hostnames; the result covers CSP deployments on 1,680,867 hosts with 26,011 unique CSP policies -- the most comprehensive study to date. We introduce the security-relevant aspects of the CSP specification and provide an in-depth analysis of its threat model, focusing on XSS protections. We identify three common classes of CSP bypasses and explain how they subvert the security of a policy. We then turn to a quantitative analysis of policies deployed on the Internet in order to understand their security benefits. We observe that 14 out of the 15 domains most commonly whitelisted for loading scripts contain unsafe endpoints; as a consequence, 75.81% of distinct policies use script whitelists that allow attackers to bypass CSP. In total, we find that 94.68% of policies that attempt to limit script execution are ineffective, and that 99.34% of hosts with CSP use policies that offer no benefit against XSS. Finally, we propose the "strict-dynamic" keyword, an addition to the specification that facilitates the creation of policies based on cryptographic nonces, without relying on domain whitelists. We discuss our experience deploying such a nonce-based policy in a complex application and provide guidance to web authors for improving their policies.

Although studies have shown that at least one in ten Web pages contains a client-side XSS vulnerability, the prevalent causes for this class of Cross-Site Scripting have not been studied in depth. Therefore, in this paper, we present a large-scale study to gain insight into these causes. To this end, we analyze a set of 1,273 real-world vulnerabilities contained on the Alexa Top 10k domains using a specifically designed architecture, consisting of an infrastructure which allows us to persist and replay vulnerabilities to ensure a sound analysis. In combination with a taint-aware browsing engine, we can therefore collect important execution trace information for all flaws. Based on the observable characteristics of the vulnerable JavaScript, we derive a set of metrics to measure the complexity of each flaw. We subsequently classify all vulnerabilities in our data set accordingly to enable a more systematic analysis. In doing so, we find that although a large portion of all vulnerabilities have a low complexity rating, several incur a significant level of complexity and are repeatedly caused by vulnerable third-party scripts. In addition, we gain insights into other factors related to the existence of client-side XSS flaws, such as missing knowledge of browser-provided APIs, and find that the root causes for Client-Side Cross-Site Scripting range from unaware developers to incompatible first- and third-party code.

In recent years, the Web witnessed a move towards sophis- ticated client-side functionality. This shift caused a signifi- cant increase in complexity of deployed JavaScript code and thus, a proportional growth in potential client-side vulnera- bilities, with DOM-based Cross-site Scripting being a high impact representative of such security issues. In this paper, we present a fully automated system to detect and validate DOM-based XSS vulnerabilities, consisting of a taint-aware JavaScript engine and corresponding DOM implementation as well as a context-sensitive exploit generation approach. Using these components, we conducted a large-scale analysis of the Alexa top 5000. In this study, we identified 6167 unique vulnerabilities distributed over 480 domains, show- ing that 9,6% of the examined sites carry at least one DOM- based XSS problem.

This paper presents "BetterAuth", an authentication protocol for Web applications. Its design is based on the experiences of two decades with the Web. BetterAuth addresses existing attacks on Web authentication, ranging from network attacks to Cross-site Request Forgery up to Phishing. Furthermore, the protocol can be realized completely in standard JavaScript. This allows Web applications an early adoption, even in a situation with limited browser support.