Sebastian Lekies

25 million flows later: large-scale detection of DOM-based XSS

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.

From Facepalm to Brain Bender: Exploring Client-Side Cross-Site Scripting

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.

Biting the hand that serves you: a closer look at client-side flash proxies for cross-domain requests

Client-side Flash proxies provide an interface for JavaScript applications to utilize Flashs cross-domain HTTP capabilities. However, the subtle differences in the respective implementations of the same-origin policy and the insufficient security architecture of the JavaScript-to-Flash interface lead to potential security problems. We comprehensively explore these problems and conduct a survey of five existing proxy implementation. Furthermore, we propose techniques to avoid the identified security pitfalls and to overcome the untrustworthy interface between the two technologies

DEMACRO: defense against malicious cross-domain requests

In the constant evolution of the Web, the simple always gives way to the more complex. Static webpages with click-through dialogues are becoming more and more obsolete and in their place, asynchronous JavaScript requests, Web mash-ups and proprietary plug-ins with the ability to conduct cross-domain requests shape the modern user experience. Three recent studies showed that a significant number of Web applications implement poor cross-domain policies allowing malicious domains to embed Flash and Silverlight applets which can conduct arbitrary requests to these Web applications under the identity of the visiting user. In this paper, we confirm the findings of the aforementioned studies and we design DEMACRO, a client-side defense mechanism which detects potentially malicious cross-domain requests and de-authenticates them by removing existing session credentials. Our system requires no training or user interaction and imposes minimal performance overhead on the users browser.