by Ben Stock & Martin Johns & Sebastian Lekies

When it comes to web security, there is the one policy to rule them all: The Same-origin Policy. Thanks to this policy, sites hosted on disjunct origins are nice and cleanly separated, thus preventing the leakage of sensitive information into the hands of unauthorized parties. Unfortunately, HTML predates the Same-origin Policy and, thus, was not designed with the origin-based security model in mind. In consequence, HTML tags can freely reference cross-domain locations and include cross-domain content in their hosting web pages.

In this talk, we will present an attack, resulting from this circumstance, that has been widely overlooked in the past but affects a surprisingly high number of Web sites: Information leakage via cross-domain script inclusion.

Modern web sites frequently generate JavaScript on-the-fly via server-side scripting, incorporating personalized user data in the process. Thanks to HTML's general ignorance of the Same-origin Policy, an attacker is able to include such dynamic scripts into web pages under his control using script-tags pointing to the vulnerable site. This, in turn, allows him to learn many of the secrets contained in these scripts, through the scripts interaction with the page it is included in. In our experiments, we were able to obtain personal information such as name & address of the logged-in user, leak CSRF tokens, read the users emails, and occasionally fully compromise the user's account. All possible by simply including a script-URL into one of our web pages.

To systematically investigate the issue, we conducted a study on its prevalence in a set of 150 top-ranked domains, in which we observed that a third of the examined sites utilize dynamic JavaScript. Using our attack techniques, we able to leak sensitive data from more than 80% of these sites via remote script inclusion. In the talk we will present the study in general, and the most interesting cases in detail, showing the wide range of possible attack variations along with a bag of tricks how the including page can be prepared to efficiently leak a script's secrets. Furthermore, we present an efficient detection mechanism, in the form of a browser extension, as well as defensive measure, which enable robust protection.

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