IceShield: Detection and Mitigation of Malicious Websites with a Frozen DOM

Mario Heiderich, Tilman Frosch, Thorsten Holz

14th International Symposium on Recent Advances in Intrusion Detection (RAID), Menlo Park, CA, September 2011


Due to its flexibility and dynamic character, JavaScript has become an important tool for attackers. The widespread scripting language often helps them to perform a broad variety of malicious activities, for example to initiate drive-by download exploits or to execute clickjack- ing attacks. Current defense mechanisms as well as reactive analysis and forensic approaches are often slow or complicated to set up and conduct since an attacker can use many different ways to obfuscate the code or make it hard to obtain a copy of the code in the first place.

In this paper, we introduce a novel approach to analyze this class of attacks by demonstrating how dynamic analysis of websites can be accomplished directly in the browser. We present IceShield, a JavaScript based tool that enables in-line dynamic code analysis as well as de-obfuscation, and a set of heuristics to detect attempts of attacking either a website or the user accessing its contents. Special care needs to be taken to implement the instrumentation in a robust and tamper resistant way since an attacker should not be able to bypass our detection process. We show how features of ECMA Script 5 can be used to freeze object properties, so they cannot be modified during runtime. We implemented a prototype version of IceShield and demonstrate that it detects malicious websites with a small overhead even on devices with limited computing power such as smartphones. Furthermore, IceShield can mitigate detected attacks by changing suspicious elements, so they do not cause harm anymore, thus actually protecting users from such attacks.


Tags: javascript, Malware, web security