This repository contains the code for the DIMVA'18 paper: "JaSt: Fully Syntactic Detection of Malicious (Obfuscated) JavaScript".
Please note that in its current state, the code is a Poc and not a fully-fledged production-ready API.

Our implementation, which aims at detecting malicious JavaScript samples, is divided into several packages with distinct functionalities:

• js for the detection of valid JavaScript code;
• features for the extraction of specific features from JavaScript inputs;
• clustering for the classification and clustering of JavaScript documents.

Detection of JavaScript samples respecting the grammar defined by ECMA-International, detection of broken JavaScript, and files not written in JavaScript.
To use this tool: python3 <path-of-js/is_js.py> --help.

An AST-based analysis of JavaScript samples can be performed. This study is based on a frequency analysis of the n-grams present in the considered files.

• Detection of malicious JavaScript documents.
  To use this tool:

1. python3 <path-of-clustering/learner.py> --help;
2. python3 <path-of-clustering/updater.py> --help;
3. python3 <path-of-clustering/classifier.py> --help.

• Clustering of JavaScript samples into k (configurable) families.
  To use this tool: python3 <path-of-clustering/cluster.py> --help.

• The system requirements are given in install.sh;
• To launch the main function of a package, see the previous Section Features;
• Application examples of our modules are given in examples.pdf;
• The complete documentation can be consulted using Python's build-in function help(<name-of-the-module>), or can be generated in HTML format with pydoc: python3 <path-of-pydoc> -w <name-of-the-module>.

If you use JaSt for academic research, you are highly encouraged to cite the following paper:

@inproceedings{fass2018jast,
    author="Fass, Aurore and Krawczyk, Robert P. and Backes, Michael and Stock, Ben",
    title="{\textsc{JaSt}: Fully Syntactic Detection of Malicious (Obfuscated) JavaScript}",
    booktitle="Proceedings of the International Conference on Detection of Intrusions and Malware, and Vulnerability Assessment~(DIMVA)",
    year="2018"
}

JavaScript is a browser scripting language initially created to enhance the interactivity of web sites and to improve their user-friendliness. However, as it offloads the work to the user's browser, it can be used to engage in malicious activities such as Crypto Mining, Drive-by Download attacks, or redirections to web sites hosting malicious software. Given the prevalence of such nefarious scripts, the anti-virus industry has increased the focus on their detection. The attackers, in turn, make increasing use of obfuscation techniques, so as to hinder analysis and the creation of corresponding signatures. Yet these malicious samples share syntactic similarities at an abstract level, which enables to bypass obfuscation and detect even unknown malware variants.

In this paper, we present JaSt, a low-overhead solution that combines the extraction of features from the abstract syntax tree with a random forest classifier to detect malicious JavaScript instances. It is based on a frequency analysis of specific patterns, which are either predictive of benign or of malicious samples. Even though the analysis is entirely static, it yields a high detection accuracy of almost 99.5% and has a low false-negative rate of 0.54%.

This is a research work and its intend is to publish the results, to make the results verifiable, and to promote information security. It does not necessarily reflect the official policy or position of the Bundesamt für Sicherheit in der Informationstechnik (BSI). Although the author has made every effort to ensure that the information in this publication was correct, the author does not assume and hereby disclaims any liability to any party for any loss, damage, or disruption caused by errors or omissions, whether such errors or omissions result from negligence, accident, or any other cause.

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Esprima, created and maintained by Ariya Hidayat has been used to perform both lexical and syntactic analysis of JavaScript files.

"Copyright JS Foundation and other contributors, https://js.foundation/

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

• Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."