FirehaK / Yobi

Yara Based Detection Engine for web browsers

Yobi

Install Yobi Here

Yara Based Detection for web browsers

Yobi is a basic firefox extension which allows to run public or private YARA rules on all scripts and pages rendered by the browser. Yobi saves files that trigger its rules and allows further inspection of them.

Yobi is completly serverless - no telemtry or other information is collected.

Manual Installation

1. clone the repo.
2. Go to about:debugging in firefox or other Gecko based browser, click "This Firefox"-> Load Temporary Add on and select manifest.json.
3. Done!

What can Yobi do?

1. Capture any file requested by the web browser and identified as malicious by a YARA rule.
2. Use custom YARA rules.
3. Download the malicious files (as zip, default password is "infected").
4. Query the file hash in VirusTotal.

YARA rules

YARA rules are fetched from a repository of JS rules I created: js-yara-rules. The repo consists of free JS rules I found on the internet and some I wrote myself. Feel free to create pull requests for additional rellevant rules.

You can change the yara rules the extension uses under Add-ons->Yobi->Preferences

Right now, YARA version 4.0.5 is used. libyara-wasm will be updated shortly and Yobi will then run the latest YARA verions.

Yobi's Inner Workings

Execution Flow

Yobi uses the Gecko webrequests feature browser.webRequest.onBeforeRequest which enables it to intercept any request and response. Yobi saves the buffer and forward it. The YARA rules run asynchronously to that and alert whether a match is found.

Dependencies

Yobi Depends on the following libraries:

1. libyara-wasm - A porting of the whole YARA engine to wasm
2. SJCL - JS encryption library used for calculating sha256.
3. jszip - A compact JS library to create zip files. used PR 6969 that added the option to encrypt the archive.
4. Bootstrap
5. jQuery

Why doesn't Yobi block the malicious scripts?

Preventing any script to run before running YARA rules on it would create a significant delay for the user.=

Continuing Development

This version is still very basic and should serve as a prototype only. Please open issues and pull request for new features or bugs you encounter.

Contact and Feedback

Contact me via twitter - @imp0rtp3

Screenshots