AntiSamy 1.0 Released - Protecting web applications from malicious HTML and CSS

Posted by Gavin Terrill on Dec 03, 2007 10:00 PM

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Cross Site Scripting (XSS) is a major security issue facing developers who wish to allow their users to submit content containing HTML and CSS. A new project on OWASP known as the "AntiSamy" project, aims to offer a comprehensive, policy driven, API that validates and sanitizes input, as well as providing user feedback on the filtering process. The project's home page describes the API:

Technically, it is an API for ensuring user-supplied HTML/CSS is in compliance within an application's rules. Another way of saying that could be: It's an API that helps you make sure that clients don't supply malicious cargo code in the HTML they supply for their profile, comments, etc. that gets persisted on the server. The term malicious code in terms of web applications is usually regarded only as JavaScript. Cascading Stylesheets are only considered malicious when they invoke the JavaScript engine. However, there are many situations where "normal" HTML and CSS can be used in a malicious manner.

What sets this API apart, according to lead developer Arshan Dabirsiaghi, is its user friendly approach:

The methodology of AntiSamy is unique in that it is built on a positive security model in both the format of the HTML document and the content within the document. It's also unique in that it attempts to help the user tune their input to pass validation in a cooperative spirit, rather than treating users as potential attackers which is how all contemporary security mechanisms work.

In the paper "Towards Malicious Code Detection and Removal" (PDF), Dabirsiaghi describes the phases involved in the filtering process:

1. Pre-Processing. Use of NekoHTML to perform HTML Sanitization.
2. Processing. Tag/CSS Validation Rules are applied depth first using three processing modes - Filter, Truncate and Validate. Filter actions remove tags that are not allowed, but retains their content. Truncating removes forbidden tag attributes and child nodes. Validation involves matching rules in the policy file with tag/attribute combinations, ensuring only valid tags are permitted.
3. Remediation. If validation fails during processing, the policy file is consulted to determine how to handle the tag and its contents. Options include removing the tag and its content, filtering out the tag and leaving the content, and removing the attribute from the tag.

The first release includes of AntiSamy includes a Java implementation, with .Net and PHP versions available soon.

Integration into a Java application is simple:

import org.owasp.validator.html.*;

Policy policy = new Policy(POLICY_FILE_LOCATION);
AntiSamy as = new AntiSamy();
CleanResults cr = as.scan(dirtyInput, policy);
MyUserDAO.storeUserProfile(cr.getCleanHTML()); // some custom function

The CleanResults class provides methods to access useful information about the filtering process:

• getErrorMessages() - a list of String error messages
• getCleanHTML() - the clean, safe HTML output
• getCleanXMLDocumentFragment() - the clean, safe XMLDocumentFragment which is reflected in
• getCleanHTML()
• getScanTime() - returns the scan time in seconds

Downloads of AntiSamy, available under a BSD style license, are available from the Google code project page.

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   problems running the code

   Dec 4, 2007 10:15 AM by Vojtech Kolencik

   Well, I wanted to try this and I haven't found the predefined policy files anywhere. Also, I think the only place to find out how the policy files should be written is the source code. Also, the Policy class must be instantiated using a static factory method, as the constructor used in this article (as well as on the project's homepage) is declared private.

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   Re: problems running the code

   Dec 4, 2007 10:39 AM by Arshan Dabirsiaghi

   Vojtech, Sorry you've been having issues. You're right about the constructor, I will have to change those code snippets. Also, the policy files are linked from a page on my blog. Or you can navigate directly to the test page which contains the actual policy files. The Google Project page is quite buggy, and I haven't been able to upload anything since I uploaded the rest of the project. As soon as I can upload the policy files - I will upload them to the project. Until then, please use the URLs above. Feel free to email me (arshan.dabirsiaghi [at the] gmail.com) directly if you have any issues. Cheers, Arshan

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   Why not simple CSS and HTML validator?

   Dec 4, 2007 10:58 AM by Balaji D Loganathan

   Hi, For HTML/CSS malicious code ? Why not simply use the online W3C CSS and HTML validator? Or even firefox extension like CSE HTML Validator? Thank you Regards Balaji D Loganathan

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   Re: problems running the code

   Dec 4, 2007 11:00 AM by Vojtech Kolencik

   Great, it works now. Thanks for the info (and for a useful library, also).

   Reply

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   Re: Why not simple CSS and HTML validator?

   Dec 4, 2007 11:02 AM by Arshan Dabirsiaghi

   This is for preventing XSS and phishing attacks, not for validating the format of an HTML document or a stylesheet. Check out the project description.

   Reply

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