OWASP AntiSamy

OWASP incubator

OWASP AntiSamy

AntiSamy was originally authored by Arshan Dabirsiaghi (arshan.dabirsiaghi [at] gmail.com) of Contrast Security with help from Jason Li (jason.li [at] owasp.org) of Aspect Security and is currently maintained by Dave Wichers (dave.wichers [at] owasp.org).

Description

The OWASP AntiSamy project is a few things. 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 get persisted on the server. The term “malicious code” in regards to web applications usually means “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. So we take care of that too.

Philosophically, AntiSamy is a departure from contemporary security mechanisms. Generally, the security mechanism and user have a communication that is virtually one way, for good reason. Letting the potential attacker know details about the validation is considered unwise as it allows the attacker to “learn” and “recon” the mechanism for weaknesses. These types of information leaks can also hurt in ways you don’t expect. A login mechanism that tells the user, “Username invalid” leaks the fact that a user by that name does not exist. A user could use a dictionary or phone book or both to remotely come up with a list of valid usernames. Using this information, an attacker could launch a brute force attack or massive account lock denial-of-service. We get that.

Unfortunately, that’s just not very usable in this situation. Typical Internet users are largely pretty bad when it comes to writing HTML/CSS, so where do they get their HTML from? Usually they copy it from somewhere out on the web. Simply rejecting their input without any clue as to why is jolting and annoying. Annoyed users go somewhere else to do their social networking.

The OWASP licensing policy allows OWASP projects to be released under any approved open source license. Under these guidelines, AntiSamy is distributed under a BSD license.

What is AntiSamy?

OWASP AntiSamy provides:

This page shows a big-picture comparison between the versions. Since it’s an unfunded open source project, the ports can’t be expected to mirror functionality exactly. If there’s something a port is missing – let us know, and we’ll try to accommodate, or write a patch!

Presentations

From OWASP & WASC AppSec U.S. 2007 Conference (San Jose, CA): AntiSamy: Picking a Fight with XSS (ppt) - by Arshan Dabirsiaghi - AntiSamy project lead

From OWASP AppSec Europe 2008 (Ghent, Belgium): The OWASP AntiSamy project (ppt) - by Jason Li - AntiSamy project contributor

From OWASP AppSec India 2008 (Delhi, India): Validating Rich User Content (ppt) - by Jason Li - AntiSamy project contributor

From Shmoocon 2009 (Washington, DC): AntiSamy - Picking a Fight with XSS (pptx) - by Arshan Dabirsiaghi - AntiSamy project lead

News and Events

[28 Sep 2017] Please update AntiSamy to 1.5.7 or later per CVE-2017-14735 and CVE-2016-10006


There are 4 steps in the process of integrating AntiSamy. Each step is detailed in the next section, but the high level overview follows:

  1. Download AntiSamy from Maven
  2. Choose one of the standard policy files that matches as close to the functionality you need:
    • antisamy-tinymce-X.X.X.xml
    • antisamy-slashdot-X.X.X.xml
    • antisamy-ebay-X.X.X.xml
    • antisamy-myspace-X.X.X.xml
    • antisamy-anythinggoes-X.X.X.xml
  3. Tailor the policy file according to your site’s rules
  4. Call the API from the code

Stage 1 - Downloading AntiSamy

First, add the dependency from Maven:

  <dependency>
    <groupId>org.owasp.antisamy</groupId>
    <projectId>antisamy</projectId>
  </dependency>

Stage 2 - Choosing a base policy file

Chances are that your site’s use case for AntiSamy is at least roughly comparable to one of the predefined policy files. They each represent a “typical” scenario for allowing users to provide HTML (and possibly CSS) formatting information. Let’s look into the different policy files:

1) antisamy-slashdot.xml

Slashdot is a techie news site that allows users to respond anonymously to news posts with very limited HTML markup. Now, Slashdot is not only one of the coolest sites around, it’s also one that’s been subject to many different successful attacks. Even more unfortunate is the fact that most of the attacks led users to the infamous goatse.cx picture (please don’t go look it up). The rules for Slashdot are fairly strict: users can only submit the following HTML tags and no CSS: <b>, <u>, <i>, <a>, <blockquote>.

Accordingly, we’ve built a policy file that allows fairly similar functionality. All text-formatting tags that operate directly on the font, color or emphasis have been allowed.

2) antisamy-ebay.xml

eBay is the most popular online auction site in the universe, as far as I can tell. It is a public site so anyone is allowed to post listings with rich HTML content. It’s not surprising that given the attractiveness of eBay as a target that it has been subject to a few complex XSS attacks. Listings are allowed to contain much more rich content than, say, Slashdot- so it’s attack surface is considerably larger. The following tags appear to be accepted by eBay (they don’t publish rules): <a>, ...

3) antisamy-myspace.xml

MySpace was, at the time this project was born, arguably the most popular social networking site. Users were allowed to submit pretty much all HTML and CSS they want - as long as it doesn’t contain JavaScript. MySpace was using a word blacklist to validate users’ HTML, which is why they were subject to the infamous Samy worm: Article: The MySpace Worm that Changed the Internet Forever and another In Samy’s own words. The Samy worm, which used fragmentation attacks combined with a word that should have been blacklisted (eval) - was the inspiration for the project.

4) antisamy-anythinggoes.xml

I don’t know of a possible use case for this policy file. If you wanted to allow every single valid HTML and CSS element (but without JavaScript or blatant CSS-related phishing attacks), you can use this policy file. Not even MySpace was this crazy. However, it does serve as a good reference because it contains base rules for every element, so you can use it as a knowledge base when using tailoring the other policy files.

Stage 3 - Tailoring the policy file

Smaller organizations may want to deploy AntiSamy in a default configuration, but it’s equally likely that a site may want to have strict, business-driven rules for what users can allow. The discussion that decides the tailoring should also consider attack surface - which grows in relative proportion to the policy file.

You may also want to enable/modify some “directives”, which are basically advanced user options. This page tells you what the directives are and which versions support them.

Stage 4 - Calling the AntiSamy API

Using AntiSamy is easy. Here is an example of invoking AntiSamy with a policy file:

import org.owasp.validator.html.*;

Policy policy = Policy.getInstance(POLICY_FILE_LOCATION);

AntiSamy as = new AntiSamy();
CleanResults cr = as.scan(dirtyInput, policy);

MyUserDAO.storeUserProfile(cr.getCleanHTML()); // some custom function

There are a few ways to create a Policy object. The getInstance() method can take any of the following:

  • a String filename
  • a File object
  • an InputStream

Policy files can also be referenced by filename by passing a second argument to the AntiSamy:scan() method as the following examples show:

AntiSamy as = new AntiSamy();
CleanResults cr = as.scan(dirtyInput, policyFilePath);

Finally, policy files can also be referenced by File objects directly in the second parameter:

AntiSamy as = new AntiSamy();
CleanResults cr = as.scan(dirtyInput, new File(policyFilePath));

Stage 5 - Analyzing CleanResults

The CleanResults object provides a lot of useful stuff.

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