Category:OWASP AntiSamy Project
OWASP AntiSamy Project
OWASP AntiSamy is a library for HTML and CSS encoding.
AntiSamy was originally authored by Arshan Dabirsiaghi (arshan.dabirsiaghi [at the] gmail.com) with help from Jason Li (jason.li [at the] owasp.org), both of Aspect Security (http://www.aspectsecurity.com/).
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 (further explained in the membership FAQ) 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!
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
This project can be purchased as a print on demand book from Lulu.com
There's 4 steps in the process of integrating AntiSamy. Each step is detailed in the next section, but the high level overview follows:
- Download AntiSamy from its home on Google Code
- Choose one of the standard policy files that matches as close to the functionality you need:
- Tailor the policy file according to your site's rules
- Call the API from the code
Stage 1 - Downloading AntiSamy
The following instructions are for AntiSamy Java, the main version. For instructions on the .NET version, see the .NET page.
Which package you download depends on what you want to do with AntiSamy. If you'd like to extend it or review the code, download the source package antisamy-X.X.X-src.jar. If you're looking to integrate AntiSamy, you can either download the library or use Maven to include it in your build. If you want to use Maven, here's an example POM for including AntiSamy. If you want a jar file, then download the antisamy-X.X.X.jar (which, before version 1.2 was confusingly called "antisamy-standalone-X.X.X.jar"), which only contains AntiSamy library. This will be the preferred choice for mature enterprise environments who don't want to be caught in classpath issues which may be introduced by the current version.
The second option, only available for versions before 1.2, is to download antisamy-standalone-X.X.X.jar, which contains not only the AntiSamy code, but all necessary supporting libraries. This should only be used by applications that don't use the libraries AntiSamy ships with as they might introduce classpath and versioning issues. This option is no longer available after version 1.2.
You must also download required dependencies, which are documented in the Developer Guide.pdf file.
You can Download AntiSamy from its home on Google Code
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:
Slashdot (http://www.slashdot.org/) 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.
eBay (http://www.ebay.com/) 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>,...
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 abnormally 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
getScanTime() - returns the scan time in seconds
There are two ways of getting information on AntiSamy. The mailing list, and contacting the project lead directly.
OWASP AntiSamy mailing list
The first is the mailing list which is located at https://lists.owasp.org/mailman/listinfo/owasp-antisamy. The list was previously private and the archives have been cleared with the release of version 1.0. We encourage all prospective and current users and bored attackers to join in the conversation. We're happy to brainstorm attack scenarios, discuss regular expressions and help with integration.
Emailing the project lead
For content which is not appropriate for the public mailing list, you can alternatively contact the project lead, Arshan Dabirsiaghi, at [arshan.dabirsiaghi] at [aspectsecurity.com].
Visit the Google Code issue tracker.
This section details the status of the various ports of AntiSamy.
Daniel Bower created a Grails plugin for AntiSamy.
A .NET port of AntiSamy is available now at the OWASP AntiSamy .NET page. The project was funded by a Summer of Code 2008 grant and was developed by Jerry Hoff.
This port is no longer under active development, and is looking for a few good developers to help make it feature-synchronized with the .NET version. If it doesn't suit your needs, consider Microsoft's AntiXSS library.
A beta Python version is currently being prototyped by a few different groups. As more information becomes available, we will post it here. If you are interested in helping, please contact the mailing list.
Although a PHP version was initially planned, we now suggest HTMLPurifier for safe rich input validation for PHP applications.
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