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AntiSamy

A library for performing fast, configurable cleansing of HTML coming from untrusted sources. Supports Java 8+.

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 mean "JavaScript." Mostly, Cascading Stylesheets are only considered malicious when they invoke JavaScript. However, there are many situations where "normal" HTML and CSS can be used in a malicious manner.

IMPORTANT! - API breaking changes in 1.7.0

Throughout the development of the 1.6.x series, we have identified and deprecated a number of features and APIs. All of these deprecated items have been removed in the 1.7.0 release. These changes were all tracked in ticket: #195. Each of the changes are described below:

CssHandler had 2 constructors which dropped the LinkedList<URI> embeddedStyleSheets parameter. Both constructors now create an empty internal LinkedList<URI> and the method getImportedStylesheetsURIList() can be used to get a reference to it, if needed. This feature is rarely used, and in fact direct invocation of these constructors is also rare, so this change is unlikely to affect most users of AntiSamy. When used, normally an empty list is passed in as this parameter value and that list is never used again.

  • The CssHandler(Policy, LinkedList<URI>, List<String>, ResourceBundle) signature was dropped

    • It was replaced with: CssHandler(Policy, List<String>, ResourceBundle)
  • The CssHandler(Policy, LinkedList<URI>, List<String>, String, ResourceBundle) signature was dropped

    • It was replaced with: CssHandler(Policy, List<String>, ResourceBundle, String). NOTE: The order of the last 2 parameters to this method was reversed.
  • Support for XHTML was dropped. AntiSamy now only supports HTML. As we believe this was a rarely used feature, we don't expect this to affect many AntiSamy users.

  • XML Schema validation is now required on AntiSamy policy files and cannot be disabled. You must make your policy file schema compliant in order to use it with AntiSamy.

  • The policy directive noopenerAndNoreferrerAnchors is now ON by default. If it is disabled, AntiSamy issues a nag, encouraging you to enable it.

Note: Since 1.7.4 some outputs may differ due to upgrading the HTML parser dependency, consider this if you were using previous versions and get different outputs.

Deprecating support for external stylesheets

The AntiSamy team has decided that supporting the ability to allow embedded remote CSS is dangerous and so we are deprecating this feature and it will be removed in a future release. It is expected that there are very few, if any, users of this feature.

We have added a log WARNing if this feature is invoked. If you are, please disable/remove this feature by switching to the primary CssScanner constructor that does not enable this feature.

How to Use

1. Import the dependency

First, add the dependency from Maven:

<dependency>
   <groupId>org.owasp.antisamy</groupId>
   <artifactId>antisamy</artifactId>
   <version>LATEST_VERSION</version>
</dependency>

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. 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.

  1. antisamy-ebay.xml

eBay is the most popular online auction site in the universe, as far as we 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.

  1. antisamy-myspace.xml

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

  1. antisamy-anythinggoes.xml

We 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.

Logging

AntiSamy now includes the slf4j-simple library for its logging, but AntiSamy users can import and use an alternate slf4j compatible logging library if they prefer. They can also then exclude slf4j-simple if they want to.

WARNING: AntiSamy's use of slf4j-simple, without any configuration file, logs messages in a buffered manner to standard output. As such, some or all of these log messages may get lost if an Exception, such as a PolicyException is thrown. This can likely be rectified by configuring slf4j-simple to log to standard error instead, or use an alternate slf4j logger that does so.

3. Tailoring the policy file

You 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.

Example policies can be adapted and tested based on the requirements for each tag. The supported tag actions that can be specified are:

  • filter: remove tags, but keep content.
  • validate: keep content as long as it passes rules.
  • remove: remove tag and contents.
  • truncate: remove tag attributes and all child tags except por its text content if any.
  • encode: similar to filter but it encodes the tag for HTML to preserve it as raw text and its children are moved up one level in the hierarchy.

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));

5. Analyzing CleanResults

The CleanResults object provides a lot of useful stuff.

  • getCleanHTML() - the clean, safe HTML output
  • getCleanXMLDocumentFragment() - the clean, safe XMLDocumentFragment which is reflected in getCleanHTML()
  • getErrorMessages() - a list of String error messages -- if this returns 0 that does not mean there were no attacks!
  • getNumberOfErrors() - the number of error messages -- Again, 0 does not mean the input was safe!
  • getScanTime() - returns the scan time in seconds

Important Note: There has been much confusion about the getErrorMessages() method. The getErrorMessages() method (nor getNumberOfErrors()) does not subtly answer the question "is this safe input?" in the affirmative if it returns an empty list. You must always use the sanitized input and there is no way to be sure the input passed in had no attacks.

The serialization and deserialization process that is critical to the effectiveness of the sanitizer is purposefully lossy and will filter out attacks via a number of attack vectors. Unfortunately, one of the tradeoffs of this strategy is that AntiSamy doesn't always know in retrospect that an attack was seen. Thus, the getErrorMessages() and getNumberOfErrors() APIs are there to help users understand whether their well-intentioned input meets the requirements of the system, not help a developer detect if an attack was present.

Other Documentation

Additional documentation is available on this GitHub project's wiki page: https://github.com/nahsra/antisamy/wiki and the OWASP AntiSamy Project Page: https://owasp.org/www-project-antisamy/

Contributing to AntiSamy

Find an Issue?

If you have found a bug, then create an issue in the AntiSamy repo: https://github.com/nahsra/antisamy/issues

Find a Vulnerability?

If you have found a vulnerability in AntiSamy, first search the issues list (see above) to see if it has already been reported. If it has not, then please contact Dave Wichers (dave.wichers at owasp.org) directly. Please do not report vulnerabilities via GitHub issues as we wish to keep our users secure while a patch is implemented and deployed. If you wish to be acknowledged for finding the vulnerability, then please follow this process.

More detail is available in the file: SECURITY.md.

How to Build

You can build and test from source pretty easily:

$ git clone https://github.com/nahsra/antisamy
$ cd antisamy
$ mvn package

License

Released under the BSD-3-Clause license as specified here: LICENSE.