sfXssSafePlugin - 0.8.0

Output Rich Text With Cross Site Scripting Protection

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sfXssSafePlugin - Output Rich Text With Cross Site Scripting Protection

Overview

Between the ESC_RAW and ESC_HTMLENTITIES, symfony lacks one escaping level allowing to strip dangerous HTML code but leave the tags that just structure a content or apply a format to it.

The sfXssSafePlugin allows to clean a string to prevent XSS attacks. It provides a new escaping strategy, ESC_XSSSAFE, to escape tainted HTML strings entered by users. This escaping strategy removes all "dangerous" tags and attributes but keeps the safe ones. The plugin embarks the HTML Putifier library and is fully unit-tested.

Installation

  • Install the plugin

    symfony plugin-install http://plugins.symfony-project.com/sfXssSafePlugin

  • Alternatively, if you don't have PEAR installed, you can download the latest package attached to this plugin's wiki page and extract it under your project's plugins/ directory

Usage

First of all, your application escaping strategy must be set to both or on to use this plugin. Check your settings.yml for the escaping_strategy parameter.

Include the helper in the templates where you want to use the new escaping strategy:


As explained in the symfony book, the methods of escaped objects accept an additionnal parameter for the escaping strategy. In addition to the core ESC_HTMLENTITIES and ESC_RAW, you can now use the ESC_XSSSAFE strategy.

// In the action 
$this->post->setContent('this is not nice <script>alert("XSS");</script>');

// In the template
<?php echo $post->getContent(ESC_XSSSAFE); ?>
 => 'this is not nice'

Configuration

You can change the HTML Purifier configuration in your application's app.yml file, under the sfXssSafePlugin section. Refer to the plugin's app.sample.yml and to the HTML Purifier documentation for further information.

Here is the default configuration used by the plugin if you don't add anything to your app.yml:

all:
  sfXssSafePlugin:
    definition:

      HTML:
        TidyLevel:              medium   # Values : "none", "light", "medium", "heavy"
        Doctype:                null     # Accepts valid Doctypes, like 'XHTML 1.0 Transitional'
        Trusted:                false

      Core:
        Encoding:               UTF-8    # This directive only accepts ISO-8859-1 if iconv is not enabled
        RemoveInvalidImg:       true
        EscapeInvalidChildren:  false
        EscapeInvalidTags:      false

      CSS:
        AllowImportant:         false

      Filter:
        YouTube:                false    # Allow YouTube video embeded

      AutoFormat:
        AutoParagraph:          false

      URI:
        Disable:                false
        DisableExternal:        false

      Output:
        TidyFormat:             false

More configuration

Although HTML Purifier contains a large set of elements and attributes, you can customize others elements or attributes.

The plugin contains a sample which show how to allow to display flash movies :

  • First add the following configuration in app.yml file :

    all: sfXssSafePlugin: definition:

      HTML:
        DefinitionID:              'allow flash movies' 
        DefinitionRev:             1
    
      AutoFormat:
        Custom:                    AddParam # injector : call class "HTMLPurifier_Injector_AddParam"
        Element:
    
          * param:
            type:                  false
            contents:              Empty
            attr_includes:         false
            attr:
              'name':              Text
              'value':             Text
    
    
          * object:
            type:                  Inline
            contents:              'Optional: param | Flow | #PCDATA'
            attr_includes:          false
            attr:
              'type*':             'Enum#application/x-shockwave-flash'
              'width*':            Pixels
              'height*':           Pixels
              'data':              Text
              'bgcolor*':          Text
              'quality*':          Text
    
    
          * embed:
            type:                  Block
            contents:              Empty
            attr_includes:         false
            attr:
              'type*':             'Enum#application/x-shockwave-flash'
              'width*':            Pixels
              'height*':           Pixels
              'src*':              URI
              'flashvars':         Text
              'allowscriptaccess': 'Enum#never'
              'enablejsurls':      'Enum#false'
              'enablehref':        'Enum#false'
              'bgcolor':           Text
              'align':             Text
              'quality':           Text
              'wmode':             Text
              'pluginspage':       URI
              'saveembedtags':     Text
              'salign':            Text
              'scale':             Text
              'name':              Text
    

HTML.DefinitionID is set to a unique identifier for your custom HTML definition. This prevents it from clobbering other custom definitions on the same installation. HTML.DefinitionRev is a revision integer of your HTML definition. AutoFormat.Element adds param, object and embed elements, with theirs allowed attributes.

  • The same way you can add an attribute to an element with AutoFormat.Attribute :

    all: sfXssSafePlugin: definition: AutoFormat: Attribute:

          * a:
            attr_name:             target
            def:                   'Enum#_blank,_self,_target,_top'
    
  • Then you can clean more the added element overloading HTML Purifier. For this, move the sample file sfXssSafeObject.class.php in the an autoloaded directory (lib of your project for instance).

The file contains the classes of transformation for each element. There is also an auto-format injector that we called AddParam in our example.

Finally, clear your cache to enable the autoloading Symfony feature to find the new classes.

Changelog

2008-11-08 | 0.8

  • heristop : Migration to HTML Purifier 3.2

2008-06-27 | 0.6.1 Beta

  • heristop : Migration to HTML Purifier 3.1.1

2008-06-20 | 0.6.0 Beta

  • heristop : Adding customizable elements

2008-05-19 | 0.5.0 Beta

  • heristop: Initial version