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guideAdvanced Content Filter

Advanced Content Filter was introduced in CKEditor 4.1.

# What is Advanced Content Filter (ACF)?

ACF is a highly configurable CKEditor core feature available since CKEditor 4.1. It limits and adapts input data (HTML code added in source mode or by the editor.setData method, pasted HTML code, etc.) so it matches the editor configuration in the best possible way. It may also deactivate features which generate HTML code that is not allowed by the configuration.

Advanced Content Filter works in two modes:

  • automatic – The filter is configured by the editor features (like plugins, buttons, and commands) that are enabled with configuration options such as plugins, extraPlugins, and toolbar.
  • custom – The filter is configured by the allowedContent option and only features that match this setting are activated.

In both modes it is possible to extend the filter configuration by using the extraAllowedContent setting or to trim it down by using the disallowedContent setting.

ACF does not replace a security filter for your website content. If the content that is to be loaded into CKEditor 4 comes from untrusted sources (e.g. the users of your website), you should always filter it on the server side to avoid potential XSS issues — just like you would do it for any other content intended to be published on your website.

Configuring ACF to accept additional tags and attributes that are unsupported by CKEditor 4 features may result in XSS vulnerabilities. For example, if you decide to allow all attributes in HTML elements, you will allow users to enter onclick, onload or onerror handlers. Although ACF is not a security filter, leaving it in default, automatic mode should minimize the risk of XSS issues.

If you want to disable Advanced Content Filter, set allowedContent to true. All available editor features will be activated and input data will not be filtered. Note that you cannot use disallowedContent when ACF is disabled.

# Automatic Mode

Advanced Content Filter works in automatic mode when the allowedContent setting is not provided. During the editor initialization, the editor features add their rules to the filter. As a result, only the content that may be edited using the currently loaded features is allowed, and all the rest is filtered out.

Please note that this means that ACF is tightly connected with the editor configuration. Take the official CKEditor 4 presets (Basic, Standard and Full). Each one of them includes a different set of features (toolbar buttons, keyboard shortcuts, content styles) and as a result, the same content pasted into editor instances with these configurations will look completely different, as CKEditor 4 will adjust it to what is available in a particular setup.

Whenever you adjust your editor configuration, for example by using the removePlugins and removeButtons methods or customizing the Format and Styles drop-down lists, these changes will affect the filter and the automatic ACF mode will make the editor remove content corresponding to disabled features.

# Automatic Mode Example

Consider the following configuration for an editor with ACF working in the default, automatic mode:

config.removePlugins = 'image,table,tabletools,horizontalrule';
config.removeButtons = 'Anchor,Underline,Strike,Subscript,Superscript';
config.format_tags = 'p;h1;h2;pre';

In this setup several tags will not be allowed in the editor because there is no plugin or button that is responsible for creating and editing this kind of content. This pertains to elements such as <img> (Image feature), <table> and its descendants (Table and Table Tools plugins) and <hr> (Horizontal Rule feature) as well as <u>, <s>, <sub> and <sup> that are normally provided by the Basic Styles plugin, but whose buttons were removed in the configuration. The Format drop-down list was trimmed down, too, so unsupported formats will also be removed.

See the Advanced Content Filter – Automatic Mode sample for a live demonstration.

If you want to configure the editor to work in automatic mode, but need to enable additional HTML tags, attributes, styles, or classes, use the extraAllowedContent configuration option. Since CKEditor 4.4 you can also disallow some of the automatically allowed content by using the disallowedContent option.

# Custom Mode

Advanced Content Filter works in custom mode when the allowedContent setting is defined. This configuration option tells the filter which HTML elements, attributes, styles, and classes are allowed. Based on defined rules (called Allowed Content Rules) the editor filters input data and decides which features can be activated.

Allowed Content Rules may be set in two different formats: the compact string format and the more powerful object format. Read about Allowed Content Rules in the Allowed Content Rules article.

In custom mode the content filter configuration affects the availability of editor features (toolbar buttons, dialog window tabs and fields, keyboard shortcuts, content styles). If a feature is not explicitely defined as allowed, it will be disabled in the editor UI and any corresponding content will be removed.

# Custom Mode Example

Suppose that allowedContent is configured as follows:

config.allowedContent =
    'h1 h2 h3 p blockquote strong em;' +
    'a[!href];' +

This will have the following effect:

  • h1 h2 h3 p blockquote strong em – Only these tags are accepted by the editor. Any tag attributes will be discarded.
  • a[!href] – The href attribute is obligatory for the <a> tag. Tags without this attribute are discarded. No other attribute will be accepted for the <a> tag.
  • img(left,right)[!src,alt,width,height] – The src attribute is obligatory for the <img> tag. The alt, width, height and class attributes are accepted, but class must be either class="left" or class="right".
  • Several toolbar buttons and dialog window fields that are responsible for the features which were not explicitly listed as allowed will be removed. In the Standard editor preset this will mean that, for example, the Strike-through, Numbered List, Bulleted List, Anchor, Table and Horizontal Line toolbar buttons will be gone, just like most of the fields of the Image Properties dialog window and formats from the Format drop-down list.

See the Advanced Content Filter – Custom Mode sample for a live demonstration.

# Content Transformations

Advanced Content Filter not only removes disallowed HTML elements, their classes, styles, and attributes, but it also tries to unify input data by transforming one form of an element to another, preferred form.

Consider the Bold feature. In HTML code it may be represented by <strong>, <b>, or even a <span style="font-weight:700|800|900|bold"> element. Suppose that the allowedContent setting contains only a rule for the <b> element. Does this mean that when pasting the <strong> or <span> element they will be removed?

No. The editor will transform all of them to <b> elements. As a result the editor will contain only <b> elements and the visual form of pasted content will be preserved. Exactly the same will happen if you leave the default allowedContent value (in automatic mode) — all Bold feature forms will be unified to the preferred <strong> form.

Suppose that the 'img[!src,alt,width,height]' setting (<img> tag with a required src and three optional attributes) was added to Allowed Content Rules. An image size should be set with attributes, so for example a pasted image whose size was set with styles will be transformed to an image with attributes (note that it will not be possible in all scenarios — only pixel-based size can be transformed).

The content transformation feature is fully automatic and there is no need to configure it. The only thing you have to do is set the allowedContent option or use the default value (automatic mode).

Currently, we have defined content transformations for only a handful of editor features, but their number will increase in future releases.

# Filtering Pasted and Dropped Content

An additional filter (called paste filter) can be configured to handle pasted and dropped content. It will be applied independently from ACF, so it can be used as the only filter (with disabled ACF), or as a complementary filter (for example to filter pasted content with stricter rules than all other content).

In browsers where it is possible to recognize whether the content comes (was copied or dragged) from an editor, the paste filter will be applied only to content that does not come from any editor. Currently content source can be recognized in modern browsers. In older browsers (in particular: Internet Explorer) and Edge, if the paste filter is enabled, it is applied to all pasted and dropped content.

Note: By default this filter is enabled in all Blink and Webkit based browsers, because they keep messy HTML content in the clipboard. In these browsers the paste filter is configured to 'semantic-content' which means that it will strip all inline styles and classes (because these are classes used on the source page, not inside the editor).

Read more about the paste filter in the pasteFilter documentation.

# Supplying Paste Tools

Plugins derived from the Paste Tools family, like Paste from Word and Paste from Google Docs, ensure that their filtering mechanisms clean up content features provided by 3rd party applications. They are mostly the first filter against incorrect, non-semantic HTML pasted into the editor. These plugins fix the HTML structure and clean up meaningless code, but at the same time try to preserve as much information that can be correctly consumed further and displayed in the editor as it is possible. A significant part of the content preparation is handled by the CKEditor 4 plugin ecosystem with predefined ACF rules that are used to semantically improve the HTML content.

As an example, when pasting an image from a Microsoft Word document, you should enable one of the image plugins like Image or Enhanced Image in your editor configuration. Otherwise, the image will be removed from content by the default ACF configuration.

However, you can still enhance this behavior by adding custom ACF rules and explicitly defining that images should be preserved despite missing an image plugin that would support it:

config.extraAllowedContent = 'img';

# Example: Removing Font Styles

One of the features that have recently been deprecated in favor of ACF is pasteTools_removeFontStyles that was responsible for removing all font-related formatting styles. Here is how the same result can now be accomplished with ACF:

config.disallowedContent = 'span{font,font-size,font-family}';

Using the above rule with the disallowedContent setting will result in removing all font styles from the pasted HTML code, reflecting the same logic as the deprecated pasteTools_removeFontStyles option.

# Advanced Content Filter Demos

The following samples are available for two ACF modes:

# Further Reading

Refer to the following resources for more information about content filtering: