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sanitize

lib/sanitize/README.md

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sanitize

sanitize is a Crystal library for transforming HTML/XML trees. It's primarily used to sanitize HTML from untrusted sources in order to prevent XSS attacks and other adversities.

It builds on stdlib's XML module to parse HTML/XML. Based on libxml2 it's a solid parser and turns malformed and malicious input into valid and safe markup.

Installation

  1. Add the dependency to your shard.yml:

    yaml
    dependencies:
  sanitize:
    github: straight-shoota/sanitize

  2. Run shards install

Sanitization Features

The Sanitize::Policy::HTMLSanitizer policy applies the following sanitization steps. Except for the first one (which is essential to the entire process), all can be disabled or configured.

  • Turns malformed and malicious HTML into valid and safe markup.
  • Strips HTML elements and attributes not included in the safe list.
  • Sanitizes URL attributes (like href or src) with customizable sanitization policy.
  • Adds rel="nofollow" to all links and rel="noopener" to links with target.
  • Validates values of accepted attributes align, width and height.
  • Filters class attributes based on a whitelist (by default all classes are rejected).

Usage

Transformation is based on rules defined by Sanitize::Policy implementations.

The recommended standard policy for HTML sanitization is Sanitize::Policy::HTMLSanitizer.common which represents good defaults for most use cases. It sanitizes user input against a known safe list of accepted elements and their attributes.

crystal
require "sanitize"

sanitizer = Sanitize::Policy::HTMLSanitizer.common
sanitizer.process(%(<a href="javascript:alert('foo')">foo</a>)) # => %(foo)
sanitizer.process(%(<p><a href="foo">foo</a></p>)) # => %(<p><a href="foo" rel="nofollow">foo</a></p>)
sanitizer.process(%()) # => %()
sanitizer.process(%(<table><tr><td>foo</td><td>bar</td></tr></table>)) # => %(<table><tr><td>foo</td><td>bar</td></tr></table>)

Sanitization should always run after any other processing (for example rendering Markdown) and is a must when including HTML from untrusted sources into a web page.

With Markd

A typical format for user generated content is Markdown. Even though it has only a very limited feature set compared to HTML, it can still produce potentially harmful HTML and is is usually possible to embed raw HTML directly. So Sanitization is necessary.

The most common Markdown renderer is markd, so here is a sample how to use it with sanitize:

crystal
sanitizer = Sanitize::Policy::HTMLSanitizer.common
# Allow classes with `language-` prefix which are used for syntax highlighting.
sanitizer.valid_classes << /language-.+/

markdown = <<-MD
  Sanitization with [https://shardbox.org/shards/sanitize](sanitize) is not that
  **difficult**.
  ```cr
  puts "Hello World!"
  ```
  <p><a href="javascript:alert("XSS attack!")">Hello world!</a></p>
  MD

html = Markd.to_html(markdown)
sanitized = sanitizer.process(html)
puts sanitized

The result:

html
<p>Sanitization with <a href="sanitize" rel="nofollow">https://shardbox.org/shards/sanitize</a> is not that
<strong>difficult</strong>.</p>
<pre><code class="language-cr">puts &quot;Hello World!&quot;
</code></pre>
<p>Hello world!</p>

Limitations

Sanitizing CSS is not supported. Thus style attributes can't be accepted in a safe way. CSS sanitization features may be added when a CSS parsing library is available.

Security

If you want to privately disclose security-issues, please contact straightshoota on Keybase or [email protected] (PGP: DF2D C9E9 FFB9 6AE0 2070 D5BC F0F3 4963 7AC5 087A).

Contributing

  1. Fork it (https://github.com/straight-shoota/sanitize/fork)
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request

Contributors