Copyright (c) 2014, David Kitchen <david@buro9.com> All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: * Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. * Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. * Neither the name of the organisation (Microcosm) nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Package bluemonday provides a way of describing a whitelist of HTML elementsand attributes as a policy, and for that policy to be applied to untrustedstrings from users that may contain markup. All elements and attributes not onthe whitelist will be stripped.
The default bluemonday.UGCPolicy().Sanitize() turns this:
Hello <STYLE>.XSS{background-image:url("javascript:alert('XSS')");}</STYLE><A CLASS=XSS></A>World
Into the more harmless:
Hello World
And it turns this:
<a href="javascript:alert('XSS1')" onmouseover="alert('XSS2')">XSS<a>
Into this:
XSS
Whilst still allowing this:
<a href="http:www.google.com/"> <img src="https:ssl.gstatic.com/accounts/ui/logo_2x.png"/> </a>
To pass through mostly unaltered (it gained a rel="nofollow"):
<a href="http:www.google.com/" rel="nofollow"> <img src="https:ssl.gstatic.com/accounts/ui/logo_2x.png"/> </a>
The primary purpose of bluemonday is to take potentially unsafe user generatedcontent (from things like Markdown, HTML WYSIWYG tools, etc) and make it safefor you to put on your website.
It protects sites against XSS (http:en.wikipedia.org/wiki/Cross-site_scripting)and other malicious content that a user interface may deliver. There are manyvectors for an XSS attack (https:www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet)and the safest thing to do is to sanitize user input against a known safe listof HTML elements and attributes.
Note: You should always run bluemonday after any other processing.
If you use blackfriday (https:github.com/russross/blackfriday) orPandoc (http:johnmacfarlane.net/pandoc/) then bluemonday should be run afterthese steps. This ensures that no insecure HTML is introduced later in yourprocess.
bluemonday is heavily inspired by both the OWASP Java HTML Sanitizer(https:code.google.com/p/owasp-java-html-sanitizer/) and the HTML Purifier(http:htmlpurifier.org/).
We ship two default policies, one is bluemonday.StrictPolicy() and can bethought of as equivalent to stripping all HTML elements and their attributes asit has nothing on its whitelist.
The other is bluemonday.UGCPolicy() and allows a broad selection of HTMLelements and attributes that are safe for user generated content. Note thatthis policy does not whitelist iframes, object, embed, styles, script, etc.
The essence of building a policy is to determine which HTML elements andattributes are considered safe for your scenario. OWASP provide an XSSprevention cheat sheet ( https:www.google.com/search?q=xss+prevention+cheat+sheet )to help explain the risks, but essentially:
1. Avoid whitelisting anything other than plain HTML elements 2. Avoid whitelisting `script`, `style`, `iframe`, `object`, `embed`, `base` elements 3. Avoid whitelisting anything other than plain HTML elements with simple values that you can match to a regexp