1// Copyright (c) 2014, David Kitchen <david@buro9.com>
2//
3// All rights reserved.
4//
5// Redistribution and use in source and binary forms, with or without
6// modification, are permitted provided that the following conditions are met:
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9//   list of conditions and the following disclaimer.
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12//   this list of conditions and the following disclaimer in the documentation
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15// * Neither the name of the organisation (Microcosm) nor the names of its
16//   contributors may be used to endorse or promote products derived from
17//   this software without specific prior written permission.
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29
30/*
31Package bluemonday provides a way of describing a whitelist of HTML elements
32and attributes as a policy, and for that policy to be applied to untrusted
33strings from users that may contain markup. All elements and attributes not on
34the whitelist will be stripped.
35
36The default bluemonday.UGCPolicy().Sanitize() turns this:
37
38    Hello <STYLE>.XSS{background-image:url("javascript:alert('XSS')");}</STYLE><A CLASS=XSS></A>World
39
40Into the more harmless:
41
42    Hello World
43
44And it turns this:
45
46    <a href="javascript:alert('XSS1')" onmouseover="alert('XSS2')">XSS<a>
47
48Into this:
49
50    XSS
51
52Whilst still allowing this:
53
54    <a href="http://www.google.com/">
55      <img src="https://ssl.gstatic.com/accounts/ui/logo_2x.png"/>
56    </a>
57
58To pass through mostly unaltered (it gained a rel="nofollow"):
59
60    <a href="http://www.google.com/" rel="nofollow">
61      <img src="https://ssl.gstatic.com/accounts/ui/logo_2x.png"/>
62    </a>
63
64The primary purpose of bluemonday is to take potentially unsafe user generated
65content (from things like Markdown, HTML WYSIWYG tools, etc) and make it safe
66for you to put on your website.
67
68It protects sites against XSS (http://en.wikipedia.org/wiki/Cross-site_scripting)
69and other malicious content that a user interface may deliver. There are many
70vectors for an XSS attack (https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet)
71and the safest thing to do is to sanitize user input against a known safe list
72of HTML elements and attributes.
73
74Note: You should always run bluemonday after any other processing.
75
76If you use blackfriday (https://github.com/russross/blackfriday) or
77Pandoc (http://johnmacfarlane.net/pandoc/) then bluemonday should be run after
78these steps. This ensures that no insecure HTML is introduced later in your
79process.
80
81bluemonday is heavily inspired by both the OWASP Java HTML Sanitizer
82(https://code.google.com/p/owasp-java-html-sanitizer/) and the HTML Purifier
83(http://htmlpurifier.org/).
84
85We ship two default policies, one is bluemonday.StrictPolicy() and can be
86thought of as equivalent to stripping all HTML elements and their attributes as
87it has nothing on its whitelist.
88
89The other is bluemonday.UGCPolicy() and allows a broad selection of HTML
90elements and attributes that are safe for user generated content. Note that
91this policy does not whitelist iframes, object, embed, styles, script, etc.
92
93The essence of building a policy is to determine which HTML elements and
94attributes are considered safe for your scenario. OWASP provide an XSS
95prevention cheat sheet ( https://www.google.com/search?q=xss+prevention+cheat+sheet )
96to help explain the risks, but essentially:
97
98    1. Avoid whitelisting anything other than plain HTML elements
99    2. Avoid whitelisting `script`, `style`, `iframe`, `object`, `embed`, `base`
100       elements
101    3. Avoid whitelisting anything other than plain HTML elements with simple
102       values that you can match to a regexp
103*/
104package bluemonday
105