HtmlSanitizer

HtmlSanitizer is a .NET library for cleaning HTML fragments from constructs that can lead to XSS attacks. It uses the excellent C# jQuery port CsQuery to parse, manipulate, and render HTML and CSS.

Because HtmlSanitizer is based on a robust HTML parser it can also shield you from deliberate or accidental "tag poisoning" where invalid HTML in one fragment can corrupt the whole document leading to broken layout or style.

In order to facilitate different use cases, HtmlSanitizer can be customized at several levels:

Configure allowed HTML tags through the property AllowedTags. All other tags will be stripped.
Configure allowed HTML attributes through the property AllowedAttributes. All other attributes will be stripped.
Configure allowed CSS property names through the property AllowedCssProperties. All other styles will be stripped.
Configure allowed URI schemes through the property AllowedSchemes. All other URIs will be stripped.
Configure HTML attributes that contain URIs (such as "src", "href" etc.) through the property UriAttributes.
Provide a base URI that will be used to resolve relative URIs against.
Cancelable events are raised before a tag, attribute, or style is removed.

Tags allowed by default

a, abbr, acronym, address, area, article, aside, b, bdi, big, blockquote, br, button, caption, center, cite, code, col, colgroup, data, datalist, dd, del, details, dfn, dir, div, dl, dt, em, fieldset, figcaption, figure, font, footer, form, h1, h2, h3, h4, h5, h6, header, hr, i, img, input, ins, kbd, keygen, label, legend, li, main, map, mark, menu, menuitem, meter, nav, ol, optgroup, option, output, p, pre, progress, q, rp, rt, ruby, s, samp, section, select, small, span, strike, strong, sub, summary, sup, table, tbody, td, textarea, tfoot, th, thead, time, tr, tt, u, ul, var, wbr

Attributes allowed by default

abbr, accept, accept-charset, accesskey, action, align, alt, autocomplete, autosave, axis, bgcolor, border, cellpadding, cellspacing, challenge, char, charoff, charset, checked, cite, clear, color, cols, colspan, compact, contenteditable, coords, datetime, dir, disabled, draggable, dropzone, enctype, for, frame, headers, height, high, href, hreflang, hspace, ismap, keytype, label, lang, list, longdesc, low, max, maxlength, media, method, min, multiple, name, nohref, noshade, novalidate, nowrap, open, optimum, pattern, placeholder, prompt, pubdate, radiogroup, readonly, rel, required, rev, reversed, rows, rowspan, rules, scope, selected, shape, size, span, spellcheck, src, start, step, style, summary, tabindex, target, title, type, usemap, valign, value, vspace, width, wrap

Note: to prevent classjacking and interference with classes where the sanitized fragment is to be integrated, the class attribute is not in the whitelist by default. It can be added as follows:

var sanitizer = new HtmlSanitizer();
sanitizer.AllowedAttributes.Add("class");
var sanitized = sanitizer.Sanitize(html);

CSS properties allowed by default

background, background-attachment, background-color, background-image, background-position, background-repeat, border, border-bottom, border-bottom-color, border-bottom-style, border-bottom-width, border-collapse, border-color, border-left, border-left-color, border-left-style, border-left-width, border-right, border-right-color, border-right-style, border-right-width, border-spacing, border-style, border-top, border-top-color, border-top-style, border-top-width, border-width, bottom, caption-side, clear, clip, color, content, counter-increment, counter-reset, cursor, direction, display, empty-cells, float, font, font-family, font-size, font-style, font-variant, font-weight, height, left, letter-spacing, line-height, list-style, list-style-image, list-style-position, list-style-type, margin, margin-bottom, margin-left, margin-right, margin-top, max-height, max-width, min-height, min-width, opacity, orphans, outline, outline-color, outline-style, outline-width, overflow, padding, padding-bottom, padding-left, padding-right, padding-top, page-break-after, page-break-before, page-break-inside, quotes, right, table-layout, text-align, text-decoration, text-indent, text-transform, top, unicode-bidi, vertical-align, visibility, white-space, widows, width, word-spacing, z-index

URI schemes allowed by default

http, https

Note: Protocol-relative URLs (e.g. //github.com) are allowed by default (as are other relative URLs).

to allow mailto: links:

sanitizer.AllowedSchemes.Add("mailto");

Default attributes that contain URIs

action, background, dynsrc, href, lowsrc, src

Usage

Install the HtmlSanitizer NuGet package. Then:

var sanitizer = new HtmlSanitizer();
var html = @"<script>alert('xss')</script><div onload=""alert('xss')"""
    + @"style=""background-color: test"">Test<img src=""test.gif"""
    + @"style=""background-image: url(javascript:alert('xss')); margin: 10px""></div>";
var sanitized = sanitizer.Sanitize(html, "http://www.example.com");
Assert.That(sanitized, Is.EqualTo(@"<div style=""background-color: test"">"
    + @"Test<img style=""margin: 10px"" src=""http://www.example.com/test.gif""></div>");

License

MIT X11

wobu/HtmlSanitizer