HtmlSanitizer
HtmlSanitizer is a class 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.
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
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
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>");