HtmlSanitizer is a .NET library for cleaning HTML fragments from constructs that can lead to XSS attacks. It uses AngleSharp 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.
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
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);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
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");action, background, dynsrc, href, lowsrc, src
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>");