xss

What is xss?

The xss npm package is a library designed to sanitize input from users to prevent Cross-Site Scripting (XSS) attacks. It filters input from the user and escapes or removes any potentially malicious scripts, ensuring that the output is safe to display on web pages.

What are xss's main functionalities?

HTML Filtering

This feature allows you to filter out any HTML tags and content that could lead to XSS attacks, leaving only the safe content.

const xss = require('xss');
let html = '<script>alert("xss");</script><div>safe content</div>';
let safeHtml = xss(html);
console.log(safeHtml); // Output: '<div>safe content</div>'

Custom Rule Configuration

This feature allows you to define custom rules for what HTML tags and attributes are allowed, giving you fine-grained control over the sanitization process.

const xss = require('xss');
let options = {
  whiteList: {
    a: ['href', 'title', 'target'],
    p: [],
    div: []
  },
  stripIgnoreTag: true
};
let html = '<a href="http://example.com" onclick="stealCookies()">Link</a>';
let safeHtml = xss(html, options);
console.log(safeHtml); // Output: '<a href="http://example.com">Link</a>'

Escape HTML

This feature provides a method to escape HTML, converting HTML special characters to their corresponding entities, which is useful when you want to display the original HTML code on the web page without rendering it.

const xss = require('xss');
let html = '<div>hello</div>';
let escapedHtml = xss.escapeHtml(html);
console.log(escapedHtml); // Output: '&lt;div&gt;hello&lt;/div&gt;'

Other packages similar to xss

sanitize-html

sanitize-html is another popular HTML sanitization library that allows you to specify allowed tags and attributes. It is similar to xss but offers a different API and additional options for sanitization.

dompurify

dompurify is a DOM-only XSS sanitizer for HTML, MathML, and SVG. It's different from xss in that it works in the browser and uses the DOM to sanitize input, which can be more effective in some cases.

README

Sanitize untrusted HTML (to prevent XSS) with a configuration specified by a Whitelist.

---

xss is a module used to filter input from users to prevent XSS attacks. (What is XSS attack?)

Project Homepage: http://jsxss.com

Try Online: http://jsxss.com/en/try.html

中文版文档

---

Features

• Specifies HTML tags and their attributes allowed with whitelist
• Handle any tags or attributes using custom function.

Reference

• XSS Filter Evasion Cheat Sheet
• Data URI scheme
• XSS with Data URI Scheme

Benchmark (for references only)

• the xss module: 22.53 MB/s
• xss() function from module validator@0.3.7: 6.9 MB/s

For test code please refer to benchmark directory.

They are using xss module

• nodeclub - A Node.js bbs using MongoDB - https://github.com/cnodejs/nodeclub
• cnpmjs.org - Private npm registry and web for Enterprise - https://github.com/cnpm/cnpmjs.org

Install

NPM

npm install xss

Bower

bower install xss

Or

bower install https://github.com/leizongmin/js-xss.git

Usages

On Node.js

var xss = require("xss");
var html = xss('<script>alert("xss");</script>');
console.log(html);

On Browser

Shim mode (reference file test/test.html):

<script src="https://rawgit.com/leizongmin/js-xss/master/dist/xss.js"></script>
<script>
  // apply function filterXSS in the same way
  var html = filterXSS('<script>alert("xss");</scr' + "ipt>");
  alert(html);
</script>

AMD mode - shim:

<script>
  require.config({
    baseUrl: "./",
    paths: {
      xss: "https://rawgit.com/leizongmin/js-xss/master/dist/xss.js",
    },
    shim: {
      xss: { exports: "filterXSS" },
    },
  });
  require(["xss"], function (xss) {
    var html = xss('<script>alert("xss");</scr' + "ipt>");
    alert(html);
  });
</script>

Notes: please don't use the URL https://rawgit.com/leizongmin/js-xss/master/dist/xss.js in production environment.

Command Line Tool

Process File

You can use the xss command line tool to process a file. Usage:

xss -i <input_file> -o <output_file>

Example:

xss -i origin.html -o target.html

Active Test

Run the following command, them you can type HTML code in the command-line, and check the filtered output:

xss -t

For more details, please run $ xss -h to see it.

Custom filter rules

When using the xss() function, the second parameter could be used to specify custom rules:

options = {}; // Custom rules
html = xss('<script>alert("xss");</script>', options);

To avoid passing options every time, you can also do it in a faster way by creating a FilterXSS instance:

options = {}; // Custom rules
myxss = new xss.FilterXSS(options);
// then apply myxss.process()
html = myxss.process('<script>alert("xss");</script>');

Details of parameters in options would be described below.

Whitelist

By specifying a whiteList, e.g. { 'tagName': [ 'attr-1', 'attr-2' ] }. Tags and attributes not in the whitelist would be filter out. For example:

// only tag a and its attributes href, title, target are allowed
var options = {
  whiteList: {
    a: ["href", "title", "target"],
  },
};
// With the configuration specified above, the following HTML:
// <a href="#" onclick="hello()"><i>Hello</i></a>
// would become:
// <a href="#">&lt;i&gt;Hello&lt;/i&gt;</a>

For the default whitelist, please refer xss.whiteList.

Customize the handler function for matched tags

By specifying the handler function with onTag:

function onTag(tag, html, options) {
  // tag is the name of current tag, e.g. 'a' for tag <a>
  // html is the HTML of this tag, e.g. '<a>' for tag <a>
  // options is some addition informations:
  //   isWhite    boolean, whether the tag is in whitelist
  //   isClosing  boolean, whether the tag is a closing tag, e.g. true for </a>
  //   position        integer, the position of the tag in output result
  //   sourcePosition  integer, the position of the tag in input HTML source
  // If a string is returned, the current tag would be replaced with the string
  // If return nothing, the default measure would be taken:
  //   If in whitelist: filter attributes using onTagAttr, as described below
  //   If not in whitelist: handle by onIgnoreTag, as described below
}

Customize the handler function for attributes of matched tags

By specifying the handler function with onTagAttr:

function onTagAttr(tag, name, value, isWhiteAttr) {
  // tag is the name of current tag, e.g. 'a' for tag <a>
  // name is the name of current attribute, e.g. 'href' for href="#"
  // isWhiteAttr whether the attribute is in whitelist
  // If a string is returned, the attribute would be replaced with the string
  // If return nothing, the default measure would be taken:
  //   If in whitelist: filter the value using safeAttrValue as described below
  //   If not in whitelist: handle by onIgnoreTagAttr, as described below
}

Customize the handler function for tags not in the whitelist

By specifying the handler function with onIgnoreTag:

function onIgnoreTag(tag, html, options) {
  // Parameters are the same with onTag
  // If a string is returned, the tag would be replaced with the string
  // If return nothing, the default measure would be taken (specifies using
  // escape, as described below)
}

Customize the handler function for attributes not in the whitelist

By specifying the handler function with onIgnoreTagAttr:

function onIgnoreTagAttr(tag, name, value, isWhiteAttr) {
  // Parameters are the same with onTagAttr
  // If a string is returned, the value would be replaced with this string
  // If return nothing, then keep default (remove the attribute)
}

Customize escaping function for HTML

By specifying the handler function with escapeHtml. Following is the default function (Modification is not recommended):

function escapeHtml(html) {
  return html.replace(/</g, "&lt;").replace(/>/g, "&gt;");
}

Customize escaping function for value of attributes

By specifying the handler function with safeAttrValue:

function safeAttrValue(tag, name, value) {
  // Parameters are the same with onTagAttr (without options)
  // Return the value as a string
}

Customize CSS filter

If you allow the attribute style, the value will be processed by cssfilter module. The cssfilter module includes a default css whitelist. You can specify the options for cssfilter module like this:

myxss = new xss.FilterXSS({
  css: {
    whiteList: {
      position: /^fixed|relative$/,
      top: true,
      left: true,
    },
  },
});
html = myxss.process('<script>alert("xss");</script>');

If you don't want to filter out the style content, just specify false to the css option:

myxss = new xss.FilterXSS({
  css: false,
});

For more help, please see https://github.com/leizongmin/js-css-filter

Quick Start

Filter out tags not in the whitelist

By using stripIgnoreTag parameter:

• true filter out tags not in the whitelist
• false: by default: escape the tag using configured escape function

Example:

If stripIgnoreTag = true is set, the following code:

code:
<script>
  alert(/xss/);
</script>

would output filtered:

code:alert(/xss/);

Filter out tags and tag bodies not in the whitelist

By using stripIgnoreTagBody parameter:

• false|null|undefined by default: do nothing
• '*'|true: filter out all tags not in the whitelist
• ['tag1', 'tag2']: filter out only specified tags not in the whitelist

Example:

If stripIgnoreTagBody = ['script'] is set, the following code:

code:
<script>
  alert(/xss/);
</script>

would output filtered:

code:

Filter out HTML comments

By using allowCommentTag parameter:

• true: do nothing
• false by default: filter out HTML comments

Example:

If allowCommentTag = false is set, the following code:

code:<!-- something -->
END

would output filtered:

code: END

Examples

Allow attributes of whitelist tags start with data-

var source = '<div a="1" b="2" data-a="3" data-b="4">hello</div>';
var html = xss(source, {
  onIgnoreTagAttr: function (tag, name, value, isWhiteAttr) {
    if (name.substr(0, 5) === "data-") {
      // escape its value using built-in escapeAttrValue function
      return name + '="' + xss.escapeAttrValue(value) + '"';
    }
  },
});

console.log("%s\nconvert to:\n%s", source, html);

Result:

<div a="1" b="2" data-a="3" data-b="4">hello</div>
convert to:
<div data-a="3" data-b="4">hello</div>

Allow tags start with x-

var source = "<x><x-1>he<x-2 checked></x-2>wwww</x-1><a>";
var html = xss(source, {
  onIgnoreTag: function (tag, html, options) {
    if (tag.substr(0, 2) === "x-") {
      // do not filter its attributes
      return html;
    }
  },
});

console.log("%s\nconvert to:\n%s", source, html);

Result:

<x
  ><x-1>he<x-2 checked></x-2>wwww</x-1
  ><a>
    convert to: &lt;x&gt;<x-1>he<x-2 checked></x-2>wwww</x-1><a></a></a
></x>

Parse images in HTML

var source =
  '<img src="img1">a<img src="img2">b<img src="img3">c<img src="img4">d';
var list = [];
var html = xss(source, {
  onTagAttr: function (tag, name, value, isWhiteAttr) {
    if (tag === "img" && name === "src") {
      // Use the built-in friendlyAttrValue function to escape attribute
      // values. It supports converting entity tags such as &lt; to printable
      // characters such as <
      list.push(xss.friendlyAttrValue(value));
    }
    // Return nothing, means keep the default handling measure
  },
});

console.log("image list:\n%s", list.join(", "));

Result:

image list: img1, img2, img3, img4

Filter out HTML tags (keeps only plain text)

var source = "<strong>hello</strong><script>alert(/xss/);</script>end";
var html = xss(source, {
  whiteList: {}, // empty, means filter out all tags
  stripIgnoreTag: true, // filter out all HTML not in the whitelist
  stripIgnoreTagBody: ["script"], // the script tag is a special case, we need
  // to filter out its content
});

console.log("text: %s", html);

Result:

text: helloend

License

Copyright (c) 2012-2018 Zongmin Lei(雷宗民) <[e-mail]>
http://ucdok.com

The MIT License

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Changelog

v1.0.10 (2021-10-08)

• Fix: #239 stripCommentTag DoS attack