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.

---

NOTE: The format of custom configuration (except Whitelist) from version 0.0.X was changed a lot since version 0.1.X. To use a newer version, it's suggested to read the following guidelines carefully.

中文版文档

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

This module is needed for situations that allows users to input HTML for typesetting or formatting, including fourms, blogs, e-shops, etc.

The xss module controls the usage of tags and their attributes, according to the whitelist. It is also extendable with a series of APIs privided, which make it become more flexible, compares with other modules.

Project Homepage: https://github.com/leizongmin/js-xss

---

Features

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

Reference

• XSS与字符编码的那些事儿 ---科普文
• 腾讯实例教程：那些年我们一起学XSS
• XSS Filter Evasion Cheat Sheet
• Data URI scheme
• XSS with Data URI Scheme

Benchmark (for references only)

• the xss module: 8.2 MB/s
• xss() function from module validator@0.3.7: 4.4 MB/s

For test code please refer to benchmark directory.

Unit Test

Run npm test command in the source directary.

Active Test

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

$ xss -t

Command Line Tool

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

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

Usages

In Node.js

To install:

$ npm install xss

Simple usage:

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

In browsers

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

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="#">Hello</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 tag 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
}

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:

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>

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 whilelist
  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

The MIT License

Donate

We made xss in our spare time because it's fun to build things. We hope you find it useful.

If you like this package and want to support future development—or just say thank you, please consider making a donation of $5, $10 or $20 (or whatever you can afford) to help us continue to improve it.

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