Owasp Xss Prevention Cheat Sheet

Owasp Xss Prevention Cheat Sheet Github
Owasp Xss Prevention Cheat Sheet Free
Cross Site Scripting Cheat Sheet

Content Security Policy Cheat Sheet¶ Introduction¶. This article brings forth a way to integrate the defense in depth concept to the client-side of web applications. By injecting the Content-Security-Policy (CSP) headers from the server, the browser is aware and capable of protecting the user from dynamic calls that will load content into the page currently being visited.
This cheat sheet lists a series of XSS attacks that can be used to bypass certain XSS defensive filters. Please note that input filtering is an incomplete defense for XSS which these tests can be used to illustrate. Basic XSS Test Without Filter Evasion.

This document has for objective to provide some tips to handle Injection into Java application code.

Sample codes used in tips are located here.

Injection in OWASP Top 10 is defined as following:

Consider anyone who can send untrusted data to the system, including external users, internal users, and administrators.

The following point can be applied, in a general way, to prevent Injection issue:

Apply Input Validation (using whitelist approach) combined with Output Sanitizing+Escaping on user input/output.
If you need to interact with system, try to use API features provided by your technology stack (Java / .Net / PHP...) instead of building command.

The Session Management General Guidelines previously available on this OWASP Authentication Cheat Sheet have been integrated into the Session Management Cheat Sheet. Password Managers Password managers are programs, browser plugins or web services that automate management of large number of different credentials, including memorizing.

Additional advices are provided on this cheatsheet.

Examples in this section will be provided in Java technology (see Maven project associated) but advices are applicable to others technologies like .Net / PHP / Ruby / Python...

SQL

Symptom

Injection of this type occur when the application use untrusted user input to build a SQL query using a String and execute it.

How to prevent

Use Query Parameterization in order to prevent injection.

Example

References

JPA

Symptom

Injection of this type occur when the application use untrusted user input to build a JPA query using a String and execute it. It's quite similar to SQL injection but here the altered language is not SQL but JPA QL.

How to prevent

Use Java Persistence Query Language Query Parameterization in order to prevent injection.

Example

References

Operating System

Symptom

Injection of this type occur when the application use untrusted user input to build a Operating System command using a String and execute it.

How to prevent

Owasp Xss Prevention Cheat Sheet Github

Use technology stack API in order to prevent injection.

Example

References

XML: XPath Injection

Symptom

Injection of this type occur when the application use untrusted user input to build a XPath query using a String and execute it.

How to prevent

Use XPath Variable Resolver in order to prevent injection.

Example

Variable Resolver implementation.

Code using it to perform XPath query.

References

HTML/JavaScript/CSS

Symptom

Injection of this type occur when the application use untrusted user input to build a HTTP response and sent it to browser.

How to prevent

Either apply strict input validation (whitelist approach) or use output sanitizing+escaping if input validation is not possible (combine both every time is possible).

Example

References

XSS)

LDAP

A dedicated cheatsheet has been created.

NoSQL

Symptom

Injection of this type occur when the application use untrusted user input to build a NoSQL API call expression.

How to prevent

As there many NoSQL database system and each one use a API for call, it's important to ensure that user input received and used to build the API call expression do not contains any character that have a special meaning in the target API syntax. This in order to avoid that it will be used to escape the initial call expression in order to create another one based on crafted user input. It's also important to not use string concatenation to build API call expression but use the API to create the expression.

Example - MongoDB

References

Log Injection

Symptom

Log Injection occurs when an application includes untrusted data in an application log message (e.g., an attacker can cause an additional log entry that looks like it came from a completely different user, if they can inject CRLF characters in the untrusted data). More information about this attack is available on the OWASP Log Injection page.

How to prevent

To prevent an attacker from writing malicious content into the application log, apply defenses such as:

Filter the user input used to prevent injection of Carriage Return (CR) or Line Feed (LF) characters.
Limit the size of the user input value used to create the log message.
Make sure all XSS defenses are applied when viewing log files in a web browser.

Example using Log4j2

Configuration of a logging policy to roll on 10 files of 5MB each, and encode/limit the log message using the Pattern encode{}{CRLF}, introduced in Log4j2 v2.10.0, and the -500m message size limit.:

Usage of the logger at code level:

Example using Logback with the OWASP Security Logging library

Configuration of a logging policy to roll on 10 files of 5MB each, and encode/limit the log message using the CRLFConverter, provided by the OWASP Security Logging Project, and the -500msg message size limit:

You also have to add the OWASP Security Logging dependency to your project.

Usage of the logger at code level:

References

PatternLayout (See the encode{}{CRLF} function)

Log Forging - See the Logback section about the CRLFConverter this library provides.

Dominique Righetto - dominique.righetto@owasp.org

Dave Wichers - dave.wichers@owasp.org

Last revision (mm/dd/yy): 10/8/2018

1Introduction
2XSS Prevention Rules
- 2.4RULE #3 - JavaScript Escape Before Inserting Untrusted Data into JavaScript Data Values
  - 2.4.1RULE #3.1 - HTML escape JSON values in an HTML context and read the data with JSON.parse

This article provides a simple positive model for preventing XSS using output escaping/encoding properly. While there are a huge number of XSS attack vectors, following a few simple rules can completely defend against this serious attack. This article does not explore the technical or business impact of XSS. Suffice it to say that it can lead to an attacker gaining the ability to do anything a victim can do through their browser.

Both reflected and stored XSS can be addressed by performing the appropriate validation and escaping on the server-side. DOM Based XSS can be addressed with a special subset of rules described in the DOM based XSS Prevention Cheat Sheet.

For a cheatsheet on the attack vectors related to XSS, please refer to the XSS Filter Evasion Cheat Sheet. More background on browser security and the various browsers can be found in the Browser Security Handbook.

Before reading this cheatsheet, it is important to have a fundamental understanding of Injection Theory.

A Positive XSS Prevention Model

This article treats an HTML page like a template, with slots where a developer is allowed to put untrusted data. These slots cover the vast majority of the common places where a developer might want to put untrusted data. Putting untrusted data in other places in the HTML is not allowed. This is a 'whitelist' model, that denies everything that is not specifically allowed.

Given the way browsers parse HTML, each of the different types of slots has slightly different security rules. When you put untrusted data into these slots, you need to take certain steps to make sure that the data does not break out of that slot into a context that allows code execution. In a way, this approach treats an HTML document like a parameterized database query - the data is kept in specific places and is isolated from code contexts with escaping.

This document sets out the most common types of slots and the rules for putting untrusted data into them safely. Based on the various specifications, known XSS vectors, and a great deal of manual testing with all the popular browsers, we have determined that the rules proposed here are safe.

The slots are defined and a few examples of each are provided. Developers SHOULD NOT put data into any other slots without a very careful analysis to ensure that what they are doing is safe. Browser parsing is extremely tricky and many innocuous looking characters can be significant in the right context.

Why Can't I Just HTML Entity Encode Untrusted Data?

HTML entity encoding is okay for untrusted data that you put in the body of the HTML document, such as inside a <div> tag. It even sort of works for untrusted data that goes into attributes, particularly if you're religious about using quotes around your attributes. But HTML entity encoding doesn't work if you're putting untrusted data inside a <script> tag anywhere, or an event handler attribute like onmouseover, or inside CSS, or in a URL. So even if you use an HTML entity encoding method everywhere, you are still most likely vulnerable to XSS. You MUST use the escape syntax for the part of the HTML document you're putting untrusted data into. That's what the rules below are all about.

You Need a Security Encoding Library

Writing these encoders is not tremendously difficult, but there are quite a few hidden pitfalls. For example, you might be tempted to use some of the escaping shortcuts like ' in JavaScript. However, these values are dangerous and may be misinterpreted by the nested parsers in the browser. You might also forget to escape the escape character, which attackers can use to neutralize your attempts to be safe. OWASP recommends using a security-focused encoding library to make sure these rules are properly implemented.

Microsoft provides an encoding library named the Microsoft Anti-Cross Site Scripting Library for the .NET platform and ASP.NET Framework has built-in ValidateRequest function that provides limited sanitization.

The OWASP Java Encoder Project provides a high-performance encoding library for Java.

The following rules are intended to prevent all XSS in your application. While these rules do not allow absolute freedom in putting untrusted data into an HTML document, they should cover the vast majority of common use cases. You do not have to allow all the rules in your organization. Many organizations may find that allowing only Rule #1 and Rule #2 are sufficient for their needs. Please add a note to the discussion page if there is an additional context that is often required and can be secured with escaping.

Do NOT simply escape the list of example characters provided in the various rules. It is NOT sufficient to escape only that list. Blacklist approaches are quite fragile. The whitelist rules here have been carefully designed to provide protection even against future vulnerabilities introduced by browser changes.

RULE #0 - Never Insert Untrusted Data Except in Allowed Locations

The first rule is to deny all - don't put untrusted data into your HTML document unless it is within one of the slots defined in Rule #1 through Rule #5. The reason for Rule #0 is that there are so many strange contexts within HTML that the list of escaping rules gets very complicated. We can't think of any good reason to put untrusted data in these contexts. This includes 'nested contexts' like a URL inside a javascript -- the encoding rules for those locations are tricky and dangerous. If you insist on putting untrusted data into nested contexts, please do a lot of cross-browser testing and let us know what you find out.

Most importantly, never accept actual JavaScript code from an untrusted source and then run it. For example, a parameter named 'callback' that contains a JavaScript code snippet. No amount of escaping can fix that.

RULE #1 - HTML Escape Before Inserting Untrusted Data into HTML Element Content

Rule #1 is for when you want to put untrusted data directly into the HTML body somewhere. This includes inside normal tags like div, p, b, td, etc. Most web frameworks have a method for HTML escaping for the characters detailed below. However, this is absolutely not sufficient for other HTML contexts. You need to implement the other rules detailed here as well.

<div>...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...

Escape the following characters with HTML entity encoding to prevent switching into any execution context, such as script, style, or event handlers. Using hex entities is recommended in the spec. In addition to the 5 characters significant in XML (&, <, >, ', '), the forward slash is included as it helps to end an HTML entity.

RULE #2 - Attribute Escape Before Inserting Untrusted Data into HTML Common Attributes

Rule #2 is for putting untrusted data into typical attribute values like width, name, value, etc. This should not be used for complex attributes like href, src, style, or any of the event handlers like onmouseover. It is extremely important that event handler attributes should follow Rule #3 for HTML JavaScript Data Values.

<div attr=...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...>content

inside UNquoted attribute

<div attr='...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...'>content

inside single quoted attribute

<div attr='...ESCAPE UNTRUSTED DATA BEFORE PUTTING HERE...'>content

inside double quoted attribute

Except for alphanumeric characters, escape all characters with ASCII values less than 256 with the &#xHH; format (or a named entity if available) to prevent switching out of the attribute. The reason this rule is so broad is that developers frequently leave attributes unquoted. Properly quoted attributes can only be escaped with the corresponding quote. Unquoted attributes can be broken out of with many characters, including [space] % * + , - / ; < = > ^ and |.

RULE #3 - JavaScript Escape Before Inserting Untrusted Data into JavaScript Data Values

Rule #3 concerns dynamically generated JavaScript code - both script blocks and event-handler attributes. The only safe place to put untrusted data into this code is inside a quoted 'data value.' Including untrusted data inside any other JavaScript context is quite dangerous, as it is extremely easy to switch into an execution context with characters including (but not limited to) semi-colon, equals, space, plus, and many more, so use with caution.

Please note there are some JavaScript functions that can never safely use untrusted data as input - EVEN IF JAVASCRIPT ESCAPED!

For example:

Except for alphanumeric characters, escape all characters less than 256 with the xHH format to prevent switching out of the data value into the script context or into another attribute. DO NOT use any escaping shortcuts like ' because the quote character may be matched by the HTML attribute parser which runs first. These escaping shortcuts are also susceptible to 'escape-the-escape' attacks where the attacker sends ' and the vulnerable code turns that into ' which enables the quote.

If an event handler is properly quoted, breaking out requires the corresponding quote. However, we have intentionally made this rule quite broad because event handler attributes are often left unquoted. Unquoted attributes can be broken out of with many characters including [space] % * + , - / ; < = > ^ and |. Also, a </script> closing tag will close a script block even though it is inside a quoted string because the HTML parser runs before the JavaScript parser. Please note this is an aggressive escaping policy that over-encodes. If there is a guarantee that proper quoting is accomplished then a much smaller character set is needed. Please look at the OWASP Java Encoder JavaScript escaping examples for examples of proper JavaScript use that requires minimal escaping. https://www.owasp.org/index.php/OWASP_Java_Encoder_Project#tab=Use_the_Java_Encoder_Project

RULE #3.1 - HTML escape JSON values in an HTML context and read the data with JSON.parse

Owasp Xss Prevention Cheat Sheet Free

In a Web 2.0 world, the need for having data dynamically generated by an application in a javascript context is common. One strategy is to make an AJAX call to get the values, but this isn't always performant. Often, an initial block of JSON is loaded into the page to act as a single place to store multiple values. This data is tricky, though not impossible, to escape correctly without breaking the format and content of the values.

Ensure returned Content-Type header is application/json and not text/html. This shall instruct the browser not misunderstand the context and execute injected script

Bad HTTP response:

Good HTTP response

A common anti-pattern one would see:

JSON serialization

A safe JSON serializer will allow developers to serialize JSON as string of literal JavaScript which can be embedded in an HTML in the contents of the <script> tag. HTML characters and JavaScript line terminators need be escaped. Consider the Yahoo JavaScript Serializer for this task. https://github.com/yahoo/serialize-javascript

HTML entity encoding

This technique has the advantage that html entity escaping is widely supported and helps separate data from server side code without crossing any context boundaries. Consider placing the JSON block on the page as a normal element and then parsing the innerHTML to get the contents. The javascript that reads the span can live in an external file, thus making the implementation of CSP enforcement easier.

An alternative to escaping and unescaping JSON directly in JavaScript, is to normalize JSON server-side by converting '<' to 'u003c' before delivering it to the browser.

RULE #4 - CSS Escape And Strictly Validate Before Inserting Untrusted Data into HTML Style Property Values

Rule #4 is for when you want to put untrusted data into a stylesheet or a style tag. CSS is surprisingly powerful, and can be used for numerous attacks. Therefore, it's important that you only use untrusted data in a property value and not into other places in style data. You should stay away from putting untrusted data into complex properties like url, behavior, and custom (-moz-binding). You should also not put untrusted data into IE’s expression property value which allows JavaScript.

Please note there are some CSS contexts that can never safely use untrusted data as input - EVEN IF PROPERLY CSS ESCAPED! You will have to ensure that URLs only start with 'http' not 'javascript' and that properties never start with 'expression'.

For example:

Except for alphanumeric characters, escape all characters with ASCII values less than 256 with the HH escaping format. DO NOT use any escaping shortcuts like ' because the quote character may be matched by the HTML attribute parser which runs first. These escaping shortcuts are also susceptible to 'escape-the-escape' attacks where the attacker sends ' and the vulnerable code turns that into ' which enables the quote.

If attribute is quoted, breaking out requires the corresponding quote. All attributes should be quoted but your encoding should be strong enough to prevent XSS when untrusted data is placed in unquoted contexts. Unquoted attributes can be broken out of with many characters including [space] % * + , - / ; < = > ^ and |. Also, the </style> tag will close the style block even though it is inside a quoted string because the HTML parser runs before the JavaScript parser. Please note that we recommend aggressive CSS encoding and validation to prevent XSS attacks for both quoted and unquoted attributes.

RULE #5 - URL Escape Before Inserting Untrusted Data into HTML URL Parameter Values

Rule #5 is for when you want to put untrusted data into HTTP GET parameter value.

Except for alphanumeric characters, escape all characters with ASCII values less than 256 with the %HH escaping format. Including untrusted data in data: URLs should not be allowed as there is no good way to disable attacks with escaping to prevent switching out of the URL. All attributes should be quoted. Unquoted attributes can be broken out of with many characters including [space] % * + , - / ; < = > ^ and |. Note that entity encoding is useless in this context.

WARNING: Do not encode complete or relative URL's with URL encoding! If untrusted input is meant to be placed into href, src or other URL-based attributes, it should be validated to make sure it does not point to an unexpected protocol, especially Javascript links. URL's should then be encoded based on the context of display like any other piece of data. For example, user driven URL's in HREF links should be attribute encoded. For example:

RULE #6 - Sanitize HTML Markup with a Library Designed for the Job

If your application handles markup -- untrusted input that is supposed to contain HTML -- it can be very difficult to validate. Encoding is also difficult, since it would break all the tags that are supposed to be in the input. Therefore, you need a library that can parse and clean HTML formatted text. There are several available at OWASP that are simple to use:

HtmlSanitizer - https://github.com/mganss/HtmlSanitizer

An open-source .Net library. The HTML is cleaned with a white list approach. All allowed tags and attributes can be configured. The library is unit tested with the OWASP XSS Filter Evasion Cheat Sheet

OWASP Java HTML Sanitizer - OWASP Java HTML Sanitizer Project

For more information on OWASP Java HTML Sanitizer policy construction, see https://github.com/OWASP/java-html-sanitizer

Ruby on Rails SanitizeHelper - http://api.rubyonrails.org/classes/ActionView/Helpers/SanitizeHelper.html

The SanitizeHelper module provides a set of methods for scrubbing text of undesired HTML elements.

Other libraries that provide HTML Sanitization include:

PHP HTML Purifier - http://htmlpurifier.org/
JavaScript/Node.js Bleach - https://github.com/ecto/bleach
Python Bleach - https://pypi.python.org/pypi/bleach

RULE #7 - Prevent DOM-based XSS

For details on what DOM-based XSS is, and defenses against this type of XSS flaw, please see the OWASP article on DOM based XSS Prevention Cheat Sheet.

Bonus Rule #1: Use HTTPOnly cookie flag

Preventing all XSS flaws in an application is hard, as you can see. To help mitigate the impact of an XSS flaw on your site, OWASP also recommends you set the HTTPOnly flag on your session cookie and any custom cookies you have that are not accessed by any Javascript you wrote. This cookie flag is typically on by default in .NET apps, but in other languages you have to set it manually. For more details on the HTTPOnly cookie flag, including what it does, and how to use it, see the OWASP article on HTTPOnly.

Bonus Rule #2: Implement Content Security Policy

There is another good complex solution to mitigate the impact of an XSS flaw called Content Security Policy. It's a browser side mechanism which allows you to create source whitelists for client side resources of your web application, e.g. JavaScript, CSS, images, etc. CSP via special HTTP header instructs the browser to only execute or render resources from those sources. For example this CSP

will instruct web browser to load all resources only from the page's origin and JavaScript source code files additionaly from static.domain.tld. For more details on Content Security Policy, including what it does, and how to use it, see the OWASP article on Content_Security_Policy

Bonus Rule #3: Use an Auto-Escaping Template System

Many web application frameworks provide automatic contextual escaping functionality such as AngularJS strict contextual escaping and Go Templates. Use these technologies when you can.

Bonus Rule #4: Use the X-XSS-Protection Response Header

This HTTP response header enables the Cross-site scripting (XSS) filter built into some modern web browsers. This header is usually enabled by default anyway, so the role of this header is to re-enable the filter for this particular website if it was disabled by the user.

The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts.

Data Type	Context	Code Sample	Defense
String	HTML Body	<span>UNTRUSTED DATA</span>
String	Safe HTML Attributes	<input type='text' name='fname' value='UNTRUSTED DATA'>	Only place untrusted data into a whitelist of safe attributes (listed below). Strictly validate unsafe attributes such as background, id and name.
String	GET Parameter	<a href='/site/search?value=UNTRUSTED DATA'>clickme</a>
String	Untrusted URL in a SRC or HREF attribute	<a href='UNTRUSTED URL'>clickme</a> <iframe src='UNTRUSTED URL' />	Canonicalize input URL Validation Safe URL verification Whitelist http and https URL's only (Avoid the JavaScript Protocol to Open a new Window) Attribute encoder
String	CSS Value	<divcolor:red;'>UNTRUSTED DATA;'>Selection</div>	CSS Hex encoding Good design of CSS Features
String	JavaScript Variable	<script>var currentValue='UNTRUSTED DATA';</script> <script>someFunction('UNTRUSTED DATA');</script>	Ensure JavaScript variables are quoted JavaScript Hex Encoding JavaScript Unicode Encoding Avoid backslash encoding (' or ' or )
HTML	HTML Body	<div>UNTRUSTED HTML</div>
String	DOM XSS	<script>document.write('UNTRUSTED INPUT: ' + document.location.hash);<script/>

Cross Site Scripting Cheat Sheet

Safe HTML Attributes include: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width

The purpose of output encoding (as it relates to Cross Site Scripting) is to convert untrusted input into a safe form where the input is displayed as data to the user without executing as code in the browser. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting.

Encoding Type	Encoding Mechanism
HTML Entity Encoding	Convert & to & Convert < to < Convert > to > Convert ' to " Convert ' to ' Convert / to /
HTML Attribute Encoding	Except for alphanumeric characters, escape all characters with the HTML Entity &#xHH; format, including spaces. (HH = Hex Value)
URL Encoding	Standard percent encoding, see: http://www.w3schools.com/tags/ref_urlencode.asp. URL encoding should only be used to encode parameter values, not the entire URL or path fragments of a URL.
JavaScript Encoding	Except for alphanumeric characters, escape all characters with the uXXXX unicode escaping format (X = Integer).
CSS Hex Encoding	CSS escaping supports XX and XXXXXX. Using a two character escape can cause problems if the next character continues the escape sequence. There are two solutions (a) Add a space after the CSS escape (will be ignored by the CSS parser) (b) use the full amount of CSS escaping possible by zero padding the value.

XSS Attack Cheat Sheet

The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid:

OWASP: XSS Filter Evasion Cheat Sheet - Based on - RSnake's: 'XSS Cheat Sheet'

Description of XSS Vulnerabilities

OWASP article on XSS Vulnerabilities

Discussion on the Types of XSS Vulnerabilities

How to Review Code for Cross-site scripting Vulnerabilities

OWASP Code Review Guide article on Reviewing Code for Cross-site scripting Vulnerabilities

How to Test for Cross-site scripting Vulnerabilities

OWASP Testing Guide article on Testing for Cross site scripting Vulnerabilities

Jeff Williams - jeff.williams[at]contrastsecurity.com
Jim Manico - jim[at]owasp.org
Neil Mattatall - neil[at]owasp.org