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README.md

Escape

Escape is a small set of methods for escaping tainted data. These escaping methods are useful in transforming user-controlled ("tainted") data into forms that are safe from being interpreted as something other than data, such as HTML with JavaScript (typical cross-site scripting attack).

At this time these escaping routines mostly focus on cross-site scripting (XSS) mitigations. Each method is good for different HTML contexts. (See below for a discussion on different contexts.)

While Coverity's static analysis product references these escaping routines and understands their behavior; however, there is no dependency on Coverity products. This library is completely standalone. Feel free to use them! Just make sure you use them correctly :)

Table of Contents

  1. Installation
  2. Usage
  3. HTML Contexts Examples
  4. Authors & License

Installation

Using Maven

To include this library into your Maven project, add the following to your pom:

<dependency>
    <groupId>com.coverity</groupId>
    <artifactId>coverity-escapers</artifactId>
    <version>1.0.0</version>
</dependency>

Manually Build and Deploy

We use maven to build the library, and you can simply do:

$ cd coverity-security-library
$ mvn package

A JAR file will be created in the coverity-escapers/target directory. You can take this JAR file coverity-escaper-1.0.0.jar and place it in the WEB-INF/lib of your application.

To use the Escape library in a JSP scriptlet, you need to import the class:

<%@ page import="com.coverity.security.Escape" %>

Build the Javadoc

The javadoc can be created directly from the Maven build:

$ cd coverity-security-library
$ mvn install
$ open ./coverity-escapers/target/apidocs/index.html

Usage

Example 1: XSS Defect in Java Servlet

Before Remediation

The servlet below takes a request parameter called index and directly inserts it into the output within an HTML context, creating an XSS defect.

public class IndexServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
                         throws ServletException, IOException {
        String param = request.getParameter("index");           
        PrintWriter out = response.getWriter();
        response.setContentType("text/html");
        out.write("<html><body>Index requested: " + param);

After Remediation

To remedy, the Escape library needs to be imported into the project and then the Escape.html method should wrap the param at the injection point.

import com.coverity.security.Escape;
// ...
public class IndexServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
                         throws ServletException, IOException {
        String param = request.getParameter("index");           
        PrintWriter out = response.getWriter();
        response.setContentType("text/html");
        out.write("<html><body>Index requested: " + Escape.html(param));

Example 2: XSS Defect in JSP EL

Before Remediation

The JSP below takes a request parameter called param.needHelp and inserts it into the page within a JavaScript single-quoted string context, within a parent HTML double-quoted attribute context, creating an XSS defect. It uses Expression Language (EL) to insert the value. While this tainted data is wrapped by the JSTL fn:escapeXml method, the defect still exists because the underlying JavaScript string context is not addressed.

<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8" %>
<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>

<!doctype html>
<html>
<head>
    <script src="/static/js/main.js"></script>
</head>
<body>
<span onmouseover="lookupHelp('${fn:escapeXml(param.needHelp)}');">
    Hello Blogger!
</span>

After Remediation

To remedy this defect, the Escape library needs to be imported into the project and then the cov:jsStringEscape EL method needs to wrap the param.needHelp at the injection point. The outer fn:escapeXml method should still be used to ensure values are properly escaped for the HTML attribute value context.

<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8" %>
<%@ taglib prefix="fn"  uri="http://java.sun.com/jsp/jstl/functions" %>
<%@ taglib prefix="cov" uri="http://coverity.com/security" %>

<!doctype html>
<html>
<head>
    <script src="/static/js/main.js"></script>
</head>
<body>
<span onmouseover="lookupHelp('${fn:escapeXml(cov:jsStringEscape(param.needHelp))}');">
    Hello Blogger!
</span>

Note that if you want to limit the number of EL functions imported, you can use the cov:htmlEscape function instead of fn:escapeXml.

Background Information

Contexts

When fixing a defect, you need to understand the current context, the safety obligations for that context, and what characters or sequences violate these obligations. A context defines a subset of a language and syntax rules. For example, the following TAINTED_DATA_HERE text occurs in an HTML double-quoted attribute context.

<span id="TAINTED_DATA_HERE">Some text here</span>

When tainted data is able to circumvent a context, it can lead to a security defect, such as a cross-site scripting (XSS), SQL injection (SQLi), etc.. For example, once outside of an HTML double-quoted attribute context, the inserted data can create a new attribute such as onmouseover. This attribute name is a DOM event handler. Browsers interpret the onmouseover attribute value as JavaScript, permitting an XSS defect.

Each context has a set of safety obligations many of which are met by not inserting characters with special meaning within that context. The purpose of this library is to assist developers by sanitizing tainted data for some common contexts. However, not all contexts are addressed by this library. Some contexts require more than character-level safety obligations and therefore it is not possible to create escapers for these. For example, when inserting characters into an HTML attribute name, not only are certain characters disallowed, but a set of names should also be disallowed since they might create an XSS defect.

Nested Contexts

A nested context occurs when more than one context exists for a given piece of data. An example is the common HTML <a> anchor element and its onclick attribute:

<a onclick="pullAuthor('TAINTED_DATA_HERE');return false;">...

In the example, there are currently two contexts that have safety obligations for TAINTED_DATA_HERE:

  • HTML double-quoted attribute
  • JavaScript single-quoted string

Common libraries exist for sanitizing user data for the first context (HTML escaping is fine). However, if the JavaScript string context is left untreated, an attacker can execute an XSS attack in it. More so, HTML entity encoding the single quote ' to &#39; is ineffective. In this context, the browser decodes the HTML entity back to a single quote when passing it to the JavaScript engine. Therefore, to fully remedy this defect, the safety obligations of the JavaScript string context need to be met before the obligations of the HTML context.

Remember that with nested contexts, order of escaping matters.

Common Contexts

HTML

The Escape library groups the following HTML contexts as one:

HTML normal element injection example:

<span>TAINTED_DATA_HERE</span>

HTML quoted attribute injection example:

<div id="TAINTED_DATA_HERE">
    <span id='TAINTED_DATA_HERE_TOO'>Testing blog</span>
</div>

The Escape library meets the security obligations of these contexts by encoding sensitive characters as HTML character references.

Escape functions to use:

  • Java/JSP scriptlet: Escape.html()
  • JSP EL: ${cov:htmlEscape()}

JavaScript Strings (Single and Double Quoted)

ECMA 262 defines the ECMAScript language, of which JavaScript is a dialect. The standard defines a string literal syntax for both ' and " strings in section 7.8.4 (of the ECMA PDF file).

Injection example:

var blogComment = 'TAINTED_DATA_HERE';
logBlogComment(blogComment, "TAINTED_DATA_HERE_TOO");

The Escape library meets the security obligations of these contexts by escaping these characters using JavaScript Unicode escaping. In addition, since JavaScript is usually embedded withing a script tag, the JavaScript string context inherits the security obligations which apply for the script tag. This is easily summarized as the tag should not be closed, and the string literal </script> should not appear in the JavaScript string. For this purpose, we also escape the / character.

Escape functions to use:

  • Java/JSP scriptlet: Escape.jsString()
  • JSP EL: ${cov:jsStringEscape()}

CSS Strings (Single and Double Quoted)

CSS Level 2, Revision 1 (CSS 2.1) defines single-quoted (', U+0027) and double-quoted (", U+0022) strings. These strings are also used within a URL quoted context and have the same obligations within that context.

Injection example:

span[id="TAINTED_DATA_HERE"] {
  background-color: #efefef;
}

The Escape library meets the security obligations of these contexts by escaping these characters using CSS Unicode escaping. Just as JavaScript string contexts are often in a parent <script> tag, CSS contexts often have a parent HTML context within the <style> tag. For the same reason as JavaScript, we also escape the / character.

Escape functions to use:

  • Java/JSP scriptlet: Escape.cssString()
  • JSP EL: ${cov:cssStringEscape()}

URIs

The URI context is comprised of numerous sub-contexts. RFC 3986 provides details on each of them. When used in HTML, the URL context includes some parent context, such as HTML, JavaScript, or CSS.

Injection examples:

<style>
    #clickme a {
      background-image: url('TAINTED_DATA_HERE');
    }
</style>
<a id="clickme" href="http://www.example.com/?test=TAINTED_DATA_HERE">Click me!</a>

When the tainted data is inserted as a query parameter, the Escape library meets the URI query parameter obligations by encoding sensitive characters using URI percent encoding.

Escape functions to use:

  • Java/JSP scriptlet: Escape.uri()
  • JSP EL: ${cov:uriEncode()}

SQL LIKE Context

SQL LIKE clauses use special characters to perform wildcard matching. When tainted data is used within a SQL LIKE clause, even is passed via a named parameter, the tainted data should have these wildcards escaped. The escaping preserves the intent of the wild cards in the LIKE clause. For example, if only a trailing percent sign (%, U+0025) is used in the clause, then if the tainted data included a leading percent sign, more results could possibly be returned, changing the intent of the query.

Injection example:

entityManager.createQuery("FROM MyEntity e WHERE e.content LIKE :like_query")
             .setParameter("like_query", "%" + TAINTED_DATA_HERE)
             .getResultList();

The Escape library meets these obligations by escaping these wildcard characters using an additional escape character, by default the at sign (@, U+0040):

entityManager.createQuery("FROM MyEntity e WHERE e.content LIKE :like_query ESCAPE '@'")
             .setParameter("like_query", "%" + Escape.sqlLikeClause(TAINTED_DATA_HERE))
             .getResultList();

Note: the Escape library does not prevent SQL injection issues. It preserves the meaning of the LIKE query by escaping only characters with special meaning in a LIKE clause.

Escape function to use:

  • Java/JSP scriptlet: Escape.sqlLikeClause()

Unquoted HTML attributes or CSS URI

HTML allows attribute values and CSS allows URI values to be used in an unquoted values, along with their single and double-quoted alternatives. We recommend not using the unquoted values in HTML or CSS. Rather, use the double or single quoted values. The reasoning is that unquoted values make it even more difficult to mitigate and are sometimes web browser specific.

Authors

The Escape library was developed by the Coverity Security Research Lab members:

License

Copyright (c) 2012, Coverity, Inc. 
All rights reserved.

Redistribution and use in source and binary forms, with or without modification, 
are permitted provided that the following conditions are met:
- Redistributions of source code must retain the above copyright notice, this 
list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright notice, this
list of conditions and the following disclaimer in the documentation and/or other
materials provided with the distribution.
- Neither the name of Coverity, Inc. nor the names of its contributors may be used
to endorse or promote products derived from this software without specific prior 
written permission from Coverity, Inc.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND INFRINGEMENT ARE DISCLAIMED.
IN NO EVENT SHALL THE COPYRIGHT HOLDER OR  CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR 
PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, 
WHETHER IN CONTRACT,  STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY 
OF SUCH DAMAGE.
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