DOM based XSS Prevention Cheat Sheet

= Introduction =

When looking at XSS (Cross Site Scripting) there are three generally recognized forms of XSS. Reflected, Persisted, and DOM based XSS. The XSS Prevention Cheatsheet has done an excellent job of addressing Reflected and Persisted XSS. This cheatsheet will address DOM (Document Object Model) based XSS and is an extension (as assumes comprehension of) the the XSS Prevention Cheatsheet.

In order to understand DOM based XSS one needs to see the fundamental difference between reflected and persisted XSS when compared to DOM based XSS. Reflected and persisted XSS exist in a higher level rendering context and DOM based XSS is primarily found in a lower level execution context. A rendering context is associated with the parsing of HTML tags and their attributes. The HTML parser of the rendering context dictates how data is presented and laid out on the page and can be further broken down into the standard contexts of HTML, HTML attribute, URL, and CSS. The JavaScript or VBScript parser of an execution context is associated with the parsing and execution of script code. Each parser has distinct and separate semantics in the way they can possibly execute script code (XSS) which make creating consistent rules for mitigating both rendering and execution based contexts difficult. The complication is compounded by the differing meanings and treatment of encoded values within each sub context (HTML, HTML attribute, URL, and CSS) within the execution context.

This paper refers to the HTML, HTML attribute, URL, and CSS Cheatsheet contexts as subcontexts because each of these contexts can be reached and set within a JavaScript execution context. In JavaScript code the main context is JavaScript but since an attacker can try to attack the other 4 contexts using equivalent JavaScript DOM methods we refer to the other contexts besides the JavaScript context as sub contexts.

The following example of an attack which occurs in the JavaScript context and HTML sub context:

One consistency, however, is the need to JavaScript encode in addition to the encoding required for the sub context in the execution context. Let’s look at the individual sub contexts of the execution context in turn.

= HTML Sub Context within the Execution Context =

There are several methods and attributes which can be used to directly render HTML content within JavaScript. These methods constitute the HTML Sub Context within the Execution Context.

Guideline
In a pure HTML execution context (not HTML Attribute) use HTML and JavaScript encoding to mitigate against attacks.

Methods
= HTML Attribute Sub Context within the Execution Context =

The HTML attribute Sub Context within the Execution context is divergent from the standard encoding rules. This is because the rule to HTML attribute encode in an HTML attribute rendering context is mitigating attacks which try to exit out of the attribute to add additional attributes and/or tags which could have executable code. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes).

For example, the general rule is to HTML Attribute encode untrusted data (data from the database, http request, user, backend system, etc.) placed in an HTML Attribute. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data.

The problem is that if companyName had the value “Johnson & Johnson”. What would be displayed in the input text field would be “Johnson &amp;amp; Johnson”. The appropriate encoding to use in the above case would be only JavaScript encoding to disallow an attacker from closing out the single quotes and in-lining code, or escaping to HTML and opening a new script > tag.

It is important to note that when setting an HTML attribute which does not execute code the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up.

= URL Attribute Sub Context within the Execution Context =

The logic which parses URLs in both execution and rendering contexts looks to be the same. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context.

If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (“http:” or “javascript:”) will be URL encoded preventing the “http” and “javascript” protocols from being invoked.

=CSS Attribute Sub Context within the Execution Context=

Normally executing JavaScript from a CSS context required either passing “javascript:attackCode” to the CSS url method or invoking the CSS expression method passing JavaScript code to be directly executed. From my experience, calling the expression function from an execution context (JavaScript) has been disabled. In order to mitigate against the CSS url method ensure that you are URL encoding the data passed to the CSS url method.

TODO: We have not been able to get the expression function working from DOM JavaScript code. Need some help.

=Event Handler and JavaScript code Sub Contexts within an Execution Context=

Putting dynamic data within JavaScript code is especially dangerous because JavaScript encoding has different semantics for JavaScript encoded data when compared to other encodings. In many cases, JavaScript encoding does not stop attacks within an execution context. For example, a JavaScript encoded string will execute even though it is JavaScript encoded.

JavaScript event handler methods are dangerous because they implicitly do an eval on the data passed to the DOM attribute. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code.

or

Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes).

However the opposite is the case with HTML encoding. HTML tag elements are well defined and do not support alternate representations of the same tag. So HTML encoding cannot be used to allow the developer to have alternate representations of the  tag for example.

HTML Encoding’s Disarming Nature
In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. Working example (no HTML encoding):

Normally encoded example (Does Not Work – DNW):

HTML encoded example to highlight a fundamental difference with JavaScript encoded values (DNW):

If HTML encoding followed the same semantics as JavaScript encoding. The line above could have possibily worked to render a link. This difference makes JavaScript encoding a less viable weapon in our fight against XSS.

=Guidelines for Developing Secure Applications Utilizing JavaScript=

DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. The guidelines below are an attempt to provide guidelines for developers when developing Web based JavaScript applications (Web 2.0) such that they can avoid XSS.

1. Untrusted data should only be treatedas displayable text. Never treat untrusted data as code or markup within JavaScript code. 2. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application (Jim Manico and Robert Hansen) 3. Use document.createElement(“…”), element.setAttribute(“…”,”value”), element.appendChild(…), etc. to build dynamic interfaces. Avoid use of HTML rendering methods: 4. Understand the dataflow of untrusted data through your JavaScript code. If you do have to use the methods above remember to HTML and them JavaScript encode the untrusted data (Stefano Di Paola). 5. There are numerous methods which implicitly eval data passed to it. Make sure that any untrusted data passed to these methods is delimited with string delimiters and enclosed within a closure or JavaScript encoded to N-levels based on usage, and wrapped in a custom function. Ensure to follow step 4 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding.

Utilizing an Enclosure (as suggested by Gaz)

The example that follows illustrates using closures to avoid double JavaScript encoding.

The other alternative is using N-levels of encoding.

N-Levels of Encoding

If your code looked like the following, you would need to only double JavaScript encode input data.

The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed in the single quotes. Then the implicit eval of setTimeout reverses another layer of JavaScript encoding to pass the correct value to "customFunction". The reason why you only need to double JavaScript encode is that the "customFunction" function did not itself pass the input to another method which implicitly or explicitly called eval. If "firstName" was passed to another JavaScript method which implicitly or explicitly called eval then "<%=doubleJavaScriptEncodedData%>" above would need to be changed to "<%=tripleJavaScriptEncodedData%>".

An important implementation note is that if the JavaScript code tries to utilize the double or triple encoded data in string comparisons, the value may be interpreted as different values based on the number of evals the data has passed through before being passed to the if comparison and the number of times the value was JavaScript encoded.

If "A" is double JavaScript encoded then the following if check will return false.

This brings up an interesting design point. Ideally, the correct way to apply encoding and avoid the problem stated above is to server side encode for the output context where data is introduced into the application. Then client-side encode (using a JavaScript encoding library) for the individual sub context (DOM methods) which untrusted data is passed to. Here is an example: It has been well noted by the group that any kind of reliance on a JavaScript library for encoding would be problematic as the JavaScript library could be subverted by attackers. One option is to wait till ECMAScript 5 so the JavaScript library could support immutable properties. Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous clousures.

An example follows:

Chris Schmidt has put together another implementation of a JavaScript encoder at http://yet-another-dev.blogspot.com/2011/02/client-side-contextual-encoding-for.html.

6. Limit the usage of dynamic untrusted data to right side operations. And be aware of data which may be passed to the application which look like code (eg. “location”, “eval”). (Achim)

If you want to change different object attributes based on user input use a level of indirection.

Instead of:

Do the following instead:

7. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel).

8. Limit access to properties objects when using object[x] accessors. (Mike Samuel). In other words use a level of indirection between untrusted input and specified object properties. Here is an example of the problem when using map types:

Although the developer writing the code above was trying to add additional keyed elements to the “myMapType” object. This could be used by an attacker to subvert internal and external attributes of the “myMapType” object.

9. Run your JavaScript in a ECMAScript 5 canopy or sand box to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens).

10. Don’t eval JSON to convert it to native JavaScript objects. Instead use JSON.toJSON and JSON.parse (Chris Schmidt).

= Common Problems Associated with Mitgating DOM Based XSS =

Complex Contexts
In many cases the context isn’t always strait forward to discern.

In the above example, untrusted data started in the rendering URL context (href attribute of an  tag) then changed to a JavaScript execution context (javascript: protocol handler) which passed the untrusted data to an execution URL sub context (window.location of myFunction). Because the data was introduced in JavaScript code and passed to a URL sub context the appropriate server side encoding would be the following:

Or if you were using ECMAScript 5 with an immutable JavaScript client-side encoding libraries you could do the following:

Insonsistencies of Encoding Libraries
There are a number of open source encoding libraries out there:


 * 1) ESAPI
 * 2) Apache Commons String Utils
 * 3) Jtidy
 * 4) Your company’s custom implementation.

Some work on a black list others ignore important characters like “<” and “>”. ESAPI is one of the few which work on a whitelist and encode all non-alpha numeric characters. It is important to use an encoding library which understands which characters can be used to exploit vulnerabilies in their respective contexts. But there are misconceptions abound related to proper encoding.

Encoding Misconceptions
Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. However, if the pages returned from your web application utilize a content type of “text/xhtml” or the file type extension of “*.xhtml” then HML encoding may not work to mitigate against XSS.

For example:

The HTML encoded value above is still executable. If that isn’t enough to keep in mind, you have to remember that encodings are lost when you retrieve them using the value attribute of a DOM element.

Let’s look at the sample page and script:

Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts.

Usually Safe Methods
One example of an attribute which is usually safe is innerText. Some papers or guides advocate its use as an alternative to innerHTML to mitigate against XSS in innerHTML. However, depending on the tag which innerText is applied, code can be executed.

Special thanks to Jim Manico, Gareth Heyes, Stefano Di Paola, Robert Hansen, Mario Heiderich, Achim Hoffmann, John Stevens, Edwardo (SirDarkCat) Alberto Vela Nava, Chris Schmidt, Mike Samuel, Jeremy Long, Jeff Williams, and many others who help make this guide possible.