The OWASP Top Ten and ESAPI – Part 1 – Cross Site Scripting (XSS)

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This article will describe how to protect your J2EE application from XSS using ESAPI. As with all of the detail articles in this series, if you need a refresher on OWASP or ESAPI, please see the intro article The OWASP Top Ten and ESAPI.

OK, so on to XSS. Here is a slightly modified definition of XSS from OWASP:
XSS flaws occur whenever an application takes untrusted (typically user supplied) data and sends it to a web browser without first validating or encoding that content. XSS allows attackers to execute script in the victim’s browser which can hijack user sessions, deface web sites, possibly introduce worms, etc.

As you can see, XSS essentially allows an attacker to splash whatever they want on the screen since the application doesn’t do any input validation or output encoding. This is not a big deal when you have benevolent users, but an attacker could, say, input some nasty JavaScript and cause quite a few problems. This is typically what happens – JavaScript is output and generally executes in the background so the user is unaware of what’s occurring.

In general there are 3 types of XSS:
1. Stored – The dangerous data is stored in a permanent data store and shown repeatedly on the site – think online forums. If an attacker were to save a forum comment that had an XSS exploit in it, that comment would then be displayed to anyone who visited the page, without requiring any further interaction from the attacker.
2. Reflected – The dangerous data is sent along with the URL typically, and is not stored in a data store but is displayed (reflected) back to the user, which launches the attack. If an attacker sent a URL in an email, and a user clicked on it b/c the site was “trusted”, say that user’s bank, but the site had an XSS exploit, the attacker could cause many issues up to and including processing transactions on behalf of the user at the bank site (more on this in a follow-on article).
3. DOM – This is the most recently named type of attack, and simply represents an entirely client side issue. While the previous 2 types of XSS have to do with the server outputting data to the browser, this has to do with the browser manipulating the DOM and data being moved in and out of context. This can result in an XSS attack if data is not properly validated or encoded. This type of XSS issue has become more prevalent with web 2.0 and the heavy proliferation of JavaScript frameworks being used to do both serious functionality and DOM manipulation in web pages.

XSS, by some accounts, is the most common vulnerability and definitely is one of the most dangerous in the wild. It is extremely prevalent due to a lack of education for the most part, but it can at times be tricky to solve correctly. There are essentially 2 options for how to deal with XSS. Both could be used individually to solve the problem entirely … in theory … but that would require detailed knowledge of every possible input/output vector both now and in the future. Since this is rarely feasible, it is recommended to use both approaches. These approaches are Input Validation and Output Encoding.

Input Validation
Input validation is simply that – checking each input for validity. This can mean many things, but in the typical and simplest case, it means to check the type and length of the data. For instance, if you are accepting a standard US zip code from a text box, you would know that the only valid type is a digit (0-9) and that the length should be 5, no more and no less. Not all cases are this simple, but many are similar.

Consider this example for what can go wrong. There is a simple search engine and on the page is the search box. A user types in a query only to find on the results page that his search terms are printed on the screen, something like – You searched for “free stuff”. That’s all well and good, but what happens when the user inputs a bit of JavaScript into the search box? If the application doesn’t handle the input/output properly, the screen will print the JavaScript out in all it’s glory, and it will get put inline in the web page response, and treated just as if the web page developer had put that bit of code in there. That’s where the problems come in – now how do we solve these issues?

Here’s an image from OWASP showing their architecture for input validation. The key here is that everything is validated, all input that doesn’t originate within the application (including user input, request headers, cookies, database data, ldap, really everything …).

ESAPI Input Validation

So how do we use this validation framework to actually validate our data. There are 2 basic types of methods in the validator interface that can be used. They are listed below:

getValidInput(java.lang.String context, java.lang.String input, java.lang.String type,
	int maxLength, boolean allowNull, ValidationErrorList errors)
isValidInput(java.lang.String context, java.lang.String input, java.lang.String type,
	int maxLength, boolean allowNull)

The first, getValidInput, returns canonicalized and validated input data along with a list of errors (ValidationErrorList) if any validation issues occurred. The second is similar, but does not return errors, and rather just returns a boolean as to whether or not the input is valid. Here are a couple real examples of these being used.

String validatedFirstName = ESAPI.validator().getValidInput("FirstName",
	myForm.getFirstName(), "FirstNameRegex", 255, false, errorList);
boolean isValidFirstName = ESAPI.validator().isValidInput("FirstName",
	myForm.getFirstName(), "FirstNameRegex", 255, false);

Both of the samples above deal with the first name field from a typical web form, but this could just as easily be a request header or parameter, or cookie value, or anything else.

In the end, there is great value in general in validating ALL of your application inputs. It will help solve the XSS issue, but will also solve other problems, including some we probably haven’t even thought up yet.

Output Encoding
On the flip side of input validation is output encoding (also known as “escaping”). Here’s a quick definition from OWASP: “Escaping” is a technique used to ensure that characters are treated as data, not as characters that are relevant to the interpreter’s parser. There are lots of different types of escaping, sometimes confusingly called output “encoding.” Some of these techniques define a special “escape” character, and other techniques have a more sophisticated syntax that involves several characters. Escaping is the primary means to make sure that untrusted data can’t be used to convey an injection attack. There is no harm in escaping data properly – it will still render in the browser properly. Escaping simply lets the interpreter know that the data is not intended to be executed, and therefore prevents attacks from working.

What does all of this mean to the developer? In order to prevent “bad” data from causing XSS issues on the screen when rendered, we can’t just out.println them to the screen – we have to “encode/escape” them. There are many libraries out now that do some form of encoding, most are minimal. c:out and jstl both do encoding – some of the struts tags, jsf, spring. All do some minimal encoding. However, ESAPI takes this to a different, but necessary level.

There are 2 issues with the previously mentioned frameworks when it comes to their encoding schemes. 1 – They don’t encode enough characters – they miss some things. 2 – They only encode for 1 context and miss the other 4. As mentioned in the cheat sheet, there are actually 5 output contexts for the browser:
1. HTML entity (this is the standard HTML output that the above frameworks at least partially handle)
2. HTML Attribute
3. JavaScript
4. CSS
5. URL

This image from OWASP shows their architecture for output validation. The important notion here is that before any data is output in the application, the context for output is considered, and the data is encoded.

ESAPI Output Encoding

All of these contexts have some special handling rules. I suggest you reference the cheat sheet in order to learn those. I’ll give a couple of simple examples here just for reference.

First, a simple example to output to an HTML entity.

//performing input validation
String cleanComment = ESAPI.validator().getValidInput("comment",
	request.getParameter("comment"), "CommentRegex", 300, false, errorList);

//check the errorList here ...

//performing output encoding for the HTML context
String safeOutput = ESAPI.encoder().encodeForHTML( cleanComment );

Now, an example of creating a URL that is safe for output.

//performing input validation
String cleanUserName = ESAPI.validator().getValidInput("userName",
	request.getParameter("userName"), "userNameRegex", 50, false, errorList);

//check the errorList here ...

//performing output encoding for the url context
String safeOutput = "/admin/"
	+ ESAPI.encoder().encodeForURL(cleanUserName);

Above, you can see that is it very simple to encode output for a given context, as long as you know the context you’re going to. It does take discipline to use this throughout your application, but it will pay you back many-fold in rewards. Additionally, ESAPI does have tag libraries that wrap each of the output encoding mechanisms available for use.

One final note regarding output encoding: you should always explicitly set the character encoding for all your pages (ISO-8859-1 or UTF8 are popular choices).

As you can see, there is quite a bit to solving XSS. It can become tricky, especially in the instances where multiple contexts are involved, like trying to safely pass a parameter to a javascript function inside an HTML attribute handler event, but it is doable. The big takeaway should be that input validation and output encoding, while they do solve XSS, are in general excellent practices that should be exercised for ALL input to an application.

A final XSS note: A great reference for understanding what is occurring in XSS and how to protect against it is the OWASP XSS Prevention Cheat Sheet. Various references in this article drew from the info on this site.

Other articles in this series:
Part 0: The OWASP Top Ten and ESAPI
Part 1: The OWASP Top Ten and ESAPI – Part 1 – Cross Site Scripting (XSS)
Part 2: The OWASP Top Ten and ESAPI – Part 2 – Injection Flaws
Part 3: The OWASP Top Ten and ESAPI – Part 3 – Malicious File Execution
Part 4: The OWASP Top Ten and ESAPI – Part 4 – Insecure Direct Object Reference
Part 5: The OWASP Top Ten and ESAPI – Part 5 – Cross Site Request Forgery (CSRF)
Part 6: The OWASP Top Ten and ESAPI – Part 6 – Information Leakage and Improper Error Handling
Part 7: The OWASP Top Ten and ESAPI – Part 7 – Broken Authentication and Session Management
Part 8: The OWASP Top Ten and ESAPI – Part 8 – Insecure Cryptographic Storage
Part 9: The OWASP Top Ten and ESAPI – Part 9 – Insecure Communications
Part 10: The OWASP Top Ten and ESAPI – Part 10 – Failure to Restrict URL Access

Note: Article updated on 11/18 per Jim Manico’s catch of improper url output encoding since ESAPI’s url encoding is intended for parameter values, not the entire url. Also added input validation as Jim’s comment pointed out was missing.

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