The OWASP Top Ten and ESAPI – Part 5 – Cross Site Request Forgery (CSRF)

This article will describe how to protect your J2EE application from Cross Site Request Forgery (CSRF/XSRF) attacks 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.

What’s the problem?

What is Cross Site Request Forgery (CSRF)? As usual, let’s check in with what OWASP says it is:

“A CSRF attack forces a logged-on victim’s browser to send a request to a vulnerable web application, which then performs the chosen action on behalf of the victim. The malicious code is often not on the attacked site. This is why it is called ‘Cross Site’. ” [from here] Additionally, they give several examples of the types of vulnerable sites including sites that “Authorize requests based only on credentials that are automatically submitted such as the session cookie if currently logged into the application, or ‘Remember me’ functionality if not logged into the application, or a Kerberos token if part of an Intranet participating in integrated logon with Active Directory”. Finally this statement is added: “Unfortunately, today, most web applications rely solely on automatically submitted credentials such as session cookies, basic authentication credentials, source IP addresses, SSL certificates, or Windows domain credentials.”

So, what does all this mean? It means that (most likely) if you aren’t protecting against CSRF, you are vulnerable to it. That’s a scary thought! Many developers don’t even know about the attack (though they should), yet they are vulnerable by default in many instances. It’s a devastating attack to many web applications. Let’s discuss in a bit of detail what it looks like. In my opinion, the easiest way to explain the attack is to walk through it in a series of steps.

1. Bob (User/Victim) logs in (or is already logged in) to “Vulnerable website X”. This website has contains some sort of e-commerce type functionality. It also allows you to store your credit card for future purchases.
2. Mary (Attacker) knows and uses website X and realizes that CSRF is a vulnerability in the application.
3. Mary crafts a bit of html and/or javascript and posts it to a forum/message board that Bob reads (any site on the internet).
The code might look something like this:


<img src="http://www.siteX.com/completePurchase.do?itemId=ABC123" />

4. Bob views the html and/or javascript by visiting the forum (though nothing visible shows up) and the code is executed. Bob has just purchased an item of the attacker's choosing without knowing it.

Ok, a few things to note about the sequence above.
a. The vulnerable site doesn't have to be e-commerce. It could be credit card related info, or adding a friend to your group. For the most part, it has to do with transactions. Usually, it's not a big deal on "view" type pages.
b. The forum/message board, while common, is not the only way to cause this. It could be done through an email, or any other number of attack vectors.
c. The actual attack here was fairly benign, but be certain, there are more nefarious attacks that exist, and are executed all the time.

Ok, so hopefully you are sufficiently concerned now based on the description above.

Where do we go from here? ....

So, now we understand what can happen if a site is vulnerable to CSRF (and most are), but what how do we solve that problem. Well, simply put, we have to have a way to know that the request is legitimate, meaning that the user intended to make the request. The most common mechanism in place today (that works) is the token. The token is a generic term that is used for implementations of the "synchronizer token pattern". There are several working implementations that use the token today, including Struts 1 and 2, Webwork, SpringMVC, and even OWASP's own CSRFGuard Project. However, since we're discussing the usage of ESAPI to solve the OWASP Top Ten, we'll cover how ESAPI does it in this article.

At this point, CSRF is actually pretty easy to solve. We know what the issue is, know how to solve it, and have a framework (ESAPI) that will help with the specifics. So, here are the steps we'll actually go through in order to protect our apps from CSRF attacks.
1. Generate new CSRF token and add it to user once on login and store user in http session.
2. On any forms or urls that should be protected, add the token as a parameter / hidden field.
3. On the server side for those protected actions, check that the submitted token matches the token from the user object in the session. (Assuming you've implemented this properly, a failure constitutes a security issue.)
4. On logout and session timeout, the user object is removed from the session and the session destroyed.

Now that we have the steps, we'll give a short snippet of code that shows each in action.

Step 1. Generate new CSRF token and add it to user once on login and store user in http session.
This code should be executed when the user logs into the application. This is done in the default ESAPI implementation, and it is stored as a member variable of the User object that gets stored in the session.


//this code is in the DefaultUser implementation of ESAPI

/** This user's CSRF token. */

private String csrfToken = resetCSRFToken();

...
public String resetCSRFToken() {

	csrfToken = ESAPI.randomizer().getRandomString(8, DefaultEncoder.CHAR_ALPHANUMERICS);

	return csrfToken;

}

Step 2. On any forms or urls that should be protected, add the token as a parameter / hidden field.
The addCSRFToken method below should be called for any url that is going to be rendered that needs CSRF protection. Alternatively if you are creating a form, or have another technique of rendering URLs (like c:url), then be sure to add a parameter or hidden field with the name "ctoken" and the value of DefaultHTTPUtilities.getCSRFToken(). That should do the work of adding the data to the url or form. We'll validate it in the next step.


//from HTTPUtilitiles interface
final static String CSRF_TOKEN_NAME = "ctoken";

//this code is from the DefaultHTTPUtilities implementation in ESAPI

public String addCSRFToken(String href) {

	User user = ESAPI.authenticator().getCurrentUser();

	if (user.isAnonymous()) {

		return href;

	}

	// if there are already parameters append with &, otherwise append with ?

	String token = CSRF_TOKEN_NAME + "=" + user.getCSRFToken();

	return href.indexOf( '?') != -1 ? href + "&" + token : href + "?" + token;

}

...

public String getCSRFToken() {

	User user = ESAPI.authenticator().getCurrentUser();

	if (user == null) return null;

	return user.getCSRFToken();

}

Step 3. On the server side for those protected actions, check that the submitted token matches the token from the user object in the session. (Assuming you've implemented this properly, a failure constitutes a security issue.)
Ensure that you call this method from your servlet or struts action or jsf controller, or whatever server side mechanism you're using to handle requests. This should be called on any request that you need to validate for CSRF protection. Notice that when the tokens do not match, it's considered a possible forged request.


//this code is from the DefaultHTTPUtilities implementation in ESAPI

public void verifyCSRFToken(HttpServletRequest request) throws IntrusionException {

	User user = ESAPI.authenticator().getCurrentUser();

	// check if user authenticated with this request - no CSRF protection required

	if( request.getAttribute(user.getCSRFToken()) != null ) {

		return;

	}

	String token = request.getParameter(CSRF_TOKEN_NAME);

	if ( !user.getCSRFToken().equals( token ) ) {

		throw new IntrusionException("Authentication failed", "Possibly forged HTTP request without proper CSRF token detected");

	}

}

Step 4. On logout and session timeout, the user object is removed from the session and the session destroyed.
In this step, logout is called. When that happens, note that the session is invalidated and the current user object is reset to be an anonymous user, thereby removing the reference to the current user and accordingly the csrf token.


//this code is in the DefaultUser implementation of ESAPI

public void logout() {

	ESAPI.httpUtilities().killCookie( ESAPI.currentRequest(), ESAPI.currentResponse(), HTTPUtilities.REMEMBER_TOKEN_COOKIE_NAME );

	HttpSession session = ESAPI.currentRequest().getSession(false);

	if (session != null) {

		removeSession(session);

		session.invalidate();

	}

	ESAPI.httpUtilities().killCookie(ESAPI.currentRequest(), ESAPI.currentResponse(), "JSESSIONID");

	loggedIn = false;

	logger.info(Logger.SECURITY_SUCCESS, "Logout successful" );

	ESAPI.authenticator().setCurrentUser(User.ANONYMOUS);

}

So there you have it. Not very complex, but solid protection against CSRF. In quick review, we covered what CSRF is, why it's a problem, and how to solve it using 4 simple steps (create token, add token to forms/urls, verify token server side, kill token on logout). There are a couple of additional notes I wanted to point out.

Note: In addition to resolving the CSRF issues in your application, you also need to resolve the XSS issues in your application (see The OWASP Top Ten and ESAPI - Part 2 - Cross Site Scripting (XSS)). If those still exist, the CSRF defenses can be circumvented.

For some additional background and information on CSRF, see the OWASP CSRF Prevention Cheat Sheet.

Well, that's it. Hope you've found this article helpful. Let me know if you have any questions or suggestions. Feel free to comment on techniques you are using to correct CSRF and if you've seen it as a large problem for your applications.

Technorati Tags: Cross Site Request Forgery, ESAPI, J2EE, Java, OWASP, Security, Validation

May 16, 2010 | Filed Under Java, Security | 1 Comment

The OWASP Top Ten and ESAPI – Part 3 – Malicious File Execution

This article will describe how to protect your J2EE application from malicious file execution attacks 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.

What’s the problem?

So what exactly is malicious file execution? It could mean different things to different people. Here, I want to limit the discussion to the execution, either immediate or delayed, of files or file handles that can be manipulated in some way by user input. This is a fairly broad definition that can cover a few different situations. Let’s first discuss a few of the different types of issues that can come up. This collection is by no means exhaustive, but should give us decent coverage of the types of issues that arise when dealing with file input.

1. Included file name is partially or wholly determined by dynamic input (think dynamic include files)
2. File is uploaded and written to disk with “as is” (no validation of uploaded file – just write to disk)
3. File name to display or retrieve is partially or wholly determined by dynamic input
4. Command / data file is uploaded (think a batch process being kicked off with data from an uploaded spreadsheet)

So, what types of issues do these particular situations present us with. In general, an attacker can manipulate this process to gain increased privileges, access data for which they are not authorized and even execute code. Since these aren’t the only 4 examples of issues that exist, we won’t specifically delve into each issue in detail, but we’ll try to extract some common problems that are present in one or more of the scenarios listed above. This set of problems is related to common file upload / processing situations.

1. Missing or insufficient input validation – This is almost always true with file processing. This also is a broad topic, since it deals both with the file name and path, as well as the file contents. Many times, this could express itself as uploading a file as is and simply writing it out with a filename that is either the name of the file as uploaded, or a name specified by the user. This could cause issues like directory traversal (“../../etc/passwd” – overwriting the passwd file – sorry for the old cliche example), or overwriting other files in the filesystem (“../../jsps/login.jsp” – overwriting the login page). Something like this could allow the attacker to break the site significantly up to and including owning the site, and even the server it’s hosted on. While most file processing code I’ve seen is not bad enough to allow something like this, some of it is just that bad .. scary.

2. No virus scanning – This goes along with input validation to some extent, but if you are uploading a file that is going to be executed in any way at any time, it should be virus scanned. There are engines out there, even free ones, that you can call out to in order to scan the file. If the scan fails, the file should be deleted, and the incident should be logged as a security incident. At that point, the normal security incident response process in your organization can be executed.

3. No size checks – One of the simplest tasks you can perform that is often overlooked is to check the file size of the uploaded file. There is most likely a reasonable limit to the size of your accepted files. For instance, a spreadsheet would have to have quite a bit of data to be any more than a few MB, but an attacker could upload files of many times more than that to try and fill up the file system, or just hang processing on the server to block out other valid users.

4. Invalid file type processing – This task is a bit more difficult, but makes sense in many cases. If you are an image hosting site, there’s no reason to accept Word files. This should involve creating a whitelist of the types of files you will accept and then verifying that only those are uploaded. This does NOT mean file extension checks. Although these can be used as a superficial first test, they are trivial to change and provide no security assurance. Often times, this check will involve output encoding (see #7).

5. Giving too much control over file name input – This occurs when users are allowed to influence the final filename that is output to the filesystem. This could happen by allowing a user to type in the desired filename or by just accepting their filename and using that on the server side. However, it is much safer to generate the filename that is used to save the file to the filesystem. Generally, this would have some securely random string of characters, as well as the author, date, time, etc. If there is a specific requirement to accept input from the user as to what the filename should be, just ensure that the name given is validated against an acceptable whitelist before writing the file out.

6. Direct object reference (DOR) problems – While the DOR issue affects more than just filenames, it is relevant here so we’ll quickly discuss it. DOR is simply an issue where the actual filename is pointed to directly. This filename could be in a hidden field, stored in a cookie, or some other place that is not directly seen by the user on the screen. Typically the filename is taken directly and used to retrieve or execute the file. This could cause issues of broken authorization (attacker changes filename to access another user’s file), elevation of privileges (touching a file belonging to a user with higher privileges or the system user), or many other problems. This topic is covered in greater detail in the next article in the series.

7. No output encoding – This issue doesn’t always exist since not every filetype can be thought of in this way. The basic issue is when a file is not validated properly before being written out. There are several filetypes that are valid, that could have extra information in them. However, of those, there are some file types which can be read in, and then written out again to ensure the “bad stuff” is not still there. This does not apply to all “bad stuff” that can be contained in those files, nor does it apply to every file type, but it can be useful when the option is available, though it rarely if ever is used in many applications.

8. Not authorizing access – This issue exists because many applications contain reasonably appropriate authentication, but horrible if any authorization. Once a user is authenticated, many times they can perform any function in the application. The classic example of this is not restricting access to a certain page, let’s say the admin screen. The home page might not show the link, but if they type the link into the url bar, there’s nothing stopping them from getting there. Depending on the type of site, this could be especially harmful with file upload. Often times, only “trusted” people are expected to be able to upload files, but if the authorization is broken, it could allow anyone to upload files at will. Again, this depends on the type of site you have – it may be perfectly fine for everyone to upload files (like flickr or youtube).

Where do we go from here? ….

So, we’ve discussed lots of issues that can crop up when dealing with uploading files, so how does ESAPI and general best practices say we should deal with these issues? Let’s take them one by one.

1. Missing or insufficient input validation – As discussed in part 2 of this series, ESAPI makes it fairly simple to do proper input validation through the framework. The code is very simple, and extensible through the addition of new regular expressions. Here is an example of how ESAPI checks the filename of the uploaded file to verify that it is valid.


if (!ESAPI.validator().isValidFileName("upload", filename, allowedExtensions, false)) {
	throw new ValidationUploadException("Upload only simple filenames with the following extensions " + allowedExtensions, "Upload failed isValidFileName check");

}

2. No virus scanning – ESAPI does not directly support virus scanning through the base APIs, but several of the antivirus vendors support API access for Java. I’ve heard that there are also free AV tools that offer this feature as well.

3. No size checks – There is no built in support for this in ESAPI. However, several of the web frameworks do provide support for this. Additionally, most if not all file upload libraries also provide support for this. Lastly, it’s trivial to perform your own check even if your specific library does not support it directly. Just load the uploaded file into a file object and call the length() method on it to determine the size of the file in bytes. Here is what ESAPI uses to prevent large files. The following is a snippet:


ServletFileUpload upload = new ServletFileUpload(factory);

upload.setSizeMax(maxBytes);

4. Invalid file type processing – This task is supported by the input validation portion of ESAPI again. First, the filename could be validated by the application. Then, each input from the file that is processed could again be validated by the application. Think of processing a spreadsheet. The filename should first be tested, then not only each line, but each cell should be checked for validity before being used by the application.

5. Giving too much control over file name input – In order to at least partially solve this issue, ESAPI has the SafeFile object. This is a trivial extension to the java.io.File object that additionally performs checks of the file and directory names to check for unsafe characters. However, this is a blacklist check. It is likely that you could do a better job if you know what characters are allowed in your filenames and only allow those (whitelist), though the characters in the ESAPI blacklist should minimally be disallowed.

6. Direct object reference (DOR) problems – Again, this will be covered in greater detail in the next article, but here I will just mention that ESAPI does have a mechanism to hide DORs. This involves essentially creating a map of direct -> indirect references and then using the indirect references for display, then mapping back during the processing. Come back for the next article for more details on how this is actually done in code.

7. No output encoding – There is no built in support for this in ESAPI. However, there are ways of doing this that *may* work for different filetypes. The most obvious one is images. If an image is uploaded, you can open the file, load the contents into an image object, then save that object. This guarantees that the image is a valid object. Otherwise, loading the image will fail. (Note: this is the expected behavior, though there have been some defects that may cause this not to work – ensure your library version functions correctly here). It may also be possible to do this with the help of some 3rd party libraries depending on the file type you are processing.

8. Not authorizing access – ESAPI does have authorization coverage in the framework, but it deserves its’ own discussion. Actually, the last article in this series will cover restricting URL access which will discuss the authorization capabilities of ESAPI. Additionally, CSRF (Cross Site Request Forgery) protections can help deal with this issue as well, and this is also going to be covered in a later article in the series.

I’d like to mention a couple of other helpful things that are more systems management related when dealing with uploaded files.

- Use a safe base directory when saving your uploaded files that is outside the standard web directory that your site is served from. This will force your application to be the only thing that can access these files. That way, an attacker can’t drop a file in an accessible directory.

- Use the file-system access controls that are offered in your operating system and possibly your web/application server. Ensure that only the appropriate users have privileges and that they have the least privilege necessary to do their jobs. This is a necessary stopgap in case your other protections fail.

- Audit properly. This can’t be stressed enough. If you audit and log properly, and then *READ* those logs, then you can notice when strange behavior occurs. This can help you locate things that need to be resolved.

Hopefully this article has been useful in helping you recognize the types of issues related to file upload and some best practices for dealing with those. As in all the other articles, I recommend trying to be as safe as possible, and only having your app do what is necessary. If you don’t need to upload files, don’t do it. If you have to do it, spend time making sure it’s as safe as you can make it. Let me know if you have other suggestions or best practices you use with file upload.

Editorial Note: I realize it’s been quite a while since I posted any articles in this series. However, I plan to finish up the series in the next few months by posting an article every week or two now that I can spend a bit more time on it. I also realize that in the gap of time I had off, the new OWASP Top Ten (2010) has been released and it supercedes the 2007 list. However, all the issues in the 2007 list either still exist in the 2010 list, are covered by it, or are still issues that may not make that high of a ranking anymore, but are still important. For these reasons, and the fact that I’d already started using the 2007 list, I’ll finish up with the 2007 list. I may go back at the end and catch any new ones from the 2010 list if relevant.

Technorati Tags: ESAPI, J2EE, Java, Malicious File Execution, OWASP, Security, Validation

May 2, 2010 | Filed Under Java, Security | 1 Comment

The OWASP Top Ten and ESAPI – Part 2 – Injection Flaws

This article will describe how to protect your J2EE application from injection (SQL and others) attacks 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.

What’s the problem

Now to discuss injection. Again, let’s begin the the definition of injection from OWASP:
“Injection flaws, particularly SQL injection, are common in web applications. Injection occurs when user-supplied data is sent to an interpreter as part of a command or query. The attacker’s hostile data tricks the interpreter into executing unintended commands or changing data.”

Here I should make a quick note about “input”. While the vast majority of attacks are usually due to malicious user-supplied input, often directly through the browser, this is not necessary for the attack to work. Dangerous (whether malicious or not) could come from a variety of sources, like the filesystem, or the database, or even other applications. The rule is, if you take in input, always understand it could be dangerous. OK, back to injection …

If you look at injection alongside XSS, you’ll notice that they are very similar types of vulnerabilities in spirit. Consider this working definition: “The application takes in input and without proper data validation or encoding, outputs that data to a destination where certain values or entities have special meaning”. This could be describing XSS or Injection. In the case of XSS, our output destination (we’ll use OWASP’s “interpreter” term from here on out) happens to be a browser. For injection, it could be a database, LDAP server, operating system or any other number of interpreters. Each of the interpreters has its’ own semantics and processing rules for what it understands and what symbols and characters have semantic meaning as input.

For the browser, for instance, the bold tag has semantic meaning, which results in making certain text bold-faced.

<b>bold-faced</b>

As for a SQL compliant database, the keyword SELECT has semantic meaning in that it performs a read of some portion of the database.


SELECT first_name, last_name FROM employees;

As we saw previously in the XSS article, it is possible for a user to insert (inject) data that will cause issues if the input is not properly validated and/or encoded (my strong suggestion is to do both). The same thing is true with injection attacks. If the input is not valid for the interpreter, or more specifically contains some commands that will be processed by the given interpreter, then the user can cause the interpreter to perform functions on the user’s behalf that were not intended to be allowed by the application.

Let’s take a classic SQL injection attack as an example to clarify the issue. Here’s some simple Java code creating a SQL string to perform an authentication check so that a user can login. Let’s assume the Java code gets the results back and tests to see if there are any results, and if so, allows the user to be authenticated.


String sqlString = "SELECT * FROM users WHERE fullname = '"
    + form.getFullName() + "' AND password = '" + form.getPassword() + "'";

So, let’s assume the user is presented with a simple login screen, and let’s also assume the user is benevolent. The user might enter something like John Melton for the username and 123pass for the password. That would create the following SQL string to be executed against the database after it is processed by Java (ie. this is what you would see as the SQL string if you watched it through the debugger, or printed it out after the values have been populated at runtime).


SELECT * FROM users WHERE username = 'John Melton' AND password = '123pass'

OK, so far, everything is great right? Sure, for those specific inputs no problem. Now, to pay homage to a good friend of mine, I’ll include his oft-quoted refrain – “The web wasn’t made for Irishmen”.

Let me explain that. For those who are unaware, many Irishmen have names like O’Brien or O’Connor. Note the apostrophe in the last name. Now, let’s show what the SQL string looks like after being processed now.


SELECT * FROM users WHERE username = 'Paul O'Malley' AND password = '789pass'

Are we still good? Nope, this time, we’re not so lucky. What will happen here is a SQLException will be thrown because the SQL cannot be processed by the database interpreter. The issue is in the extra apostrophe in the name. The apostrophe is used in SQL, among other reasons, to open and close a string representation. So, what the SQL interpreter sees in this case is that the username string is populated with Paul O and then closed, and the actual SQL string starts again, only <b> is not valid SQL syntax, so processing stops and an exception is thrown.

(Note: I’m using interpreter generically. In the cases I’m describing, it actually would get booted by the parser, but that’s quibbling :>. )

All right, now that we’ve covered the basic “it works” case and also seen there is a way to break it, how can we break it to our advantage if we wanted to attack the site? Let’s say an attacker sends these two crafted inputs.


Full Name: blah blah
Password: ' OR '1' = '1

Update: modified code above according to Ben’s comment below.

Now that’s a funny looking string to be entered in the password field – let’s see what the SQL string produced looks like.


SELECT * FROM users WHERE username = 'blah blah' AND password = ' ' OR '1' = '1'

What the above SQL actually does is returns all records from the users table that use the given username and password OR where 1 = 1, which is … EVERY RECORD!!!. This grants the attacker access to the application as an authenticated user. This was an actual issue in a real (fairly well-known at the time) web application many years ago. It would check the results that were returned and take the first one as the user to log in. In the case where they were being attacked and all records were returned, it took the first result, which like most other systems first user, was the admin account! Most decent sites have closed this issue on the login screen thankfully, but it is a very prevalent issue in systems across the world.

The above is a simple example of what can happen with SQL injection. The vast majority of reported issues with injection have to do with SQL injection. The others are not as popular, probably because they’re less common types of systems (b/c almost every app uses a DB) and SQL injection is so easy and prevalent, there’s no real driver for attackers to do anything else. Additionally, most of the sensitive data is likely stored in the DB anyhow, so we might as well attack it directly if possible.

Where do we go from here? ….

So, now that we see the problem, how do we fix it? Well there are essentially 2 reasonably reliable techniques as I see it. The first option has to do with using PreparedStatements with bound parameters, and the second option uses the ESAPI framework.

Update: From Jeff’s (the chair of OWASP) comment below, it should be clear that PreparedStatements and the ESAPI output encoding mechanisms are not mutually exclusive. They can be complementary and used together for increased assurance. It is a task left up to the development organization securing their app to make a decision over which option (or both) to choose. As with most choices, each option (PS, ESAPI, or both) has its’ advantages and disadvantages.

Note: with either technique, white-list input validation should be used since it’s relatively easy to do, and helps avoid so many types of problems. Additionally, standard security measures like least privilege for the user account connecting to the DB for your application should also be utilized.

Before getting into the detail of the 2 options, I want to point out that the ESAPI folks have put together another article called the SQL Injection Prevention Cheat Sheet and it is a great resource about this topic, specifically SQL injection.

PreparedStatements (with bound parameters)
——————
Those familiar with DB programming in java will recognize the 3 statement types in Java: Statement, PreparedStatement and CallableStatement. These 3 types represent varying levels of security. The standard Statement class is not a good idea – there are no security measures built-in and you are on your own if you try and use this class. As for the CallableStatement, it is used with stored procedures and functions. *Typically* stored procs are secure. However, it is very possible to make them insecure by not validating the input properly or even constructing dynamic sql within the procedure for execution. Finally, there is the PreparedStatement. When used properly (with a safe driver and bound variables), these are typically viewed as the most secure option.

There are 2 important things to remember about PreparedStatements when it comes to security.
1. SQL executed with PreparedStatements is only as safe as the drivers that implement the classes. The actual runtime class that implements the PreparedStatement interface is part of a 3rd party package. There are some out there that are generally thought to be safe, and some that aren’t. The most popular ones expectedly tend to be more respected (not necessarily earned, but true nonetheless). The only suggestion I can give here is that you can look at the code of popular open source drivers yourself and make up your mind about their safety. If you use a commercial driver, understand the support structure in case there is a security issue that must be resolved.
2. The code leveraging the PreparedStatements must use them properly. This involves the use of strongly typed bound parameters. This means that if you concatenate your SQL string together using dynamic input, the PreparedStatement does you no good. The safety comes into play when using the setXXX methods on the PreparedStatement class. The snippet below shows how to properly use dynamic input (which should already be validated) as input to the SQL string by doing parameter substitution. The setInt method below (if the driver is coded properly) should be DB specific and will escape any necessary characters for that DB to prevent breaking out of the interpreter and allowing the user to specify code that will be run by the DB.


myPrepStmt = conn.prepareStatement("SELECT name FROM users WHERE id = ?");
myPrepStmt.setInt(1, validatedUserId);
...
//execute statement and get results

and a query with multiple input parameters looks very similar …


myPrepStmt = conn.prepareStatement
    ("SELECT name FROM users WHERE id = ? AND date_created >= ?");
myPrepStmt.setInt(1, validatedUserId);		//first param
myPrepStmt.setDate(2, validatedStartDate);	//second param
...
//execute statement and get results

As you can see, using the PreparedStatement can solve this issue, but only when used properly. One very important thing to note here is about ORM frameworks. Most of these frameworks *can* be used securely, but you have to know what you’re doing. Hibernate, for instance, can be used securely or insecurely depending on which classes you choose. Be sure to do your homework, read solid tutorials, pick good frameworks, and above all understand what you’re using when you let a framework do some of the work for you. OK, now on to our other option …

ESAPI
——————
ESAPI takes a similar approach to solving this issue, but the code will look slightly different. The approach is very simple, and boils down to escaping/encoding all dynamic input to the SQL statement using an appropriate encoder. For this section of the article, I relied heavily on the ESAPI section of the SQL Injection Prevention Cheat Sheet at OWASP, so it’s a worthwhile time to go read it if you haven’t already. Let’s look at one quick example to show how it works. We’ll modify the second query above to use ESAPI instead of PreparedStatements.


//ESAPI version of query
Codec ORACLE_CODEC = new OracleCodec();		//we're using oracle
String query = "SELECT name FROM users WHERE id = " +
   ESAPI.encoder().encodeForSQL( ORACLE_CODEC, validatedUserId)
   + " AND date_created >= '"
   + ESAPI.encoder().encodeForSQL( ORACLE_CODEC, validatedStartDate) +"'";
myStmt = conn.createStatement(query);
...
//execute statement and get results

As you can see above, all that we did was encoded the input data (already validated) for SQL by using the OracleCodec, since we are using the Oracle database for our application. This has some obvious drawbacks if you are using multiple databases, and can use the ORM features of cross-database query generation, but that is rare for many apps. Most applications know the 1 database they are going to be running on, and I would argue that’s a good thing from a security perspective because then you can better understand the security ins and outs of that one DB.

Looking at the code, it’s fairly simple to encode most anything – you just have to be diligent to do so. As for the other types of injection, you may have to get your hands dirty and write an encoder for your specific application, or you may not. You’ll just have to check what the ESAPI team has already done. Additionally, it might be possible to get on the mailing list and request a new codec. Certainly, if you have any that you’ve written, I’m sure the dev team would love to at least see them, and possibly include them – you could be helping others out!.

Clearly, using the ESAPI framework, it’s possible to safely and securely prevent injection from the DB as well as other types of interpreters if you use the framework properly.

In summary, I’ll say that while I think either of these techniques work from a purely technical perspective, there are a couple of things you should consider before implementing protection. First, PreparedStatements are far more prevalent in industry. They are a solid solution to the SQL injection problem, and are widely understood and available. They’ve also been very widely tested both for performance and security. Essentially, they are a known entity. However, they do not solve any of the other types of injection. The equivalent to PreparedStatements do not exist for XML or XPath injection or others. You’d have to handle them differently. As for ESAPI, it will also work well, and has been tested for security and performance, but not nearly as much. Additionally, it does not have the industry reach that PreparedStatements do. However, ESAPI has a solution for the other types of injection as well, and a clear model for implementing new encoding mechanisms for new interpreter types. You can even write your own custom encoders if you choose. No matter which you choose (or both), my hope is that you’ve seen there is a need to be responsible and protect resources from injection. These solutions make it fairly trivial to implement solid protection for applications against injection.

Update: Added inline comments referencing changes made regarding Jeff and Ben’s comments below.

Technorati Tags: ESAPI, Injection, J2EE, Java, OWASP, Prepared Statement, Security, SQL Injection, Validation

December 1, 2009 | Filed Under Java, Security | 7 Comments

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