In February, I wrote a three part series on how we fixed JForum on coderanch to protect from CSRF.  In included;

1. Analysis
2. Extending OWASP
3. Problems

Remaining problems

Unfortunately, there were three remaining problems.

1. Some mobile devices weren’t able to handle the JavaScript which added the tokens.  Meaning our site didn’t work on all mobile devices.
2. The CSRF token was in the URL of thread links which meant people were sharing those links with tokens in them.  This isn’t a significant security risk, but it does confuse Google which is bad for SEO.  It is a tiny security risk in that if someone posts their current CSRF token, that user can be targeted for a CSRF attack until that token expires.
3. Some people take a really long time to write a post or have lunch in the middle and the token expires.  Giving them a CSRF error page when they finally finish.

This blog post shows how we solved these problems.

Ending requirement for JavaScript to set CSRF token and getting token out of URLS

In general, there are three options for setting a CSRF token in forums/URLs: JavaScript, a filter to change the HTML at runtime before it gets served to the client or hard coding the token.

Based on our experience with mobile devices, we decided not to go with option1 (JavaScript.)  I had used a technique similar to option 2 (changing the HTML as it goes by) to transform our JForum URLs to a different format.  This turned out to be more complex on a forum where users routinely post code.  The same problem would occur here so I decided against that option.  We don’t want to be adding CSRF tokens to code user’s post in questions.  And we certainly don’t want someone to be able to inject a CSRF attack in a post!

This left me with option 3 – hard coding the token.  There were a few steps to implementing this option:

1. The JavaScript solution added the token to URLs in addition to forms.  This wasn’t “good” in the first place in that URLs shouldn’t be changing database state.  I had recognized this as a shortcoming back in February but lacked the time to fix it then.  There’s a representative list of these pages/URLs on github.  Many were fixed by a one to one conversion of links to forms. (with POST as the method.)  A few were fixed with a generic form on the page and JavaScript that calls it.  We used the generic JavaScript form for some admin links to save bandwidth. Most of these aren’t available in the mobile view anyway.  And moderators weren’t the ones having JavaScript issues in the first place – probably because we have newer mobile devices.

   </span></span>function submitActionForForum(actionVerb, forumId) {
   var action = document.actionForm.action;
   
   action = action.replace("ACTION", actionVerb);
   
   action = action.replace("FORUM_ID", forumId);
   
   document.actionForm.action = action;
   
   document.actionForm.submit();
   
   }
   
   

2. At this point we don’t need to add tokens to anchors.
3. Remove AddJavaScriptServletFilter and JavaScriptServlet so tokens are no longer generated by JavaScript.
4. Add token to all forms:

   <input type="hidden" name="OWASP_CSRFTOKEN" value="${OWASP_CSRFTOKEN!""}" />
   
   

5. For forms containing multipart/form-data (there were less than 10 of these), add to the URL:(this is needed because the multipart request is only parsed once – later on in the process and the parameter isn’t available to us in the filter)

   ?OWASP_CSRFTOKEN=${OWASP_CSRFTOKEN!""} 

6. In parallel to the previous two steps, I wrote a unit test to ensure I didn’t miss any required tokens AND to alert anyone adding a form to the codebase in the future about the need to add a token.  Unit test available on github.

This post goes through how we fixed CSRF (cross site request forgery) in JForum, issues encountered and approach.  It is useful reading for anyone who needs to protect against CSRF on their website.

Background

Stock JForum has a number of security vulnerabilities.  We’ve fixed a lot of the XSS ones.  We hadn’t fixed CSRF as of early January 2013.  (It is fixed now – don’t bother trying.)  We had captchas enabled for creating ids.  And I think our working theory was that other CSRF problems would be annoying, but easy to undo.

On January 14th, we learned there was going to be an announcement of XSS and CSRF vulnerabilities  on January 16th (later changed to February – High-Tech Bridge asked me to post this before their announcement.)  I did a code review and learned that someone could exploit a CSRF attack to cause all sorts of damage including deleting the forums – which would require a database restore and likely lost posts from the duration.  Eek.  Nothing like an urgent need to have a release.

What is CSRF

Cross Site Request Forgery is a security attack where the attacker tries to do something in your name.  For example, if you open an email (with images enabled) or go to another website in a tab of your browser while being logged into your bank in another tab, the bad guy could try to move money between accounts for you.  Not something you want.  In a forum, the bad guy could post spam or ads in your name.  Or even delete the forum.  OWASP has an excellent overview of CSRF along with suggestions of “fixes” that do and don’t work.

Analysis

The first thing I did was write code (AllJForumActions.java) to output the names of all actions in JForum that can be accessed from a browser.  To see how much analysis was required.   In stock JForum, the 112 such method names are listed here..  In JavaRanch/CodeRanch forked JForum, there were 191.  (we’ve added a lot of functionality)

I then classified them in a property file by whether they were safe (read only) or needed protection.  I wrote a unit test to check this property file:

• has all action names represented
• doesn’t have any action names more than once
• only lists valid case.

Choosing a CSRF filter

Ok.  Analysis done.  Now on to coding. CSRF is a common problem.  I’d like to use an existing tested filter and just adapt it to JForum.

1) Google code CSRF Filter (StatelessCookieFilter)

From website: this one says it is better than the OWASP one by being simpler and using cookies rather than session (memory) usage.

Analysis

• It is simpler, I’ll give them that.
• I’m not terribly concerned with the memory implication.  It’s just one token per user.  We store other things in the session in memory.
• Disadvantage: the code requires you to manual add the CSRF token to all POST forms.  That’s time consuming and error prone.
• HUGE disadvantage: the filter assumes that only POST methods need to be protected.  This is only true if you strictly enforce not accepting GET requests for any requests that change state.  While I agree all code should be that way, it isn’t in the real world.  Many frameworks (including JForum) treat get and post requests the same by directing them to the same code.  If I was using this code, I’d need to change it avoid the check for POST.  (albeit not a big deal since it is less than 100 lines long).  We’d need to protect both get and post actions and exclude the expected GET URLs from the filter.  This checks the expected POST actions regardless of their actual submit type.

3) OWASP filter

Well documented.   Supports many types of checking.  Sets up Javascript DOM injection of token so don’t have to hand edit each page.  It supports a choice of only injecting the token into forms or whether to include src/href links as well.  This is great as I know that each and every update to the database isn’t in a form.  I’m also confident the developers know what they are doing because the documentation says:

 However, most JavaEE web applications respond in the exact same manner to HTTP requests and their associated parameters regardless of the HTTP method. The risk associated with not protecting GET requests in this situation is perceived greater than the risk of exposing the token in protected GET requests. As a result, the default value of this attribute is set to true. Developers that are confident their server-side state changing controllers will only respond to POST requests (i.e. discarding GET requests) are strongly encouraged to disable this property.

Comparing this to the google code which only checks POST, it is a very easy decision which one to go with.  The one that actually secures you against CSRF in the face of code that isn’t perfectly written.  OWASP CSRF Guard!

Starting out with OWASP CSRF Guard Version 3

The documentation was clear and self explanatory so let’s get into the problems I encountered:

1. Minor problem: the OWASP CSRF Guard source code from github isn’t the same with the download offered on owasp.  It was an easy Ant build from the github version.  I rebuilt because I needed to call a method that existed in the github version but not the download/packaged one
2. Major problem: JForum doesn’t consistently use URL names for actions.  There are a lot of Ajax calls that pass the “url name” as a parameter so I can’t filter these out through the OWASP API.    And the “unprotectedPages” logic in the OWASP filter is deep within the component in a final class.  Very secure, but I can’t extend it so must find another approach.

See part 1 for how we got here  and part 2 for how we changed the OWASP filter.

Code cleanup and problems

There is some poorly written code in JForum that CSRF now prevents from working.  In these cases, I needed to clean up our code.  For example:

1. Links/anchors shouldn’t be used to update state.  They should only be used for gets.  The OWASP filter is realistic enough to recognize links are in fact used for updates in the real world and accomodates them.  It doesn’t accommodate code like

   onclick="document.location = '${contextPath}/${jforumMainServletMapping}/${moduleName}/...';" />

   In the cases this was used for actions that required CSRF protection, I needed to refactor to use a form.  In a few cases, I created a one to one form.  On others where there were a lot of actions or it wasn’t conducive, I used a common form

   <!-- CSRF token messes up submission if forum id isn't in URL -->
   
   function submitActionForForum(actionVerb, forumId) {
   
   var action = document.actionForm.action;
   
   action = action.replace("ACTION", actionVerb);
   
   action = action.replace("FORUM_ID", forumId);
   
   document.actionForm.action = action;
   
   document.actionForm.submit();
   
   }
   
   </script>
   
   <#-- general purpose form for the submit buttons on the screen; see JS for ACTION and FORUM_ID values -->
   
   <form method="post" name="actionForm" action="${contextPath}/${jforumMainServletMapping}/${moduleName}/ACTION/FORUM_ID">
   
   </form>
   
   ...
   
   onClick="submitActionForForum('up', ${forum.id})"
   

2. Some pages (like the first one I happened to test) didn’t have </head> so dynamically adding the JavaScript which sets the CSRF token wasn’t working.  I wrote a unit test to identify such files and ensure we don’t create any more.  We also added the missing HTML headers.  Most of them were on the admin pages.

Note: all code is sanitized to remove references to javaranch/coderanch.

The story continues in part 4 – deciding not to use JavaScript for the solution.