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Using Plack::Middleware::CSRFBlock and jQuery to deal with Cross Site Request Forgery

·842 words·4 mins·
CSRF GET jQuery perl Plack Plack::Middleware POST
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At $work, our flagship application was recently audited for potential security issues. One of the items which raised a red flag was the fact that we weren’t dealing with the threat of CSRF (Cross Site Request Forgery). The solution which we decided to implement was to add a CSRF token to all POST requests. This token should only be known to the app and the end user. Passing it along with a POST request gives some measure of assurance that a POST by the user is intentional and so can help to reduce the risk of CSRF.

On the Perl side of things, it was actually easy to implement. Since the app is deployed under Plack, CSRF tokens could easily be implemented with Plack::Middleware::CSRFBlock, albeit a slightly modified version (which I still need to contact the author about). This module parses outgoing HTML which is produced by your app. If it finds a form which can POST, it creates a hidden form input field inside the form and inserts a CSRF token into it.

So, that takes care of your traditional forms in HTML, but it doesn’t detect your fancy jQuery Ajaxy goodness. It turns out, that’s fairly easy to handle in one place as well:

var token = 'some_random_secret';
// some defaults for all ajax calls
    cache: false,
    data:  {'csrf_token': token}

And with that, we re-submitted ourselves to the Burp Suite scanning software, hoping for a clean bill of health. Again, it was not meant to be. This time, the Burp Suite pointed out that the CSRF token is not very secret when it is attached to the params of a GET request. It can show up in browser history, web server logs, etc. That is sub-optimal to be sure. As far as fixing this goes, I was still looking for a solution to fix this in one place. The app is big enough on JS that I wasn’t about to scour for every GET and fix this manually. It turns out that jQuery has a solution for this as well, courtesy of StackOverflow:

var token = 'some_random_secret';
// some defaults for all ajax calls
    cache: false

$.ajaxPrefilter(function (options, originalOptions, jqXHR) {
    // do not send data for GET/PUT/DELETE
    if(originalOptions.type !== 'POST' || options.type !== 'POST') {
    } = $.param($.extend(, { csrf_token: token }));

This allows us to mangle only the params of POST requests, leaving GET, PUT and DELETE untouched. That’s basically it. There were still some additional hacks we had to put into place in order to get the tokens to work with 3rd party plugins (like tinyMCE), but for the most part this was a very clean and simple solution. It didn’t clutter the app with a lot of extra code (Perl or JS) which will need to be maintained. If you can fix something without seeing a big bump in lines of code, people will be more inclined to thank you down the line.

I should add the caveat that using Plack::Middleware to solve this particular problem may not scale well for your particular case. The app in question doesn’t deal with high volumes of traffic, so the tradeoff of some extra computation versus a comprehensive HTML audit made it a very tidy solution for us.



Date: 07/19/2012 10:32:21 PM

I can’t really see a reason why having the CSRF token appear in the browser history nor in the server logs is a big problem. CSRF is an attack against the user him/her self and a CSRF token is only vulnerable if it falls into the hands of some other site in the timeframe that it’s valid. If another site can get your server logs or the user’s browser history you/they have bigger problems then having a short lived CSRF token potentially being used against them.

Author: Olaf Alders

Date: 07/20/2012 03:10:48 AM

I think the issue with the server logs is the CSRF token appearing in a referring URL in someone else’s server logs. Specifically the report says:

“Sensitive information within URLs may be logged in various locations, including the user’s browser, the web server, and any forward or reverse proxy servers between the two endpoints. URLs may also be displayed on-screen, bookmarked or emailed around by users. They may be disclosed to third parties via the Referer header when any off-site links are followed. Placing session tokens into the URL increases the risk that they will be captured by an attacker.”

It was given a severity of “medium” in the grand scheme of things. Without hijacking the session the token isn’t helpful to the attacker, but I guess if there’s not a lot of overhead to keeping the token out of the URL, it makes sense. The other part of the exercise for us is removing enough red flags from the report so that the app passes inspection. The report is being run as part of the approval procedure for a application, so the cleaner it looks, the faster the app gets approved.


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