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Billy (BK) Rios

Simple Lesson on Secure Cookies

Posted: September 9th, 2008, 4:05am CDT by xssniper

Tags: Security [edit]

I recently read a paper written by Sandro Gauci from Enable Security entitled “Surf Jacking – HTTPS will not save you”. You can find the paper here.

It’s an interesting read and extremely relevant to today’s web applications. The heart of the paper describes some simple tricks to force a session cookie to be sent over a non encrypted channel. These tricks are possible if the secure flag isn’t set for the session cookie. These types of attacks have been discussed before. Side Jacking is probably the most well known (and most widely used) attack against leaked cookies.

<RANT> It bugs me that we’re still dealing with issues like this. Despite having a simple and effective means to ensure that session cookies are only sent over secure channels, application owners choose to ignore the secure (and HTTPONLY) flag when developing their applications. Later, as the application matures, developers find that their application has taken a significant dependency on this insecure behavior and what was once a simple fix now becomes a huge design change (which equals $$$). The true victim’s to these poor security decisions are the users who are left scratching their heads when their accounts get pwnd while using the WiFi at Joes Coffee shop. </RANT>

I believe the secure flag is symbolic of the current state of web application security… the countermeasures to the issues we are facing are known, simple, and effective… yet we continue to struggle on wide scale implementation because we’ve taken dependencies on insecure behavior. SSL certs are another great example of this. Every major browser has a way to bypass the security provided by SSL certs. Browsers MUST offer this bypass because if they didn’t, it would break the web… but i digress.

There is a bright spot when it comes to the protecting cookies. Cookies are stored and protected by the browser (as any decent web app hacker should know!). So, when an application server issues a “SET-COOKIE” header, it’s merely a recommendation as to how the browser should use the cookie. Each cookie is maintained by the browser and all the flags (secure, path, domain, httponly, expires…etc) associated with cookies are enforced ENTIRELY by the browser. So, if an application server sets a cookie WITHOUT the secure flag, I can tell my browser to disregard the servers recommendation and add the secure flag which ensures that the cookie will only be sent over secure channels. This is really simple stuff, so seasoned web app hackers can stop here. Everyone else can continue reading.

I’ve set up a page on here that simply sets a cookie in the following manner:

Set-Cookie: XSSniper=BKRios; expires=CURRENTDATE

Examining the Cookie in FireFox shows the following:

As you can see, we have a cookie named XSSNIPER and the SECURE flag was NOT set by the server. In fact, my server will NEVER set the secure flag for the XSSNIPER cookie. Now if I want to force my browser to enforce the secure flag for the XSSNIPER cookie, I can do so by entering the following Javascript into address bar.

javascript:var cookies=unescape(document.cookie);var split=cookies.split(";");for (i = 0; i <split.length;i++){document.cookie=split[i]+";expires=Thu,1-Jan-1970 00:00:00 GMT;";document.cookie=split[i]+";secure;"}document.location="http://xs-sniper.com/blog";

The Javascript above expires all of the current cookies (only on the client side, if you had a session established with the server it would still be maintained) and sets every cookie for the current domain to secure. I realize the Javascript is pretty ghetto, this should ideally be handled by application, but we could also use a browser plugin with a nice UI and fine grained control over each cookie attribute… Hmmmm a tool to prevent Surf/Side Jacking attacks… I wonder what I would call it… Any ideas Nate?

After we run the Javascript, we can take another look at the Cookie info presented by Firefox:

As you can see, the cookie will only be sent over encrypted connections and the cookie now expires at the end of the session (no more persistence). We’ve turned the XSSNIPER cookie into a SECURE cookie, despite the fact that the server never specified this behavior.

Now, this approach does have it cons… Servers typically recommend a particular cookie setting because the application was designed to work/anticipate/depend on those characteristics. This will probably break some application functionality, but broken functionality will show you exactly where your cookie would have been leaked

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