Home > Security > Exploring ReU: Rewriting URLs For Fun And XSRF/HTTPS

Exploring ReU: Rewriting URLs For Fun And XSRF/HTTPS

So I’ve been musing lately about a scheme I call ReU. It would allow client side rewriting of URLs, to make the addition of XSRF tokens much easier. As a side effect, it should also make it much easier to upgrade the links on a site from http to https. Does it work? Is it a good idea? I’m not at all sure. But I think it’s worth talking about:


There is a consistent theme to both XSS and XSRF attacks: XS. The fact that foreign sites can access local resources, with all the credentials of the user, is a fundamental design issue. Random web sites on the Internet should not be able to retrieve Page 3 of 5 of a shopping cart in the same way they can retrieve this blog entry! The most common mechanism recommended to prevent this sort of cross-site chicanery is to use a XSRF token, like so:


The token is randomized, so now links from the outside world fail despite having appropriate credentials in the cookie.

In some people’s minds, this is quite enough. It only needs to be possible to deploy secure solutions; cost of deployment is so irrelevant, it’s not even worth being aware of. The problem is, somebody’s got to fix this junk, and it turns out that retrofitting a site with new URLs is quite tricky. As Martin Johns wrote:

…all application local URLs have to be rewritten for using the randomizer object. While standard HTML forms and hyperlinks pose no special challenge, prior existing JavaScript may be harder to deal with. All JavaScript functions that assign values to document.location or open new windows have to be located and modified. Also all existing onclick and onsubmit events have to be rewritten. Furthermore, HTML code might include external referenced JavaScript libraries, which have to be processed as well. Because of these problems, a web application that is protected by such a solution has to be examined and tested thoroughly.

Put simply, web pages are pieced together from pieces developed across sprints, teams, companies, and languages. No one group knows the entire story — but if anyone screws up and leaves the token off, the site breaks.

The test load to prevent those breakages from reoccurring are a huge part of why delivering secure solutions is such a slow and expensive process.

So I’ve been thinking: If upgrading the security of URLs is difficult server side, because the code there tends to be fragmented, could we do something on the client? Could the browser itself be used to add XSRF tokens to URLs in advance of their retrieval or navigation, in a way that basically came down to a single script embedded in the HEAD segment of an HTML file?

Would not such a scheme be useful not only for adding XSRF tokens, but also changing http links to https, paving the way for increasing the adoption of TLS?

The answer seems to be yes. A client side rewriter — ReU, as I call it — might very well be a small, constrained, but enormously useful request of the browser community. I see the code looking something vaguely like:

var session_token=aaa241ee1298371;
var session_domain="foo.com";

function analyzeURL(url, domnode, flags)
   domain = getDomain(url);
   // only add token to local resources
   if(isInside(session_domain, domain)) { 
      addParam(url, "token", session_token); }
   return url;


Now, something that could totally kill this proposal, is if browsers that didn’t support the extension suddenly broke — or, worse, if users with new browsers couldn’t experience any of the security benefits of the extension, lest older users have their functionality broken. Happily, I think we can avoid this trap, by encoding in the stored cookie whether the user’s browser supports ReU. Remember, for the XSRF case we’re basically trying to figure out in what situation we want to ignore a browser’s request despite the presence of a valid cookie. So if the cookie reports to us that it came from a user without the ability to automatically add XSRF tokens, oh well. We let that subset through anyway.

As long as the bad guys don’t have the ability to force modern browsers to appear as insecure clients, we’re OK.

Now, there’s some history here. Jim Manico pointed me at CSRFGuard, a project from the OWASP guys. This also tries to “upgrade” URLs through client side inspection. The problem is that real world frameworks really do assemble URLs dynamically, or go through paths that CSRFGuard can’t interrupt. These aren’t obscure paths, either:

No known way to inject CSRF prevention tokens into setters of document.location (ex: document.location=”http://www.owasp.org”;)
Tokens are not injected into HTML elements dynamically generated using JavaScript “createElement” and “setAttribute”

Yeah, there’s a lot of setters of document.location in the field. Ultimately, what we’re asking for here is a new security boundary for browsers — a direct statement that, if the client’s going somewhere or grabbing something, this inspector is going to be able to take a look at it.

There’s also history with both the Referer and Origin headers, which I spoke about (along with the seeds of this research) in my Interpolique talk. The basic concept with each is that the navigation source of a request — say http://www.google.com — would identify itself in some manner in the HTTP header of the subsequent request. The problem for Referer is that there are just so many sources of requests, and it’s best effort whether they show up or not (let alone whether they show up correctly).

Origin was its own problem. Effectively, they added this entire use case where images and links on a shared forum wouldn’t get access to Origin headers. It was necessary for their design, because their security model was entirely static. But for everything that’s *not* an Internet forum, the security model is effectively random.

With ReU, you write the code (or at least import the library) that determines whether or not a token is applied. And as a bonus, you can quickly upgrade to HTTPS too!

There is at least one catastrophic error case that needs to be handled. Basically, those pop-under windows that advertisers use, are actually really useful for all sorts of interesting client side attacks. See, those pop-unders usually retain a connection to the window they came from, and (here’s the important part) can renavigate those windows on demand. So there’s a possible scenario in which you go to a malicious site, it spawns the pop-under, you go to another site, you log in, and then the malicious page drags your real logged in window to a malicious URL at the otherwise protected site.

It’s a navigation event, and so a naive implementation of ReU would fire, adding the token. Yikes.

The solution is straightforward — guarantee that, at least by default, any event that fired the rewriter came from a local source. But that brings up a much trickier question:

Do we try to stop clickjacking with this mechanism as well? IFrames are beautiful, but they are also the source of a tremendous number of events that comes from rather untrusted parties. It’s one thing to request a URL rewriter that can special case external navigation events. It’s possibly another to get a default filter against events sourced from actions that really came from an IFrame parent.

It is very, very possible to make technically infeasible demands. People think this is OK, because it counts as “trying”. The problem is that “trying” can end up eating your entire budget for fixes. So I’m a little nervous about making ReU not fire when the event that triggered it came from the parent of an IFrame.

There are a few other things of note — this approach is rather bookmark friendly, both because the URLs still contain tokens, and because a special “bookmark handler” could be registered to make a long-lived bookmark of sorts. It’s also possible to allow other modulations for the XSRF token, which (as people note) does leak into Referer headers and the like. For example, HTTP headers could be added, or the cookie could be modified on a per request basis. Also, for navigations offsite, secrets could be removed from the referring URL before handed to foreign sites — this could close a longstanding concern with the Referer header too.

Finally, for debuggability, it’d be necessary to have some way of inspecting the URL rewriting stack. It might be interesting to allow the stack to be locked, at least in order of when things got patched in. Worth thinking about.

In summary, there’s lots of quirks to noodle on — but I’m fairly convinced that we can give web developers a “one stop shop” for implementing a number of the behaviors we in the security community believe they should be exhibiting. We can make things easier, more comprehensive, less buggy. It’s worth exploring.

Categories: Security
  1. kascha newberry
    November 29, 2011 at 8:00 pm

    Dan- I am NOT a computer wiz. However, my brother is almost completely color deficient and I bought your app for his IPhone. I was wondering- why couldn’t you adapt that to television watching? Many people watch on high def and their own tv- so why couldn’t you program the TV to adjust for color defiency?

    Just an idea.

    Kascha Newberry

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