Het Bijstere Spoor

Escaping user-input in your HTML is essential for preventing worlds #1 vulnerability.

When you're embedding user input into javascript, a simple htmlspecialchars won't cut it, you'll need to make sure you're escaping other things, like \n (line endings), and \ (slashes). Google doctype has a good list of characters in need of proper escaping to prevent users breaking your javascript.

However, when I dropped the question if a simple string replacement would be good enough, the members of the Web security mailing list gave me a different answer.

When escaping or filtering output using a blacklist (such as the one published on google doctype) browser/unicode escaping bugs are not taking into consideration. Some new vulnerability might appear in the future, which would immediately open a hole in your app. For this reason its wiser to go with a much more defensive white-list approach, essentially only letting things through you know is safe.

Introducing Reform

Reform is a tool that does exactly this. Reform allows you to escape your data for a javascript, xml, html or vbscript (yes it still exists) context. It provides libraries for Java, .NET, PHP, Perl, Python, Javascript and ASP. Pretty cool!

One dislike I have is that it only considers I really small set of unicode codepoints safe, especially when dealing with non-latin languages this is going to add a great deal to the bandwidth usage and the legibility of your sourcecode. One would think there has to be more ranges considered 'safe'.

PHP example:

<?php
  // Assuming the Reform class is included..
 
  echo '<script type="text/javascript"> var myString = ', Reform::JsString($userInput), '; </script>';
 
?>

I made a couple of changes in the PHP version, specifically:

• Prepended the 'static' keyword to every method to make it work in PHP5's strict mode.
• Removed the UTF-8 checks, I'm in a controlled environment, mbstring is installed, and the internal encoding is utf-8.
• Added a parameter to Reform::JsString to not automatically put the string between quotes (').

Via: Jeremiah Grossman.

A proposal for a Site Security Policy has been proposed by mozilla employee Brandon Sterne. This is an extremely important specification for the web, and could be a big step ahead for security on the web.

Over the last decade websites have transformed into feature-rich web applications, with the introduction of Javascript and XMLHTTPRequest, Flash and whatnot. While great for user experience, this has also brought huge security implications, resulting in over 80% of all documented security vulnerabilities in 2007 being carried out using XSS.

While implementing all this new fancy stuff, browser vendors have been slacking thinking about the security implications and essentially the responsibility of safe browsing has been put on the user. (Remember the 90's advice of disabling cookies while browsing?). Browsers have become better over time, but one single XSS hole on any site can still have devastating effects for you and your users.

With the demand for web development and web developers still way higher than the supply, education is in a sorry state too. In job interviews I've conducted it's been rare to find a junior who knows of the concept XSS, and finding one who can explain the implications of CSRF, is, well, I've yet to meet one. I don't claim to be a security expert myself, but I feel everybody who's in the profession of web development, should at least be aware of the basic attack vectors and how to prevent them.

“If builders built buildings the way programmers wrote programs, then the first woodpecker that came along would destroy civilization.”

- Weinberg’s Second Law

With the Site Security Policy we're given the ability to lock down certain types of behaviours. It allows us to disable javascript included from unknown domains (a whitelist approach), and HTTP requests initiated by external domains, essentially fixing the CSRF problem completely. Additionally, it defines a way for a browser to log attempts to violate the policy.

For the actual implementation details, I'd suggest just reading the spec, even though its still a proposal, it's good reading material.

So whats next?

The spec needs to be finished. While the current policy is distributed through HTTP headers, some people seem to prefer an external file, like how crossdomain.xml or robots.txt is implemented. The latter would have my vote, because the policy can then be easily cached, which can save some bytes in the end. It would also be easier for people to upload a policy file to a server where there's no scripting available and allows the policy to be enforced for a complete domain, instead having to add it to each and every script.

And last but not least, browser vendors would need to implement it. Sterne works at Mozilla, so that's a good sign already. Personally, I can't wait.

Yep, thats a long title, but they are all related to each other in some way. In the first few paragraphs I will explain what cookies are and XSS. You might want to skip ahead if you already know what this is.

Sessions, Cookies

HTTP is stateless. This means that every request to the server is a 'new' one and normally there is no relation to a first or second request. To allow maintaining a session or 'state' between multiple requests, HTTP cookies are used.

A cookie is basically a HTTP header with a tiny piece of information that gets re-sent with every request to the server. A popular way to make use of this is through PHP's$_SESSION system. This sends a cookie with a unique id to the client that allows PHP to retain a users' information across pages.

XSS

If you allow users to for example comment on one of your pages and allow (certain) html, it is sometimes possible to inject a piece of javascript. There are many tricks to evade the so-called html sanitizers.. strip_tags() is PHP's built-in sanitizer, but it doesn't work really well.. if, for example, you would allow users to use a <p> tag, which might seem harmless, there would be tricks to abuse the style="" or onclick="" attributes, just to name a few.

XSS and cookies

So how do you abuse javascript and cookies combined?

Because with for example PHP's session system, you can use the contents of the cookie to steal someone's session. The hacker would be logged in as you and might able to change your password and log you out afterwards. The contents of the cookies is stored in the javascript variable document.cookie.

HttpOnly, a solution

Microsoft came up with a way to prevent this from happening, ever since Internet Explorer 6.0 (starting from Windows XP SP1). They added an extra piece of information to a cookie, that will still allow the use of cookies in the way you are used to, but it will prevent the cookie from being read by javascript (basically it is invisible for javascript). Be sure to check out microsofts spec at MSDN

Safari and Opera quickly started supporting this. Because of this it is becoming pretty useful to use in practice. Remember that this doesn't mean you can just accept any html on your site, you should still always sanitize the bad stuff or not allow it at all! But in the case you missed something, it can make it a lot more difficult for your attacker to steal sessions.

UPDATE: Safari/Opera actually ignore it, my excuses, I didn't check my sources.

Under the hood

Normally, a cookie header will look like this:

Set-Cookie: USER=username; expires=Wednesday, 09-Nov-99 23:12:40 GMT;

But with the HttpOnly, it will look like this:

Set-Cookie: USER=username; expires=Wednesday, 09-Nov-99 23:12:40 GMT; HttpOnly

It's a small change, and all normal browsers should still accept this even if they don't understand the HttpOnly part. There is an exception though, and it goes by the name of IE 5 for mac. This browser won't understand the cookie and totally ignores it. Personally I don't support this browser for any application anymore, as there are too many bugs in this browser. But if your boss wants it, this might prevent you from using HttpOnly

PHP support

A guy named Scott MacVicar created a patch for PHP that will add an extra parameter to set_cookie() to enable HttpOnly for your cookies. The patch should also enable this by default for the session system.

If the patch will get accepted we will likely be able to use this in PHP 6.0 and perhaps even PHP 5.2, I'm looking forward to that. There is a chance though, because of the IE5/mac breakage that it eventually won't be auto-enabled for the session system.

So what about firefox?

Firefox doesn't support it, there is currently a bug open for it (Bug #178993). There was an initial solution posted over 2 years ago (January 2004). And a few other patches later on, but the mozilla folks refused all of them because they want to maintain the exact format of cookies.txt (the file they use to store cookies), because other applications might rely on that format. A few solutions for that have been posted, but it doesn't seem like a high priority for them.

A workaround for firefox (kind of)

There have been solutions for firefox that also blocked reading of cookies by javascript. Firefox has a magic function __defineGetter__ that can block reading of variables. To do this for all cookies on your site, include the following snippet on top of your html page:

HTMLDocument.prototype.__defineGetter__("cookie",function (){return null;});

However, you can't rely on this! There are still ways to get this cookie if the hacker can somehow create an iframe in your html page. The hacker has a reference to the same cookies if he uses the data: protocol in the src="" attribute. This will still make it a bit harder to steal cookies, so it's not a bad idea to implement. For a longer explanation of this workaround, check out http://www.wisec.it/sectou.php?lang=en.

The LiveJournal case

The same people who submitted the initial firefox-patch (see above) 2 years ago, also got hit by the attack in January 2006. Over 46% accounts were hijacked. This were over 900.000 stolen accounts. (check out their post about their solution.)

The reason they got hit by this is because they allowed users to use remote CSS stylesheets for their pages. CSS used to be only a specification for how html elements would look like on a page, but since a few years it has become more dynamic and now there are ways to exploit CSS with XSS attacks through for example IE's non-standard behavior: attribute and Firefox' -moz-binding: attribute (there are more you can exploit, but its outside the scope of the article). These attributes allow an author to create a custom behavior for a HTML element. The technique to do this for Firefox is called XBL.

Normally, when a page is loaded from domain A, and another (in a different frame for example) is loaded from domain B. Malicious scripts from domain B can never access cookies from domain A. This is called a 'same origin check'. This generally works in all browsers for all kinds of content, but an exception is XBL. (see bug #324253.)

Apparently this requires some major changes in how firefox works. The last comment in this bug is from February this year (2006). I hope they will wake up some day soon and make our life a little bit easier by both supporting HttpOnly and securing XBL.

So the LiveJournal attack could happen because the hackers used XBL in their CSS, which in turn accessed the HTTP cookies through javascript. The attack didn't affect users of other browsers, because: A) they support HttpOnly, B) even if they wouldn't.. the microsoft equivalent of XBL, which is .HTC files, don't work across different domains.