Yes, HTTP-Only cookies would be fine for this functionality. They will still be provided with the XmlHttpRequest's request to the server.

In the case of Stack Overflow, the cookies are automatically provided as part of the XmlHttpRequest request. I don't know the implementation details of the Stack Overflow authentication provider, but that cookie data is probably automatically used to verify your identity at a lower level than the "vote" controller method.

More generally, cookies are not required for AJAX. XmlHttpRequest support (or even iframe remoting, on older browsers) is all that is technically required.

However, if you want to provide security for AJAX enabled functionality, then the same rules apply as with traditional sites. You need some method for identifying the user behind each request, and cookies are almost always the means to that end.

In your example, I cannot write to your document.cookie, but I can still steal your cookie and post it to my domain using the XMLHttpRequest object.

XmlHttpRequest won't make cross-domain requests (for exactly the sorts of reasons you're touching on).

You could normally inject script to send the cookie to your domain using iframe remoting or JSONP, but then HTTP-Only protects the cookie again since it's inaccessible.

Unless you had compromised StackOverflow.com on the server side, you wouldn't be able to steal my cookie.

Edit 2: Question 2. If the purpose of Http-Only is to prevent JavaScript access to cookies, and you can still retrieve the cookies via JavaScript through the XmlHttpRequest Object, what is the point of Http-Only?

Consider this scenario:

• I find an avenue to inject JavaScript code into the page.
• Jeff loads the page and my malicious JavaScript modifies his cookie to match mine.
• Jeff submits a stellar answer to your question.
• Because he submits it with my cookie data instead of his, the answer will become mine.
• You vote up "my" stellar answer.
• My real account gets the point.

With HTTP-Only cookies, the second step would be impossible, thereby defeating my XSS attempt.

Edit 4: Sorry, I meant that you could send the XMLHttpRequest to the StackOverflow domain, and then save the result of getAllResponseHeaders() to a string, regex out the cookie, and then post that to an external domain. It appears that Wikipedia and ha.ckers concur with me on this one, but I would love be re-educated...

That's correct. You can still session hijack that way. It does significantly thin the herd of people who can successfully execute even that XSS hack against you though.

However, if you go back to my example scenario, you can see where HTTP-Only does successfully cut off the XSS attacks which rely on modifying the client's cookies (not uncommon).

It boils down to the fact that a) no single improvement will solve all vulnerabilities and b) no system will ever be completely secure. HTTP-Only is a useful tool in shoring up against XSS.

Similarly, even though the cross domain restriction on XmlHttpRequest isn't 100% successful in preventing all XSS exploits, you'd still never dream of removing the restriction.

I got it to work, but the solution is a bit complex, so bear with me.

What's happening

As it is, Internet Explorer gives lower level of trust to IFRAME pages (IE calls this "third-party" content). If the page inside the IFRAME doesn't have a Privacy Policy, its cookies are blocked (which is indicated by the eye icon in status bar, when you click on it, it shows you a list of blocked URLs).

_{(source: piskvor.org)}

In this case, when cookies are blocked, session identifier is not sent, and the target script throws a 'session not found' error.

(I've tried setting the session identifier into the form and loading it from POST variables. This would have worked, but for political reasons I couldn't do that.)

It is possible to make the page inside the IFRAME more trusted: if the inner page sends a P3P header with a privacy policy that is acceptable to IE, the cookies will be accepted.

How to solve it

Create a p3p policy

A good starting point is the W3C tutorial. I've gone through it, downloaded the IBM Privacy Policy Editor and there I created a representation of the privacy policy and gave it a name to reference it by (here it was policy1).

NOTE: at this point, you actually need to find out if your site has a privacy policy, and if not, create it - whether it collects user data, what kind of data, what it does with it, who has access to it, etc. You need to find this information and think about it. Just slapping together a few tags will not cut it. This step cannot be done purely in software, and may be highly political (e.g. "should we sell our click statistics?").

(e.g. "the site is operated by ACME Ltd., it uses anonymous per-session identifiers for its operation, collects user data only if explicitly permitted and only for the following purposes, the data is stored only as long as necessary, only our company has access to it, etc. etc.").

(When editing with this tool, it's possible to view errors/omissions in the policy. Also very useful is the tab "HTML Policy": at the bottom, it has a "Policy Evaluation" - a quick check if the policy will be blocked by IE's default settings)

The Editor exports to a .p3p file, which is an XML representation of the above policy. Also, it can export a "compact version" of this policy.

Link to the policy

Then a Policy Reference file (http://example.com/w3c/p3p.xml) was needed (an index of privacy policies the site uses):

<META>
  <POLICY-REFERENCES>
    <POLICY-REF about="/w3c/example-com.p3p#policy1">
      <INCLUDE>/</INCLUDE>
      <COOKIE-INCLUDE/>
    </POLICY-REF>
  </POLICY-REFERENCES>
</META>

The <INCLUDE> shows all URIs that will use this policy (in my case, the whole site). The policy file I've exported from the Editor was uploaded to http://example.com/w3c/example-com.p3p

Send the compact header with responses

I've set the webserver at example.com to send the compact header with responses, like this:

HTTP/1.1 200 OK 
P3P: policyref="/w3c/p3p.xml", CP="IDC DSP COR IVAi IVDi OUR TST"
// ... other headers and content

policyref is a relative URI to the Policy Reference file (which in turn references the privacy policies), CP is the compact policy representation. Note that the combination of P3P headers in the example may not be applicable on your specific website; your P3P headers MUST truthfully represent your own privacy policy!

Profit!

In this configuration, the Evil Eye does not appear, the cookies are saved even in the IFRAME, and the application works.

Edit: What NOT to do, unless you like defending from lawsuits

Several people have suggested "just slap some tags into your P3P header, until the Evil Eye gives up".

The tags are not only a bunch of bits, they have real world meanings, and their use gives you real world responsibilities!

For example, pretending that you never collect user data might make the browser happy, but if you actually collect user data, the P3P is conflicting with reality. Plain and simple, you are purposefully lying to your users, and that might be criminal behavior in some countries. As in, "go to jail, do not collect $200".

A few examples (see p3pwriter for the full set of tags):

• NOI : "Web Site does not collected identified data." (as soon as there's any customization, a login, or any data collection (***** Analytics, anyone?), you must acknowledge it in your P3P)
• STP: Information is retained to meet the stated purpose. This requires information to be discarded at the earliest time possible. Sites MUST have a retention policy that establishes a destruction time table. The retention policy MUST be included in or linked from the site's human-readable privacy policy." (so if you send STP but don't have a retention policy, you may be committing fraud. How cool is that? Not at all.)

I'm not a lawyer, but I'm not willing to go to court to see if the P3P header is really legally binding or if you can promise your users anything without actually willing to honor your promises.