3rd-Party Cookies, DOM Storage and Privacy

We’re in the process of launching our new publisher integration feature at DotSpots and needed to gain an understanding of how browsers deal with “third-party cookies”, or cookies that are set on domains that differ from the top-level domain.

Each of the major browsers, Firefox, Chrome, Safari, Opera and Internet Explorer have their own quirks about how cookies are accepted, which vary wildly depending on whether they were set by a top-level page, or a page in a third-party iframe.

Executive summary: Based on this study of browsers, the ideal method of storing information in iframes is a combination of localStorage for modern browsers and persistent cookies with a privacy policy for downlevel IE and Firefox versions. The default privacy settings are permissive enough on most of the old browsers to make this approach feasible. Earlier Safari versions that don’t support localStorage are out-of-luck, but the market share is too small to worry about.

These are the browsers I tested:

  • Firefox Default (checked ‘accept third-party cookies’)
  • Firefox (unchecked ‘accept third-party cookies’)
  • Chrome Default (allow all cookies)
  • Chrome (accept cookies only from sites I visit)
  • Safari Default (only from sites I visit)
  • Safari (accept cookies always)
  • Opera Default (accept cookies)
  • Opera (only from the site I visit)
  • IE6 Default
  • IE7 Default
  • IE8 Default

All browsers that support localStorage support setting and retrieving storage values from any frame. This includes Firefox, Chrome, Safari and IE8. HTML5 localStorage is by far the most reliable way to store information at this time for browsers that support it. There is one small difference: in Firefox, blocking all cookies will also block localStorage. In Chrome and Safari, blocking cookies does not block localStorage.

Chrome and Safari are based on the same WebKit engine and, as expected, share the same cookie policies for the same modes. Chrome defaults to the more permissive ‘Allow all cookies’ setting, while Safari defaults to ‘Allow Cookies from Sites I Visit’. When third-party cookies are disabled, frames can read cookies set by top-level pages but not write them.

Firefox defaults to a permissive mode in which cookies can be set and retrieved from all locations. Unlike WebKit browsers, disallowing third-party cookies means that a third-party iframe cannot read or write cookies at all.

Opera also defaults to permissive mode. In “accept cookies only from the site I visit” mode, it behaves the same way as Firefox does when third-party cookies are disallowed.

Internet Explorer is a bit finicky about privacy. If you add a basic privacy policy header to your responses, cookies will be accepted from iframes. Without the policy, most cookies can’t be set or retrieved in iframes at all (the exception is that iframes can read session cookies set at the top level). The following P3P header is sufficient to fix cookies in iframes.

P3P: CP="CAO PSA OUR"

To configure the header in Apache, you can use a simple mod_header line:

        
Header append P3P "CP=\"CAO PSA OUR\""

For those interested, here is a breakdown of cookie handling by browser and mode. Unlike cookies, HTML5 localStorage has no known limitations, so it has been omitted from the following charts:

WebKit, Allow All Cookies (Chrome default):

Set / Can be read Top level Iframe
Top level X X
Iframe X X

WebKit, Only From Sites I Visit (Safari default):

Set / Can be read Top level Iframe
Top level X X
Iframe

Firefox, default:

Set / Can be read Top level Iframe
Top level X X
Iframe X X

Firefox, unchecked ‘accept third-party cookies’:

Set / Can be read Top level Iframe
Top level X
Iframe

Opera, default:

Set / Can be read Top level Iframe
Top level X X
Iframe X X

Opera, Only From Sites I Visit:

Set / Can be read Top level Iframe
Top level X
Iframe

IE6/IE7/IE8 default mode, without Privacy policy:

Set / Can be read Top level Iframe
Top level X *
Iframe

* session cookies set by the top-level page may be read by iframes, but persistent cookies may not

IE6/IE7/IE8 default mode, with Privacy policy:

Set / Can be read Top level Iframe
Top level X X
Iframe X X

4 Responses to “3rd-Party Cookies, DOM Storage and Privacy”

  1. Helder Silva says:

    It still doesn’t work with me. IE8 and with private policy as you defined it still rejects my session cookie :|

    Can you give me some help?

    Thanks.

  2. Riley Carter says:

    Internet Explorer 8 have been my most used browser this year, it is definitely stable and fast loading too. `

  3. i really love to attend in parties because it is fun an you met a lot of new friends.**-

  4. Aaed Kayal says:

    Thanks , Great Work , Worth Reading , Keep the Good Work