The mess with X-Frame-Options: ALLOWALL
Seemingly a quite new invention, X-Frame-Options option "ALLOWALL", comes amid the standardization process of X-Frame-Options header.
X-Frame-Options is a poor man's replacement for CSP that has been around since 2009 and initially it had just two values: DENY and SAMEORIGIN. Later on someone has added ALLOW-FROM and these three values together found their way into an upcoming IETF standard (draft-ietf-websec-x-frame-options).
So I was very suprised to see Google (!) AdSense servers return a previously unseen value of ALLOWALL, that is documented nowhere else — except for this article (from now on) and a recent (2 days ago) request to add it to WebKit (Bug 110857 and patch). Here's description of the change:
DoubleClick, among others, serves ALLOWALL as a 'X-Frame-Options' value with the intent of (shock!) allowing a resource to be framed by all origins. Given its prevelance, and the fact that IE supports the header, we shouldn't call it out as invalid.
I don't quite get it. If you allow your content to be framed, you just do not set X-Frame-Options. What is even even more surprising, all that the patch does is to disable console warnings in WebKit, without really changing any frames logic.
Invalid 'X-Frame-Options' header encountered when loading 'https://googleads.g.doubleclick.net/pagead/drt/s?v=r20120211': 'ALLOWALL' is not a recognized directive. The header will be ignored.
Here's full HTTP response from https://googleads.g.doubleclick.net/pagead/drt/si captured in the wild:
cache-control:private content-length:0 content-type:text/html; charset=UTF-8 date:Thu, 28 Feb 2013 16:55:19 GMT expires:Thu, 28 Feb 2013 16:55:19 GMT p3p:policyref="https://googleads.g.doubleclick.net/pagead/gcn_p3p_.xml", CP="CURa ADMa DEVa TAIo PSAo PSDo OUR IND UNI PUR INT DEM STA PRE COM NAV OTC NOI DSP COR" server:safe set-cookie:_drt_=OPT_OUT; expires=Fri, 01-Mar-2013 16:55:19 GMT; path=/; domain=.doubleclick.net; HttpOnly status:200 OK version:HTTP/1.1 x-content-type-options:nosniff x-frame-options:ALLOWALL x-xss-protection:1; mode=block