|February 7th, 2021|
mail.example.comthe same site? I'd say yes, since they're probably run by the same people. What about
example-b.github.io? I'd say no, since GitHub allows anyone to register pages like
username.github.io. I can make my judgments as a human, but what should the browser do? Should
www.example.combe able to set a cookie that will be sent to
It is a bit of a hack, but the way browsers deal with this is a big
list: the Public Suffix List.
The PSL contains, for example,
github.io, which tell us that
example.github.io are independent sites. On the
other hand, any subdomains are not separate sites:
Have a look, it's pretty hairy: public_suffix_list.dat
Browsers are somewhat ashamed of the hackiness of
and nervous about the security risk of omissions, and so have
generally used a much stricter concept of
introducing functionality. For example,
https://a.example.com cannot write to
localStorage in a way visible to
https://b.example.com. As browsers work to prevent
cross-site tracking, however, with privacy changes such as cache partitioning, the
origin model is too strict. These mitigations generally
use the PSL, and I wanted to look back at its origins.
HTTP was originally completely stateless. This poses challenges if
you want to implement per-user functionality, like a shopping cart.
Netscape's solution, which the world adopted, was cookies. If you
specification, it has some discussion of how to prevent someone
setting a cookie on all of
Only hosts within the specified domain can set a cookie for a domain and domains must have at least two (2) or three (3) periods in them to prevent domains of the form: ".com", ".edu", and "va.us". Any domain that fails within one of the seven special top level domains listed below only require two periods. Any other domain requires at least three. The seven special top level domains are: "COM", "EDU", "NET", "ORG", "GOV", "MIL", and "INT".This simple heuristic worked reasonably well at the time: it understands that
example.co.ukare independent sites, separate from other
There were, even from the beginning, cases that this heuristic did not
handle. My library growing up was
ideally would not have shared cookies with anything else under
lib.ma.us. In 2000, however, ICAAN announced
seven more TLDs, and initially browsers did not allow anyone to set
example.info etc. It wasn't too bad, since
you could still set a cookie on
www.example.info, but you
couldn't share it with
In 2005-2006, Mozilla decided to replace their inconsistent collection
of heuristics and exceptions with an explicit list (b319643,
effective_tld_names.dat. You can see the first public
github (Mar 2007).
The next round of cookie standardization, RFC 6265 in 2011, recommended projects use it:
NOTE: A "public suffix" is a domain that is controlled by a public registry, such as "com", "co.uk", and "pvt.k12.wy.us". This step is essential for preventing attacker.com from disrupting the integrity of example.com by setting a cookie with a Domain attribute of "com". Unfortunately, the set of public suffixes (also known as "registry controlled domains") changes over time. If feasible, user agents SHOULD use an up-to-date public suffix list, such as the one maintained by the Mozilla project at <http://publicsuffix.org/>.
This still doesn't explain how
github.io got on the list:
that's not a public registry, the way
co.uk is. The
first private registry to be added was
November 2009 (b531252):
The domain operaunite.com is used by Opera's new Unite feature (a small web server built into Opera 10.10,
http://unite.opera.com/). Each instance of Opera Unite have a name
server.username.operaunite.com. While some restrictions are being implemented in Unite, there are still some ways to set cookies for the
operaunite.comdomain, and we would like to restrict the impact by adding this domain to the public suffix list.
appspot.com for App
blogspot.com for Blogger (b593818),
though the Blogger change was rolled back for two years (b598911,
These changes seem to have been uncontroversial; I don't see any
pushback about how these are not "real TLDs".
As more of these came in, there was discussion about how these
were fundamentally different concepts (b712640,
2011), and the list was split into public ("BEGIN ICANN DOMAINS") and
private ("BEGIN PRIVATE DOMAINS") sections. For example, no one
should be able to get a wildcard cert for
*.github.io still makes sense.
Over the last ten years, I believe everyone has migrated to using Mozilla's list. It does take some time for updates to fully propagate, since the list is compiled into browsers, but having one place to update and one place to check for the definition of a site is pretty good.