HTTP cookie processing wrt "public suffixes" (fwd)

[Daniel Stenberg]

FYI

--

/ daniel.haxx.se

---------- Forwarded message ----------
Date: Fri, 14 Nov 2014 03:07:05
From: =JeffH <Jeff....@KingsMountain.com>
To: HTTP State <http-...@ietf.org>
Subject: [http-state] HTTP cookie processing wrt "public suffixes"

Thanks to Adam Barth for his comments on this.

In terms of the first comment on single-label domain names, I added a
definition for them and updated other related definitions.

wrt..
>> (d) The UA will not return any of the eTLD's cookies to subdomain
>> servers. Effectively, this depends upon UA configuration -- i.e., a UA
>> might be configured to not accept cookies from an eTLD server at all.
>
> Are there popular UAs that are configured that way? Presumably they'd
> break in corporate networks that use DNS names like "www"...

oops yeah, you're correct, thx. I'd misread the Note at the end of S 4.1.2.3.
on the Domain Attribute. I've removed that statement from section 2 of the
below, and made some other fixups/polishing.

any other comments from anyone?

thanks,

=JeffH

---- updated original msg ----

This is a (draft) community-service posting: The purpose is to unambiguously
state the specification of "cookie processing wrt public suffixes".

It is somewhat difficult to tease this out of the requisite specification(s)
and associated documents, e.g., [RFC6265] and the Public Suffix List [PSL], and
so here it is (corrections/comments welcome).

=JeffH
------
HTTP cookie processing wrt "public suffixes" aka "effective TLDs (eTLDs)"

Contents:
1. Terminology
2. Summarized Cookie-setting and -returning Algorithms wrt eTLDs
3. Detailed Cookie-setting and -returning Algorithms wrt eTLDs
4. Excerpts from [RFC6265]
5. References


1. Terminology:

Cookie - a unit of HTTP state management data, sent by the server to the UA as
part of (some) HTTP responses. The UA returns cookie(s) to the server on all
subsequent requests, modulo various rules (some of which are discussed herein,
the full story is given by [RFC6265]).

eTLD / effective Top Level Domain - a term closely related to "public suffix".
It is more general than the term "public suffix" -- i.e., because it implies
that the domain in question ought to be treated effectively as a TLD, but it
may actually not be an official TLD, nor might it allow for "public" subdomain
registrations (as the term "public suffix" specifically implies).

Origin - {scheme, host, port} tuple of a webapp [RFC6454], also known as a Web
Origin.

Public Suffix - A "public suffix" is a DNS domain [RFC1034] under which
Internet users can directly register names [modulo some policies]. Some
examples of public suffixes are .com, .co.uk and pvt.k12.ma.us [PSL]. Note
however, this definition does not adequately address various subtleties in
practice (hence, in part, the [DBOUND] effort). Also note that the term
"effective TLD (eTLD)" is closely related, and that the domains listed in [PSL]
are a superset of actual IANA-registered TLDs [IANA-TLDs].

single-label domain name - A DNS name consisting of a single label, e.g.,
"org", "net", "com". Note that on various networks, often in enterprise
deployments, existence of "local" single-label domains is not uncommon, e.g.,
"corp", "www". See also eTLD and TLD.

subdomain - a child domain of some given domain [RFC1034]. E.g.,
foo.example.org is a subdomain of example.org.

superdomain - the parent domain of some given domain. E.g., example.org is the
superdomain of foo.example.org.

TLD / Top-level Domain - TLDs are those single-label DNS domains directly under
the global public DNS root [IANA-TLDs] [RFC1591]. However, not all single-label
DNS names are globally-visible TLDs. See also single-label domain name.

UA / User Agent - A web browser, or other HTTP client implementation, that
implements the HTTP State spec [RFC6265], i.e., accepts cookies sent by HTTP
servers.

webapp - a web application, which has both a server-side and client-side
instances. The client-side instances, which are emitted by the server-side over
HTTP, are effectively mobile code (typically composed of
HTML+JavaScript+CSS+etc), and execute within UAs (as "web pages"), and have an
origin derived from the server-side's URI. Note that this also encompasses
"apps" that are not general purpose web browsers but that meet the definition
of UA.



2. Summarized Cookie-setting and -returning Algorithms wrt eTLDs:

(a) A server-side webapp, whose origin's host component [RFC6454] (aka domain
name) IS NOT a "public suffix"/eTLD, can "set cookies" (on UAs) for its own
domain name, or for superdomains -- unless the targeted superdomain is a eTLD.
In the latter case, the set-cookie attempt is ignored.
(b) The UA will return cookies set for a given host (aka domain) to only that
host, or, to that host and all subdomains thereof (depending on
server-specified particulars of the cookie itself).

(c) Conversely, a server-side webapp whose origin's host component IS denoted
as a "public suffix"/eTLD (i.e., it appears in [PSL] or similar compendiums)
can set cookies for itself (modulo UA configuration), but its subdomains can
not set cookies for the eTLD domain.
(d) The UA will not return any of the eTLD's cookies to subdomain servers.


3. Detailed Cookie-setting and -returning Algorithms wrt eTLDs:

Excerpts from [RFC6265] expressing the specifics underlying the cookie-setting
and -returning algorithms, and in which the effects of "public suffixes"/eTLDs
are specified, are found in the section after this one. They are referred to
(using an "S" prefix) in the following explanations of the algorithms
summarized in the preceding section:

In terms of (a), a server that is NOT a eTLD doing cookie-setting: the 2nd
portion of S 4.1.2.3 describes the cookie-setting semantics, step 5 of S 5.3
handles cookie-setting attempts for "public suffixes", step 6 of S 5.3 defines
the determination of the cookie's applicable domain and its notation in the
cookie's cookie store entry, e.g. setting the host-only-flag. The
host-only-flag governs whether the cookie will be returned to the origin server
only, or whether it will be returned to the origin server as well as its
subdomains.

In terms of (b), a UA returning cookies to non-eTLD servers: S 5.4 governs the
construction of the Cookie header for the HTTP request, which depends directly
upon the cookie entry's host-only-flag, and the domain-match algorithm (S
5.1.3).


In terms of (c), a "public suffix"/eTLD webapp doing cookie-setting: the 2nd
portion of S 4.1.2.3 describes again applies, and refers directly to S 5.3 in
this case. S 5.3 steps 5 and 6 apply and the path through them depends on
whether the UA is config'd to reject "public suffixes" or not. If not, and the
domain-attribute is empty, then in step 6 the cookie is stored in the cookie
store with its host-only-flag set to true and its domain set to the
canonicalized request-host.

In terms of (d), a UA returning cookies to a "public suffix"/eTLD server:
step 1 of S 5.4 will gather only such cookies and return them to ONLY the eTLD
server, due to the manner that they were recorded in the cookie store.


4. Excerpts from [RFC6265]:

[Note: I've included the hierarchy of [RFC6265] section titles in
order to provide context.]

4. Server Requirements
4.1. Set-Cookie
4.1.2. Semantics (Non-Normative)
4.1.2.3. The Domain Attribute
https://tools.ietf.org/html/rfc6265#section-4.1.2.3

The Domain attribute specifies those hosts to which the cookie will
be sent. For example, if the value of the Domain attribute is
"example.com", the user agent will include the cookie in the Cookie
header when making HTTP requests to example.com, www.example.com, and
www.corp.example.com. ... If the server omits the Domain attribute,
the user agent will return the cookie only to the origin server.
...
The user agent will reject cookies unless the Domain attribute
specifies a scope for the cookie that would include the origin
server. For example, the user agent will accept a cookie with a
Domain attribute of "example.com" or of "foo.example.com" from
foo.example.com, but the user agent will not accept a cookie with a
Domain attribute of "bar.example.com" or of "baz.foo.example.com".

NOTE: For security reasons, many user agents are configured to reject
Domain attributes that correspond to "public suffixes". For example,
some user agents will reject Domain attributes of "com" or "co.uk".
(See Section 5.3 for more information.)
...

5. User Agent Requirements
5.1. Subcomponent Algorithms
5.1.3. Domain Matching
https://tools.ietf.org/html/rfc6265#section-5.1.3

A string domain-matches a given domain string if at least one of the
following conditions hold:

o The domain string and the string are identical. (Note that both
the domain string and the string will have been canonicalized to
lower case at this point.)

o All of the following conditions hold:

* The domain string is a suffix of the string.

* The last character of the string that is not included in the
domain string is a %x2E (".") character.

* The string is a host name (i.e., not an IP address).


5.3. Storage Model
https://tools.ietf.org/html/rfc6265#section-5.3
...
5. If the user agent is configured to reject "public suffixes" and
the domain-attribute is a public suffix:

If the domain-attribute is identical to the canonicalized
request-host:

Let the domain-attribute be the empty string.

Otherwise:

Ignore the cookie entirely and abort these steps.

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/>.

6. If the domain-attribute is non-empty:

If the canonicalized request-host does not domain-match the
domain-attribute:

Ignore the cookie entirely and abort these steps.

Otherwise:

Set the cookie's host-only-flag to false.

Set the cookie's domain to the domain-attribute.

Otherwise:

Set the cookie's host-only-flag to true.

Set the cookie's domain to the canonicalized request-host.
...

5.4. The Cookie Header
https://tools.ietf.org/html/rfc6265#section-5.4

The user agent includes stored cookies in the Cookie HTTP request
header.
...
If the user agent does attach a Cookie header field to an HTTP
request, the user agent MUST send the cookie-string (defined below)
as the value of the header field.

The user agent MUST use an algorithm equivalent to the following
algorithm to compute the "cookie-string" from a cookie store and a
request-uri:

1. Let cookie-list be the set of cookies from the cookie store that
meets all of the following requirements:

* Either:

The cookie's host-only-flag is true and the canonicalized
request-host is identical to the cookie's domain.

Or:

The cookie's host-only-flag is false and the canonicalized
request-host domain-matches the cookie's domain.

* The request-uri's path path-matches the cookie's path.

...

* If the cookie's http-only-flag is true, then exclude the
cookie if the cookie-string is being generated for a "non-
HTTP" API (as defined by the user agent).
...


5. References:

[DBOUND] https://www.ietf.org/mailman/listinfo/dbound

[IANA-TLDs] https://www.iana.org/domains/root/db

[PSL] Public Suffix List
https://publicsuffix.org/

[RFC1034] DOMAIN NAMES - CONCEPTS AND FACILITIES
https://tools.ietf.org/html/rfc1034

[RFC1591] Domain Name System Structure and Delegation
http://tools.ietf.org/html/rfc1591
see also: https://en.wikipedia.org/wiki/Top-level_domain

[RFC6265] HTTP State Management Mechanism
https://tools.ietf.org/html/rfc6265

[RFC6454] The Web Origin Concept
https://tools.ietf.org/html/rfc6454



end

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