How do browser cookie domains work?

Although there is the RFC 2965 (Set-Cookie2, had already obsoleted RFC 2109) that should define the cookie nowadays, most browsers don’t fully support that but just comply to the original specification by Netscape.

There is a distinction between the Domain attribute value and the effective domain: the former is taken from the Set-Cookie header field and the latter is the interpretation of that attribute value. According to the RFC 2965, the following should apply:

If the Set-Cookie header field does not have a Domain attribute, the effective domain is the domain of the request.

If there is a Domain attribute present, its value will be used as effective domain (if the value does not start with a . it will be added by the client).

Having the effective domain it must also domain-match the current requested domain for being set; otherwise the cookie will be revised. The same rule applies for choosing the cookies to be sent in a request.

Mapping this knowledge onto your questions, the following should apply:

Cookie with Domain=.example.com will be available for www.example.com

Cookie with Domain=.example.com will be available for example.com

Cookie with Domain=example.com will be converted to .example.com and thus will also be available for www.example.com

Cookie with Domain=example.com will not be available for anotherexample.com

www.example.com will be able to set cookie for example.com

www.example.com will not be able to set cookie for www2.example.com

www.example.com will not be able to set cookie for .com

And to set and read a cookie for/by www.example.com and example.com, set it for .www.example.com and .example.com respectively. But the first (.www.example.com) will only be accessible for other domains below that domain (e.g. foo.www.example.com or bar.www.example.com) where .example.com can also be accessed by any other domain below example.com (e.g. foo.example.com or bar.example.com).

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The previous answers are a little outdated.

RFC 6265 was published in 2011, based on the browser consensus at that time. Since then, there has been some complication with public suffix domains. I've written an article explaining the current situation - http://bayou.io/draft/cookie.domain.html

To summarize, rules to follow regarding cookie domain:

The origin domain of a cookie is the domain of the originating request.

If the origin domain is an IP, the cookie's domain attribute must not be set.

If a cookie's domain attribute is not set, the cookie is only applicable to its origin domain.

If a cookie's domain attribute is set, the cookie is applicable to that domain and all its subdomains; the cookie's domain must be the same as, or a parent of, the origin domain the cookie's domain must not be a TLD, a public suffix, or a parent of a public suffix.

the cookie is applicable to that domain and all its subdomains;

the cookie's domain must be the same as, or a parent of, the origin domain

the cookie's domain must not be a TLD, a public suffix, or a parent of a public suffix.

It can be derived that a cookie is always applicable to its origin domain.

The cookie domain should not have a leading dot, as in .foo.com - simply use foo.com

As an example,

x.y.z.com can set a cookie domain to itself or parents - x.y.z.com, y.z.com, z.com. But not com, which is a public suffix.

a cookie with domain=y.z.com is applicable to y.z.com, x.y.z.com, a.x.y.z.com etc.

Examples of public suffixes - com, edu, uk, co.uk, blogspot.com, compute.amazonaws.com

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For an extensive coverage review the contents of RFC2965. Of course that doesn't necessarily mean that all browsers behave exactly the same way.

However in general the rule for default Path if none specified in the cookie is the path in the URL from which the Set-Cookie header arrived. Similarly the default for the Domain is the full host name in the URL from which the Set-Cookie arrived.

Matching rules for the domain require the cookie Domain to match the host to which the request is being made. The cookie can specify a wider domain match by include *. in the domain attribute of Set-Cookie (this one area that browsers may vary). Matching the path (assuming the domain matches) is a simple matter that the requested path must be inside the path specified on the cookie. Typically session cookies are set with path=/ or path=/applicationName/ so the cookie is available to all requests into the application.

Will a cookie for .example.com be available for www.example.com? Yes

Will a cookie for .example.com be available for example.com? Don't Know

Will a cookie for example.com be available for www.example.com? Shouldn't but... *

Will a cookie for example.com be available for anotherexample.com? No

Will www.example.com be able to set cookie for example.com? Yes

Will www.example.com be able to set cookie for www2.example.com? No (Except via .example.com)

Will www.example.com be able to set cookie for .com? No (Can't set a cookie this high up the namespace nor can you set one for something like .co.uk).

* I'm unable to test this right now but I have an inkling that at least IE7/6 would treat the path example.com as if it were .example.com.

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The last (third to be exactly) RFC for this issue is RFC-6265 (Obsoletes RFC-2965 that in turn obsoletes RFC-2109).

According to it if the server omits the Domain attribute, the user agent will return the cookie only to the origin server (the server on which a given resource resides). But it's also warning that some existing user agents treat an absent Domain attribute as if the Domain attribute were present and contained the current host name (For example, if example.com returns a Set-Cookie header without a Domain attribute, these user agents will erroneously send the cookie to www.example.com as well).

When the Domain attribute have been specified, it will be treated as complete domain name (if there is the leading dot in attribute it will be ignored). Server should match the domain specified in attribute (have exactly the same domain name or to be a subdomain of it) to get this cookie. More accurately it specified here.

So, for example:

cookie attribute Domain=.example.com is equivalent to Domain=example.com

cookies with such Domain attributes will be available for example.com and www.example.com

cookies with such Domain attributes will be not available for another-example.com

specifying cookie attribute like Domain=www.example.com will close the way for www4.example.com

PS: trailing comma in Domain attribute will cause the user agent to ignore the attribute =(

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I tested all the cases in the latest Chrome, Firefox, Safari in 2019.

Response to Added:

Will a cookie for .example.com be available for www.example.com? YES

Will a cookie for .example.com be available for example.com? YES

Will a cookie for example.com be available for www.example.com? NO, Domain without wildcard only matches itself.

Will a cookie for example.com be available for anotherexample.com? NO

Will www.example.com be able to set cookie for example.com? NO, it will be able to set cookie for '.example.com', but not 'example.com'.

Will www.example.com be able to set cookie for www2.example.com? NO. But it can set cookie for .example.com, which www2.example.com can access.

Will www.example.com be able to set cookie for .com? NO

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The RFCs are known not to reflect reality.

Better check draft-ietf-httpstate-cookie, work in progress.

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There are rules that determine whether a browser will accept the Set-header response header (server-side cookie writing), a slightly different rules/interpretations for cookie set using Javascript (I haven't tested VBScript).

Then there are rules that determine whether the browser will send a cookie along with the page request.

There are differences between the major browser engines how domain matches are handled, and how parameters in path values are interpreted. You can find some empirical evidence in the article How Different Browsers Handle Cookies Differently

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Will www.example.com be able to set cookie for .com?

No, but example.com.fr may be able to set a cookie for example2.com.fr. Firefox protects against this by maintaining a list of TLDs: http://securitylabs.websense.com/content/Blogs/3108.aspx

Apparently Internet Explorer doesn't allow two-letter domains to set cookies, which I suppose explains why o2.ie simply redirects to o2online.ie. I'd often wondered that.

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I was surprised to read section 3.3.2 about rejecting cookies:

https://www.rfc-editor.org/rfc/rfc2965

That says that a browser should reject a cookie from x.y.z.com with domain .z.com, because 'x.y' contains a dot. So, unless I am misinterpreting the RFC and/or the questions above, there could be questions added:

Will a cookie for .example.com be available for www.yyy.example.com? No.

Will a cookie set by origin server www.yyy.example.com, with domain .example.com, have it's value sent by the user agent to xxx.example.com? No.

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