I have two questions. I understand that if I specify the domain as .example.com (with the leading dot) in the cookie that all subdomains can share a cookie.
Can subdomain.example.com access a cookie created in example.com (without the www subdomain)?
Can example.com (without the www subdomain) access the cookie if created in subdomain.example.com?
If you set a cookie like this:
Set-Cookie: name=value
then the cookie will only apply to the request domain, and will only be sent for requests to the exact same domain, not any other subdomains. (See What is a host only cookie?)
Two different domains (e.g. example.com and subdomain.example.com, or sub1.example.com and sub2.example.com) can only share cookies if the domain attribute is present in the header:
Set-Cookie: name=value; domain=example.com
The domain attribute must "domain-match" the request URL for it to be valid, which basically means it must be the request domain or a super-domain. So this applies for both examples in the question, as well as sharing between two separate subdomains.
This cookie would then be sent for any subdomain of example.com, including nested subdomains like subsub.subdomain.example.com. (Bear in mind there are other attributes that could restrict the scope of the cookie and when it gets sent by the browser, like path or Secure).
Because of the way the domain-matching works, if you want sub1.example.com and sub2.example.com to share cookies, then you'll also share them with sub3.example.com.
See also:
www vs no-www and cookies
setcookie.net: a site where you can try it out (disclaimer: developed by me, for this question)
A note on leading dots in domain attributes: In the early RFC 2109, only domains with a leading dot (domain=.example.com) could be used across subdomains. But this could not be shared with the top-level domain, so what you ask was not possible in the older spec.
However, the newer specification RFC 6265 ignores any leading dot, meaning you can use the cookie on subdomains as well as the top-level domain.
Please everyone note that you can set a cookie from a subdomain on a domain.
(sent in the response for requesting subdomain.example.com)
Set-Cookie: name=value; Domain=example.com // GOOD
But you can't set a cookie from a domain on a subdomain.
(sent in the response for requesting example.com)
Set-Cookie: name=value; Domain=subdomain.example.com // Browser rejects cookie
Why?
According to the specifications, RFC 6265 section 5.3.6 Storage Model,
If the canonicalized request-host does not domain-match the domain-attribute: Ignore the cookie entirely and abort these steps.
and RFC 6265 section 5.1.3 Domain Matching,
Domain Matching
A string domain-matches a given domain string if at least one of the following conditions hold:
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.)
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).
So subdomain.example.com domain-matches example.com, but example.com does not domain-match subdomain.example.com
Check this answer also.
I'm not sure cmbuckley's answer is showing the full picture. What I read is:
Unless the cookie's attributes indicate otherwise, the cookie is
returned only to the origin server (and not, for example, to any
subdomains), and it expires at the end of the current session (as
defined by the user agent). User agents ignore unrecognized cookie.
RFC 6265
Also
8.6. Weak Integrity
Cookies do not provide integrity guarantees for sibling domains (and
their subdomains). For example, consider foo.example.com and
bar.example.com. The foo.example.com server can set a cookie with a
Domain attribute of "example.com" (possibly overwriting an existing
"example.com" cookie set by bar.example.com), and the user agent will
include that cookie in HTTP requests to bar.example.com. In the
worst case, bar.example.com will be unable to distinguish this cookie
from a cookie it set itself. The foo.example.com server might be
able to leverage this ability to mount an attack against
bar.example.com.
To me that means you can protect cookies from being read by subdomain/domain but cannot prevent writing cookies to the other domains. So somebody may rewrite your site cookies by controlling another subdomain visited by the same browser. Which might not be a big concern.
Awesome cookies test site provided by cmbuckley and for those that missed it in his answer like me; worth scrolling up.
Cookie Test
Here is an example using the DOM cookie API, so we can see the behavior for ourselves.
If we execute the following JavaScript code,
document.cookie = "key=value"
it appears to be the same as executing:
document.cookie = "key=value;domain=example.com"
The cookie key becomes available (only) on the domain example.com.
Now, if you execute the following JavaScript code on example.com,
document.cookie = "key=value;domain=.example.com"
the cookie key becomes available to example.com as well as subdomain.example.com.
Finally, if you were to try and execute the following on subdomain.example.com,
document.cookie = "key=value;domain=.example.com"
does the cookie key become available to subdomain.example.com? I was a bit surprised that this is allowed; I had assumed it would be a security violation for a subdomain to be able to set a cookie on a parent domain.
Be careful if you are working on localhost!
If you store your cookie in JavaScript like this:
document.cookie = "key=value;domain=localhost"
It might not be accessible to your subdomain, like sub.localhost. In order to solve this issue you need to use VirtualHost. For example, you can configure your virtual host with ServerName localhost.com, and then you will be able to store your cookie on your domain and subdomain like this:
document.cookie = "key=value;domain=localhost.com"
In both cases, yes, it can, and this is the default behaviour for both Internet Explorer and Edge.
The other answers add valuable insight, but they chiefly describe the behaviour in Chrome. It's important to note that the behaviour is completely different in Internet Explorer. CMBuckley's very helpful test script demonstrates that in (say) Chrome, the cookies are not shared between root and subdomains when no domain is specified.
However, the same test in Internet Explorer shows that they are shared. This Internet Explorer case is closer to the take-home description in CMBuckley's www-or-not-www link. I know this to be the case because we have a system that used different servicestack cookies on both the root and subdomain. It all worked fine until someone accessed it in Internet Explorer and the two systems fought over whose session cookie would win until we blew up the cache.
I do this and it works for me:
Cookie.set('token', 'some jwt-token', { expire:50000, domain: 'example.com' })
What determines the value of document.domain property on page? It seems like it's not always equal to the root url domain.
document.domain contains the complete domain (including subdomains) from which the page was downloaded. You can set it manually to a parent domain. E.g., you can set the domain of a document loaded from subdomain.example.com to example.com. <iframe>s may have a different domain than the domain in the url bar.
When setting cookie in javascript using document.cookie="dom_x=yyy;domain=www.mozilla.org;path=/", the cookie gets set in the domain www.mozilla.org.
At the same time while using document.cookie="dom_x_dot=yyy;domain:www.mozilla.org;path=/" the cookies get set in the domain .www.mozilla.org.
Is the syntax document.cookie="dom_x_dot=yyy;domain:www.mozilla.org;path=/" valid and if so when does it need to be used?
When you set a cookie domain that starts with a dot, like '.www.mozilla.org', it will be sent to that domain but also all subdomains of that domain like 'sub.www.mozilla.org'. Without the dot it will only be send to the exact domain 'www.mozilla.org'.
Are there any differences between external domains, and subdomains when using EasyXDM?
I am wondering about:
Security Issues
Cross Browser Support
Speed
Anything minor or major
I really can't find much information about this, so I figure that it must be the same?
To explain further which one is the best setup of these:
example.com <-> sub.example.com
example.com <-> sub.other.com
There is one thing that make this thing easier, I found document.domain:
There is one exception to the same origin rule. A script can set the
value of document.domain to a suffix of the current domain. If it does
so, the shorter domain is used for subsequent origin checks.
For example, assume a script in the document at
http://store.company.com/dir/other.html executes the following
statement: document.domain = "company.com";
After that statement executes, the
page would pass the origin check with
http://company.com/dir/page.html. However, by the same reasoning,
company.com could not set document.domain to othercompany.com.
Source: https://developer.mozilla.org/en-US/docs/Same_origin_policy_for_JavaScript
I have a javascript script that is being run from within an iframe that is trying to access the parent but I'm getting the following error:
Unsafe JavaScript attempt to access frame with URL mysite.com from frame with URL myothersite.com?. Domains, protocols and ports must match.
The iframe html is on a different domain but I didn't think that would matter. This is the code that is generating the JS error:
var parent_site = parent.document;
Is there a way around this?
If the parent domain is a trailing part of the iframe domain (i.e., iframe is child.parent.com and parent is parent.com) you can set the domain of the iframe document with document.domain = "parent.com" and avoid the problem.
If the domains of the parent and the iframe are unrelated there is no way to work around it.
Do some research on CORS (Cross-origin resource sharing): http://www.w3.org/TR/cors/
You essentially need to add this to your .htaccess of the parent domain:
Header set Access-Control-Allow-Origin *
Header set Access-Control-Allow-Headers x-requested-with
Ideally you'd replace the * with the domain(s) for which you want to allow access.