Share cookie between subdomain and domain

Question

I have two questions  I understand that if I specify the domain as  mydomain com  with the leading dot  in the cookie that all subdomains can share a cookie   Can subdomain mydomain com access a cookie created in mydomain com  without the www subdomain     Can mydomain com  without the www subdomain  access the cookie if created in subdomain mydomain com

User · Accepted Answer

The 2 domains mydomain com and subdomain mydomain com can only share cookies if the domain is explicitly named in the Set-Cookie header  Otherwise  the scope of the cookie is restricted to the request host   This is referred to as a  host-only cookie   See What is a host only cookie    For instance  if you sent the following header from subdomain mydomain com  then the cookie won t be sent for requests to mydomain com   Set-Cookie  name value   However if you use the following  it will be usable on both domains   Set-Cookie  name value  domain mydomain com   This cookie will be sent for any subdomain of mydomain com  including nested subdomains like subsub subdomain mydomain com   In RFC 2109  a domain without a leading dot meant that it could not be used on subdomains  and only a leading dot   mydomain com  would allow it to be used across multiple subdomains  but not the top-level domain  so what you ask was not possible in the older spec    However  all modern browsers respect the newer specification RFC 6265  and will ignore any leading dot  meaning you can use the cookie on subdomains as well as the top-level domain   In summary  if you set a cookie like the second example above from mydomain com  it would be accessible by subdomain mydomain com  and vice versa  This can also be used to allow sub1 mydomain com and sub2 mydomain com to share cookies   See also    www vs no-www and cookies cookies test script to try it out

User · Answer

Please everyone note that you can set a cookie from a subdomain on a domain    sent in the response for requesting subdomain mydomain com   Set-Cookie  name value  Domain mydomain com    GOOD   But you CAN T set a cookie from a domain on a subdomain    sent in the response for requesting mydomain com   Set-Cookie  name value  Domain subdomain mydomain 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 mydomain com  domain-matches  mydomain com   but  mydomain com  does NOT domain-match  subdomain mydomain com   Check this answer also

User · Answer

Be careful if you are working on localhost   If you store your cookie in js 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 Virtual Host  For exemple you can configure your virtual host with ServerName localhost com then you will be able to store your cookie on your domain and subdomain like this   document cookie    key value domain localhost com

User · Answer

Here is an example using the DOM cookie API  https   developer mozilla org en-US docs Web API Document cookie   so we can see for ourselves the behavior   If we execute the following JavaScript      document cookie    key value    It appears to be the same as executing      document cookie    key value domain mydomain com    The cookie key becomes available  only  on the domain mydomain com     Now  if you execute the following JavaScript on mydomain com      document cookie    key value domain  mydomain com    The cookie key becomes available to mydomain com as well as subdomain mydomain com     Finally  if you were to try and execute the following on subdomain mydomain com      document cookie    key value domain  mydomain com    Does the cookie key become available to subdomain mydomain 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

User · Answer

I m not sure  cmbuckley 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  for those that missed it in his answer like me  worth scrolling up and upvoting     http   scripts cmbuckley co uk cookies php

User · Answer

In both cases yes it can  and this is the default behaviour for both IE and Edge   The other answers add valuable insight but chiefly describe the behaviour in Chrome  it s important to note that the behaviour is completely different in IE  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 IE shows that they are shared  This IE 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 IE and the two systems fought over whose session cookie would win until we blew up the cache

User · Answer

Simple solution  setcookie  NAME    VALUE   time   3600       EXAMPLE COM     Setcookie s 5th parameter determines the  sub domains that the cookie is available to   Setting it to  EXAMPLE COM  makes it available to any subdomain  eg  SUBDOMAIN EXAMPLE COM    Reference  http   php net manual en function setcookie php

[http] Share cookie between subdomain and domain

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