do cookies expire during a session?

Question

If I have a cookie set to expire at 7pm today and I start a session at 6.30pm a request made during that session but after 7pm will contain the cookie because it's deleted when the session ends or the browser will have already deleted it?

Answer 1

Yes, cookies can expire during a session and routinely do so. Cookies expire at their time of expiration regardless of it being in a session, or not in a session. If the server wants the cookie to last for the session it should reset the cookie to be a session cookie, or set an expiration time further in the future.

The browser should not be sending any cookies which have expired to the server. This was dealt with in RFC 2965 (October 2000) which states:

Max-Age=value
      OPTIONAL. The value of the Max-Age attribute is delta-seconds,
      the lifetime of the cookie in seconds, a decimal non-negative
      integer. To handle cached cookies correctly, a client SHOULD
      calculate the age of the cookie according to the age calculation
      rules in the HTTP/1.1 specification [RFC2616]. When the age is
      greater than delta-seconds seconds, the client SHOULD discard the
      cookie. A value of zero means the cookie SHOULD be discarded
      immediately.

[Emphasis added]

And says:

Cookies that have expired should have been discarded and thus are not forwarded to an origin server.

[Emphasis added]

This RFC was in place from October 2000 through April 2011 at which point, RFC 2965 (October 2000) was replaced by RFC 6265 (April 2011). RFC 6265 changed the requirement for removing expired cookies from "SHOULD" to "MUST". RFC 6265 says:

4.  Server Requirements
...
4.1.2.1.  The Expires Attribute

   The Expires attribute indicates the maximum lifetime of the cookie,
   represented as the date and time at which the cookie expires.  The
   user agent is not required to retain the cookie until the specified
   date has passed.  In fact, user agents often evict cookies due to
   memory pressure or privacy concerns.

4.1.2.2.  The Max-Age Attribute

   The Max-Age attribute indicates the maximum lifetime of the cookie,
   represented as the number of seconds until the cookie expires.  The
   user agent is not required to retain the cookie for the specified
   duration.  In fact, user agents often evict cookies due to memory
   pressure or privacy concerns.

      NOTE: Some existing user agents do not support the Max-Age
      attribute.  User agents that do not support the Max-Age attribute
      ignore the attribute.

   If a cookie has both the Max-Age and the Expires attribute, the Max-
   Age attribute has precedence and controls the expiration date of the
   cookie.  If a cookie has neither the Max-Age nor the Expires
   attribute, the user agent will retain the cookie until "the current
   session is over" (as defined by the user agent).
...
5.  User Agent Requirements
...
5.2.1.  The Expires Attribute

   If the attribute-name case-insensitively matches the string
   "Expires", the user agent MUST process the cookie-av as follows.

   Let the expiry-time be the result of parsing the attribute-value as
   cookie-date (see Section 5.1.1).

   If the attribute-value failed to parse as a cookie date, ignore the
   cookie-av.

   If the expiry-time is later than the last date the user agent can
   represent, the user agent MAY replace the expiry-time with the last
   representable date.
   If the expiry-time is earlier than the earliest date the user agent
   can represent, the user agent MAY replace the expiry-time with the
   earliest representable date.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Expires and an attribute-value of expiry-time.

5.2.2.  The Max-Age Attribute

   If the attribute-name case-insensitively matches the string "Max-
   Age", the user agent MUST process the cookie-av as follows.

   If the first character of the attribute-value is not a DIGIT or a "-"
   character, ignore the cookie-av.

   If the remainder of attribute-value contains a non-DIGIT character,
   ignore the cookie-av.

   Let delta-seconds be the attribute-value converted to an integer.

   If delta-seconds is less than or equal to zero (0), let expiry-time
   be the earliest representable date and time.  Otherwise, let the
   expiry-time be the current date and time plus delta-seconds seconds.

   Append an attribute to the cookie-attribute-list with an attribute-
   name of Max-Age and an attribute-value of expiry-time.
...
5.3.  Storage Model
...
   A cookie is "expired" if the cookie has an expiry date in the past.

   The user agent MUST evict all expired cookies from the cookie store
   if, at any time, an expired cookie exists in the cookie store.

   At any time, the user agent MAY "remove excess cookies" from the
   cookie store if the number of cookies sharing a domain field exceeds
   some implementation-defined upper bound (such as 50 cookies).

   At any time, the user agent MAY "remove excess cookies" from the
   cookie store if the cookie store exceeds some predetermined upper
   bound (such as 3000 cookies).

   When the user agent removes excess cookies from the cookie store, the
   user agent MUST evict cookies in the following priority order:

   1.  Expired cookies.

   2.  Cookies that share a domain field with more than a predetermined
       number of other cookies.

   3.  All cookies.

   If two cookies have the same removal priority, the user agent MUST
   evict the cookie with the earliest last-access date first.

   When "the current session is over" (as defined by the user agent),
   the user agent MUST remove from the cookie store all cookies with the
   persistent-flag set to false.