7 Network Working Group D. Kristol
8 Request for Comments: 2109 Bell Laboratories, Lucent Technologies
9 Category: Standards Track L. Montulli
10 Netscape Communications
14 HTTP State Management Mechanism
18 This document specifies an Internet standards track protocol for the
19 Internet community, and requests discussion and suggestions for
20 improvements. Please refer to the current edition of the "Internet
21 Official Protocol Standards" (STD 1) for the standardization state
22 and status of this protocol. Distribution of this memo is unlimited.
26 This document specifies a way to create a stateful session with HTTP
27 requests and responses. It describes two new headers, Cookie and
28 Set-Cookie, which carry state information between participating
29 origin servers and user agents. The method described here differs
30 from Netscape's Cookie proposal, but it can interoperate with
31 HTTP/1.0 user agents that use Netscape's method. (See the HISTORICAL
36 The terms user agent, client, server, proxy, and origin server have
37 the same meaning as in the HTTP/1.0 specification.
39 Fully-qualified host name (FQHN) means either the fully-qualified
40 domain name (FQDN) of a host (i.e., a completely specified domain
41 name ending in a top-level domain such as .com or .uk), or the
42 numeric Internet Protocol (IP) address of a host. The fully
43 qualified domain name is preferred; use of numeric IP addresses is
46 The terms request-host and request-URI refer to the values the client
47 would send to the server as, respectively, the host (but not port)
48 and abs_path portions of the absoluteURI (http_URL) of the HTTP
49 request line. Note that request-host must be a FQHN.
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60 RFC 2109 HTTP State Management Mechanism February 1997
63 Hosts names can be specified either as an IP address or a FQHN
64 string. Sometimes we compare one host name with another. Host A's
65 name domain-matches host B's if
67 * both host names are IP addresses and their host name strings match
70 * both host names are FQDN strings and their host name strings match
73 * A is a FQDN string and has the form NB, where N is a non-empty name
74 string, B has the form .B', and B' is a FQDN string. (So, x.y.com
75 domain-matches .y.com but not y.com.)
77 Note that domain-match is not a commutative operation: a.b.c.com
78 domain-matches .c.com, but not the reverse.
80 Because it was used in Netscape's original implementation of state
81 management, we will use the term cookie to refer to the state
82 information that passes between an origin server and user agent, and
83 that gets stored by the user agent.
87 This document describes a way to create stateful sessions with HTTP
88 requests and responses. Currently, HTTP servers respond to each
89 client request without relating that request to previous or
90 subsequent requests; the technique allows clients and servers that
91 wish to exchange state information to place HTTP requests and
92 responses within a larger context, which we term a "session". This
93 context might be used to create, for example, a "shopping cart", in
94 which user selections can be aggregated before purchase, or a
95 magazine browsing system, in which a user's previous reading affects
96 which offerings are presented.
98 There are, of course, many different potential contexts and thus many
99 different potential types of session. The designers' paradigm for
100 sessions created by the exchange of cookies has these key attributes:
102 1. Each session has a beginning and an end.
104 2. Each session is relatively short-lived.
106 3. Either the user agent or the origin server may terminate a
109 4. The session is implicit in the exchange of state information.
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116 RFC 2109 HTTP State Management Mechanism February 1997
121 We outline here a way for an origin server to send state information
122 to the user agent, and for the user agent to return the state
123 information to the origin server. The goal is to have a minimal
124 impact on HTTP and user agents. Only origin servers that need to
125 maintain sessions would suffer any significant impact, and that
126 impact can largely be confined to Common Gateway Interface (CGI)
127 programs, unless the server provides more sophisticated state
128 management support. (See Implementation Considerations, below.)
132 The two state management headers, Set-Cookie and Cookie, have common
133 syntactic properties involving attribute-value pairs. The following
134 grammar uses the notation, and tokens DIGIT (decimal digits) and
135 token (informally, a sequence of non-special, non-white space
136 characters) from the HTTP/1.1 specification [RFC 2068] to describe
139 av-pairs = av-pair *(";" av-pair)
140 av-pair = attr ["=" value] ; optional value
143 word = token | quoted-string
145 Attributes (names) (attr) are case-insensitive. White space is
146 permitted between tokens. Note that while the above syntax
147 description shows value as optional, most attrs require them.
149 NOTE: The syntax above allows whitespace between the attribute and
152 4.2 Origin Server Role
156 The origin server initiates a session, if it so desires. (Note that
157 "session" here does not refer to a persistent network connection but
158 to a logical session created from HTTP requests and responses. The
159 presence or absence of a persistent connection should have no effect
160 on the use of cookie-derived sessions). To initiate a session, the
161 origin server returns an extra response header to the client, Set-
162 Cookie. (The details follow later.)
164 A user agent returns a Cookie request header (see below) to the
165 origin server if it chooses to continue a session. The origin server
166 may ignore it or use it to determine the current state of the
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172 RFC 2109 HTTP State Management Mechanism February 1997
175 session. It may send back to the client a Set-Cookie response header
176 with the same or different information, or it may send no Set-Cookie
177 header at all. The origin server effectively ends a session by
178 sending the client a Set-Cookie header with Max-Age=0.
180 Servers may return a Set-Cookie response headers with any response.
181 User agents should send Cookie request headers, subject to other
182 rules detailed below, with every request.
184 An origin server may include multiple Set-Cookie headers in a
185 response. Note that an intervening gateway could fold multiple such
186 headers into a single header.
188 4.2.2 Set-Cookie Syntax
190 The syntax for the Set-Cookie response header is
192 set-cookie = "Set-Cookie:" cookies
194 cookie = NAME "=" VALUE *(";" cookie-av)
197 cookie-av = "Comment" "=" value
199 | "Max-Age" "=" value
202 | "Version" "=" 1*DIGIT
204 Informally, the Set-Cookie response header comprises the token Set-
205 Cookie:, followed by a comma-separated list of one or more cookies.
206 Each cookie begins with a NAME=VALUE pair, followed by zero or more
207 semi-colon-separated attribute-value pairs. The syntax for
208 attribute-value pairs was shown earlier. The specific attributes and
209 the semantics of their values follows. The NAME=VALUE attribute-
210 value pair must come first in each cookie. The others, if present,
211 can occur in any order. If an attribute appears more than once in a
212 cookie, the behavior is undefined.
215 Required. The name of the state information ("cookie") is NAME,
216 and its value is VALUE. NAMEs that begin with $ are reserved for
217 other uses and must not be used by applications.
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228 RFC 2109 HTTP State Management Mechanism February 1997
231 The VALUE is opaque to the user agent and may be anything the
232 origin server chooses to send, possibly in a server-selected
233 printable ASCII encoding. "Opaque" implies that the content is of
234 interest and relevance only to the origin server. The content
235 may, in fact, be readable by anyone that examines the Set-Cookie
239 Optional. Because cookies can contain private information about a
240 user, the Cookie attribute allows an origin server to document its
241 intended use of a cookie. The user can inspect the information to
242 decide whether to initiate or continue a session with this cookie.
245 Optional. The Domain attribute specifies the domain for which the
246 cookie is valid. An explicitly specified domain must always start
249 Max-Age=delta-seconds
250 Optional. The Max-Age attribute defines the lifetime of the
251 cookie, in seconds. The delta-seconds value is a decimal non-
252 negative integer. After delta-seconds seconds elapse, the client
253 should discard the cookie. A value of zero means the cookie
254 should be discarded immediately.
257 Optional. The Path attribute specifies the subset of URLs to
258 which this cookie applies.
261 Optional. The Secure attribute (with no value) directs the user
262 agent to use only (unspecified) secure means to contact the origin
263 server whenever it sends back this cookie.
265 The user agent (possibly under the user's control) may determine
266 what level of security it considers appropriate for "secure"
267 cookies. The Secure attribute should be considered security
268 advice from the server to the user agent, indicating that it is in
269 the session's interest to protect the cookie contents.
272 Required. The Version attribute, a decimal integer, identifies to
273 which version of the state management specification the cookie
274 conforms. For this specification, Version=1 applies.
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287 4.2.3 Controlling Caching
289 An origin server must be cognizant of the effect of possible caching
290 of both the returned resource and the Set-Cookie header. Caching
291 "public" documents is desirable. For example, if the origin server
292 wants to use a public document such as a "front door" page as a
293 sentinel to indicate the beginning of a session for which a Set-
294 Cookie response header must be generated, the page should be stored
295 in caches "pre-expired" so that the origin server will see further
296 requests. "Private documents", for example those that contain
297 information strictly private to a session, should not be cached in
300 If the cookie is intended for use by a single user, the Set-cookie
301 header should not be cached. A Set-cookie header that is intended to
302 be shared by multiple users may be cached.
304 The origin server should send the following additional HTTP/1.1
305 response headers, depending on circumstances:
307 * To suppress caching of the Set-Cookie header: Cache-control: no-
310 and one of the following:
312 * To suppress caching of a private document in shared caches: Cache-
315 * To allow caching of a document and require that it be validated
316 before returning it to the client: Cache-control: must-revalidate.
318 * To allow caching of a document, but to require that proxy caches
319 (not user agent caches) validate it before returning it to the
320 client: Cache-control: proxy-revalidate.
322 * To allow caching of a document and request that it be validated
323 before returning it to the client (by "pre-expiring" it):
324 Cache-control: max-age=0. Not all caches will revalidate the
325 document in every case.
327 HTTP/1.1 servers must send Expires: old-date (where old-date is a
328 date long in the past) on responses containing Set-Cookie response
329 headers unless they know for certain (by out of band means) that
330 there are no downsteam HTTP/1.0 proxies. HTTP/1.1 servers may send
331 other Cache-Control directives that permit caching by HTTP/1.1
332 proxies in addition to the Expires: old-date directive; the Cache-
333 Control directive will override the Expires: old-date for HTTP/1.1
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340 RFC 2109 HTTP State Management Mechanism February 1997
345 4.3.1 Interpreting Set-Cookie
347 The user agent keeps separate track of state information that arrives
348 via Set-Cookie response headers from each origin server (as
349 distinguished by name or IP address and port). The user agent
350 applies these defaults for optional attributes that are missing:
352 VersionDefaults to "old cookie" behavior as originally specified by
353 Netscape. See the HISTORICAL section.
355 Domain Defaults to the request-host. (Note that there is no dot at
356 the beginning of request-host.)
358 Max-AgeThe default behavior is to discard the cookie when the user
361 Path Defaults to the path of the request URL that generated the
362 Set-Cookie response, up to, but not including, the
365 Secure If absent, the user agent may send the cookie over an
368 4.3.2 Rejecting Cookies
370 To prevent possible security or privacy violations, a user agent
371 rejects a cookie (shall not store its information) if any of the
374 * The value for the Path attribute is not a prefix of the request-
377 * The value for the Domain attribute contains no embedded dots or
378 does not start with a dot.
380 * The value for the request-host does not domain-match the Domain
383 * The request-host is a FQDN (not IP address) and has the form HD,
384 where D is the value of the Domain attribute, and H is a string
385 that contains one or more dots.
389 * A Set-Cookie from request-host y.x.foo.com for Domain=.foo.com
390 would be rejected, because H is y.x and contains a dot.
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399 * A Set-Cookie from request-host x.foo.com for Domain=.foo.com would
402 * A Set-Cookie with Domain=.com or Domain=.com., will always be
403 rejected, because there is no embedded dot.
405 * A Set-Cookie with Domain=ajax.com will be rejected because the
406 value for Domain does not begin with a dot.
408 4.3.3 Cookie Management
410 If a user agent receives a Set-Cookie response header whose NAME is
411 the same as a pre-existing cookie, and whose Domain and Path
412 attribute values exactly (string) match those of a pre-existing
413 cookie, the new cookie supersedes the old. However, if the Set-
414 Cookie has a value for Max-Age of zero, the (old and new) cookie is
415 discarded. Otherwise cookies accumulate until they expire (resources
416 permitting), at which time they are discarded.
418 Because user agents have finite space in which to store cookies, they
419 may also discard older cookies to make space for newer ones, using,
420 for example, a least-recently-used algorithm, along with constraints
421 on the maximum number of cookies that each origin server may set.
423 If a Set-Cookie response header includes a Comment attribute, the
424 user agent should store that information in a human-readable form
425 with the cookie and should display the comment text as part of a
426 cookie inspection user interface.
428 User agents should allow the user to control cookie destruction. An
429 infrequently-used cookie may function as a "preferences file" for
430 network applications, and a user may wish to keep it even if it is
431 the least-recently-used cookie. One possible implementation would be
432 an interface that allows the permanent storage of a cookie through a
433 checkbox (or, conversely, its immediate destruction).
435 Privacy considerations dictate that the user have considerable
436 control over cookie management. The PRIVACY section contains more
439 4.3.4 Sending Cookies to the Origin Server
441 When it sends a request to an origin server, the user agent sends a
442 Cookie request header to the origin server if it has cookies that are
443 applicable to the request, based on
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459 The syntax for the header is:
461 cookie = "Cookie:" cookie-version
462 1*((";" | ",") cookie-value)
463 cookie-value = NAME "=" VALUE [";" path] [";" domain]
464 cookie-version = "$Version" "=" value
467 path = "$Path" "=" value
468 domain = "$Domain" "=" value
470 The value of the cookie-version attribute must be the value from the
471 Version attribute, if any, of the corresponding Set-Cookie response
472 header. Otherwise the value for cookie-version is 0. The value for
473 the path attribute must be the value from the Path attribute, if any,
474 of the corresponding Set-Cookie response header. Otherwise the
475 attribute should be omitted from the Cookie request header. The
476 value for the domain attribute must be the value from the Domain
477 attribute, if any, of the corresponding Set-Cookie response header.
478 Otherwise the attribute should be omitted from the Cookie request
481 Note that there is no Comment attribute in the Cookie request header
482 corresponding to the one in the Set-Cookie response header. The user
483 agent does not return the comment information to the origin server.
485 The following rules apply to choosing applicable cookie-values from
486 among all the cookies the user agent has.
489 The origin server's fully-qualified host name must domain-match
490 the Domain attribute of the cookie.
493 The Path attribute of the cookie must match a prefix of the
497 Cookies that have expired should have been discarded and thus
498 are not forwarded to an origin server.
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511 If multiple cookies satisfy the criteria above, they are ordered in
512 the Cookie header such that those with more specific Path attributes
513 precede those with less specific. Ordering with respect to other
514 attributes (e.g., Domain) is unspecified.
516 Note: For backward compatibility, the separator in the Cookie header
517 is semi-colon (;) everywhere. A server should also accept comma (,)
518 as the separator between cookie-values for future compatibility.
520 4.3.5 Sending Cookies in Unverifiable Transactions
522 Users must have control over sessions in order to ensure privacy.
523 (See PRIVACY section below.) To simplify implementation and to
524 prevent an additional layer of complexity where adequate safeguards
525 exist, however, this document distinguishes between transactions that
526 are verifiable and those that are unverifiable. A transaction is
527 verifiable if the user has the option to review the request-URI prior
528 to its use in the transaction. A transaction is unverifiable if the
529 user does not have that option. Unverifiable transactions typically
530 arise when a user agent automatically requests inlined or embedded
531 entities or when it resolves redirection (3xx) responses from an
532 origin server. Typically the origin transaction, the transaction
533 that the user initiates, is verifiable, and that transaction may
534 directly or indirectly induce the user agent to make unverifiable
537 When it makes an unverifiable transaction, a user agent must enable a
538 session only if a cookie with a domain attribute D was sent or
539 received in its origin transaction, such that the host name in the
540 Request-URI of the unverifiable transaction domain-matches D.
542 This restriction prevents a malicious service author from using
543 unverifiable transactions to induce a user agent to start or continue
544 a session with a server in a different domain. The starting or
545 continuation of such sessions could be contrary to the privacy
546 expectations of the user, and could also be a security problem.
548 User agents may offer configurable options that allow the user agent,
549 or any autonomous programs that the user agent executes, to ignore
550 the above rule, so long as these override options default to "off".
552 Many current user agents already provide a review option that would
553 render many links verifiable. For instance, some user agents display
554 the URL that would be referenced for a particular link when the mouse
555 pointer is placed over that link. The user can therefore determine
556 whether to visit that site before causing the browser to do so.
557 (Though not implemented on current user agents, a similar technique
558 could be used for a button used to submit a form -- the user agent
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567 could display the action to be taken if the user were to select that
568 button.) However, even this would not make all links verifiable; for
569 example, links to automatically loaded images would not normally be
570 subject to "mouse pointer" verification.
572 Many user agents also provide the option for a user to view the HTML
573 source of a document, or to save the source to an external file where
574 it can be viewed by another application. While such an option does
575 provide a crude review mechanism, some users might not consider it
576 acceptable for this purpose.
578 4.4 How an Origin Server Interprets the Cookie Header
580 A user agent returns much of the information in the Set-Cookie header
581 to the origin server when the Path attribute matches that of a new
582 request. When it receives a Cookie header, the origin server should
583 treat cookies with NAMEs whose prefix is $ specially, as an attribute
584 for the adjacent cookie. The value for such a NAME is to be
585 interpreted as applying to the lexically (left-to-right) most recent
586 cookie whose name does not have the $ prefix. If there is no
587 previous cookie, the value applies to the cookie mechanism as a
588 whole. For example, consider the cookie
590 Cookie: $Version="1"; Customer="WILE_E_COYOTE";
593 $Version applies to the cookie mechanism as a whole (and gives the
594 version number for the cookie mechanism). $Path is an attribute
595 whose value (/acme) defines the Path attribute that was used when the
596 Customer cookie was defined in a Set-Cookie response header.
598 4.5 Caching Proxy Role
600 One reason for separating state information from both a URL and
601 document content is to facilitate the scaling that caching permits.
602 To support cookies, a caching proxy must obey these rules already in
603 the HTTP specification:
605 * Honor requests from the cache, if possible, based on cache validity
608 * Pass along a Cookie request header in any request that the proxy
609 must make of another server.
611 * Return the response to the client. Include any Set-Cookie response
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623 * Cache the received response subject to the control of the usual
624 headers, such as Expires, Cache-control: no-cache, and Cache-
627 * Cache the Set-Cookie subject to the control of the usual header,
628 Cache-control: no-cache="set-cookie". (The Set-Cookie header
629 should usually not be cached.)
631 Proxies must not introduce Set-Cookie (Cookie) headers of their own
632 in proxy responses (requests).
638 Most detail of request and response headers has been omitted. Assume
639 the user agent has no stored cookies.
641 1. User Agent -> Server
643 POST /acme/login HTTP/1.1
646 User identifies self via a form.
648 2. Server -> User Agent
651 Set-Cookie: Customer="WILE_E_COYOTE"; Version="1"; Path="/acme"
653 Cookie reflects user's identity.
655 3. User Agent -> Server
657 POST /acme/pickitem HTTP/1.1
658 Cookie: $Version="1"; Customer="WILE_E_COYOTE"; $Path="/acme"
661 User selects an item for "shopping basket."
663 4. Server -> User Agent
666 Set-Cookie: Part_Number="Rocket_Launcher_0001"; Version="1";
669 Shopping basket contains an item.
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679 5. User Agent -> Server
681 POST /acme/shipping HTTP/1.1
682 Cookie: $Version="1";
683 Customer="WILE_E_COYOTE"; $Path="/acme";
684 Part_Number="Rocket_Launcher_0001"; $Path="/acme"
687 User selects shipping method from form.
689 6. Server -> User Agent
692 Set-Cookie: Shipping="FedEx"; Version="1"; Path="/acme"
694 New cookie reflects shipping method.
696 7. User Agent -> Server
698 POST /acme/process HTTP/1.1
699 Cookie: $Version="1";
700 Customer="WILE_E_COYOTE"; $Path="/acme";
701 Part_Number="Rocket_Launcher_0001"; $Path="/acme";
702 Shipping="FedEx"; $Path="/acme"
705 User chooses to process order.
707 8. Server -> User Agent
711 Transaction is complete.
713 The user agent makes a series of requests on the origin server, after
714 each of which it receives a new cookie. All the cookies have the
715 same Path attribute and (default) domain. Because the request URLs
716 all have /acme as a prefix, and that matches the Path attribute, each
717 request contains all the cookies received so far.
721 This example illustrates the effect of the Path attribute. All
722 detail of request and response headers has been omitted. Assume the
723 user agent has no stored cookies.
725 Imagine the user agent has received, in response to earlier requests,
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735 Set-Cookie: Part_Number="Rocket_Launcher_0001"; Version="1";
740 Set-Cookie: Part_Number="Riding_Rocket_0023"; Version="1";
743 A subsequent request by the user agent to the (same) server for URLs
744 of the form /acme/ammo/... would include the following request
747 Cookie: $Version="1";
748 Part_Number="Riding_Rocket_0023"; $Path="/acme/ammo";
749 Part_Number="Rocket_Launcher_0001"; $Path="/acme"
751 Note that the NAME=VALUE pair for the cookie with the more specific
752 Path attribute, /acme/ammo, comes before the one with the less
753 specific Path attribute, /acme. Further note that the same cookie
754 name appears more than once.
756 A subsequent request by the user agent to the (same) server for a URL
757 of the form /acme/parts/ would include the following request header:
759 Cookie: $Version="1"; Part_Number="Rocket_Launcher_0001"; $Path="/acme"
761 Here, the second cookie's Path attribute /acme/ammo is not a prefix
762 of the request URL, /acme/parts/, so the cookie does not get
763 forwarded to the server.
765 6. IMPLEMENTATION CONSIDERATIONS
767 Here we speculate on likely or desirable details for an origin server
768 that implements state management.
770 6.1 Set-Cookie Content
772 An origin server's content should probably be divided into disjoint
773 application areas, some of which require the use of state
774 information. The application areas can be distinguished by their
775 request URLs. The Set-Cookie header can incorporate information
776 about the application areas by setting the Path attribute for each
779 The session information can obviously be clear or encoded text that
780 describes state. However, if it grows too large, it can become
781 unwieldy. Therefore, an implementor might choose for the session
782 information to be a key to a server-side resource. Of course, using
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791 a database creates some problems that this state management
792 specification was meant to avoid, namely:
794 1. keeping real state on the server side;
796 2. how and when to garbage-collect the database entry, in case the
797 user agent terminates the session by, for example, exiting.
801 Caching benefits the scalability of WWW. Therefore it is important
802 to reduce the number of documents that have state embedded in them
803 inherently. For example, if a shopping-basket-style application
804 always displays a user's current basket contents on each page, those
805 pages cannot be cached, because each user's basket's contents would
806 be different. On the other hand, if each page contains just a link
807 that allows the user to "Look at My Shopping Basket", the page can be
810 6.3 Implementation Limits
812 Practical user agent implementations have limits on the number and
813 size of cookies that they can store. In general, user agents' cookie
814 support should have no fixed limits. They should strive to store as
815 many frequently-used cookies as possible. Furthermore, general-use
816 user agents should provide each of the following minimum capabilities
817 individually, although not necessarily simultaneously:
819 * at least 300 cookies
821 * at least 4096 bytes per cookie (as measured by the size of the
822 characters that comprise the cookie non-terminal in the syntax
823 description of the Set-Cookie header)
825 * at least 20 cookies per unique host or domain name
827 User agents created for specific purposes or for limited-capacity
828 devices should provide at least 20 cookies of 4096 bytes, to ensure
829 that the user can interact with a session-based origin server.
831 The information in a Set-Cookie response header must be retained in
832 its entirety. If for some reason there is inadequate space to store
833 the cookie, it must be discarded, not truncated.
835 Applications should use as few and as small cookies as possible, and
836 they should cope gracefully with the loss of a cookie.
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847 6.3.1 Denial of Service Attacks
849 User agents may choose to set an upper bound on the number of cookies
850 to be stored from a given host or domain name or on the size of the
851 cookie information. Otherwise a malicious server could attempt to
852 flood a user agent with many cookies, or large cookies, on successive
853 responses, which would force out cookies the user agent had received
854 from other servers. However, the minima specified above should still
859 7.1 User Agent Control
861 An origin server could create a Set-Cookie header to track the path
862 of a user through the server. Users may object to this behavior as
863 an intrusive accumulation of information, even if their identity is
864 not evident. (Identity might become evident if a user subsequently
865 fills out a form that contains identifying information.) This state
866 management specification therefore requires that a user agent give
867 the user control over such a possible intrusion, although the
868 interface through which the user is given this control is left
869 unspecified. However, the control mechanisms provided shall at least
872 * to completely disable the sending and saving of cookies.
874 * to determine whether a stateful session is in progress.
876 * to control the saving of a cookie on the basis of the cookie's
879 Such control could be provided by, for example, mechanisms
881 * to notify the user when the user agent is about to send a cookie
882 to the origin server, offering the option not to begin a session.
884 * to display a visual indication that a stateful session is in
887 * to let the user decide which cookies, if any, should be saved
888 when the user concludes a window or user agent session.
890 * to let the user examine the contents of a cookie at any time.
892 A user agent usually begins execution with no remembered state
893 information. It should be possible to configure a user agent never
894 to send Cookie headers, in which case it can never sustain state with
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900 RFC 2109 HTTP State Management Mechanism February 1997
903 an origin server. (The user agent would then behave like one that is
904 unaware of how to handle Set-Cookie response headers.)
906 When the user agent terminates execution, it should let the user
907 discard all state information. Alternatively, the user agent may ask
908 the user whether state information should be retained; the default
909 should be "no". If the user chooses to retain state information, it
910 would be restored the next time the user agent runs.
912 NOTE: User agents should probably be cautious about using files to
913 store cookies long-term. If a user runs more than one instance of
914 the user agent, the cookies could be commingled or otherwise messed
919 The restrictions on the value of the Domain attribute, and the rules
920 concerning unverifiable transactions, are meant to reduce the ways
921 that cookies can "leak" to the "wrong" site. The intent is to
922 restrict cookies to one, or a closely related set of hosts.
923 Therefore a request-host is limited as to what values it can set for
924 Domain. We consider it acceptable for hosts host1.foo.com and
925 host2.foo.com to share cookies, but not a.com and b.com.
927 Similarly, a server can only set a Path for cookies that are related
930 8. SECURITY CONSIDERATIONS
934 The information in the Set-Cookie and Cookie headers is unprotected.
935 Two consequences are:
937 1. Any sensitive information that is conveyed in them is exposed
940 2. A malicious intermediary could alter the headers as they travel
941 in either direction, with unpredictable results.
943 These facts imply that information of a personal and/or financial
944 nature should only be sent over a secure channel. For less sensitive
945 information, or when the content of the header is a database key, an
946 origin server should be vigilant to prevent a bad Cookie value from
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956 RFC 2109 HTTP State Management Mechanism February 1997
961 Proper application design can avoid spoofing attacks from related
964 1. User agent makes request to victim.cracker.edu, gets back
965 cookie session_id="1234" and sets the default domain
968 2. User agent makes request to spoof.cracker.edu, gets back
969 cookie session-id="1111", with Domain=".cracker.edu".
971 3. User agent makes request to victim.cracker.edu again, and
974 Cookie: $Version="1";
976 session_id="1111"; $Domain=".cracker.edu"
978 The server at victim.cracker.edu should detect that the second
979 cookie was not one it originated by noticing that the Domain
980 attribute is not for itself and ignore it.
982 8.3 Unexpected Cookie Sharing
984 A user agent should make every attempt to prevent the sharing of
985 session information between hosts that are in different domains.
986 Embedded or inlined objects may cause particularly severe privacy
987 problems if they can be used to share cookies between disparate
988 hosts. For example, a malicious server could embed cookie
989 information for host a.com in a URI for a CGI on host b.com. User
990 agent implementors are strongly encouraged to prevent this sort of
991 exchange whenever possible.
993 9. OTHER, SIMILAR, PROPOSALS
995 Three other proposals have been made to accomplish similar goals.
996 This specification is an amalgam of Kristol's State-Info proposal and
997 Netscape's Cookie proposal.
999 Brian Behlendorf proposed a Session-ID header that would be user-
1000 agent-initiated and could be used by an origin server to track
1001 "clicktrails". It would not carry any origin-server-defined state,
1002 however. Phillip Hallam-Baker has proposed another client-defined
1003 session ID mechanism for similar purposes.
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1012 RFC 2109 HTTP State Management Mechanism February 1997
1015 While both session IDs and cookies can provide a way to sustain
1016 stateful sessions, their intended purpose is different, and,
1017 consequently, the privacy requirements for them are different. A
1018 user initiates session IDs to allow servers to track progress through
1019 them, or to distinguish multiple users on a shared machine. Cookies
1020 are server-initiated, so the cookie mechanism described here gives
1021 users control over something that would otherwise take place without
1022 the users' awareness. Furthermore, cookies convey rich, server-
1023 selected information, whereas session IDs comprise user-selected,
1028 10.1 Compatibility With Netscape's Implementation
1030 HTTP/1.0 clients and servers may use Set-Cookie and Cookie headers
1031 that reflect Netscape's original cookie proposal. These notes cover
1032 inter-operation between "old" and "new" cookies.
1034 10.1.1 Extended Cookie Header
1036 This proposal adds attribute-value pairs to the Cookie request header
1037 in a compatible way. An "old" client that receives a "new" cookie
1038 will ignore attributes it does not understand; it returns what it
1039 does understand to the origin server. A "new" client always sends
1040 cookies in the new form.
1042 An "old" server that receives a "new" cookie will see what it thinks
1043 are many cookies with names that begin with a $, and it will ignore
1044 them. (The "old" server expects these cookies to be separated by
1045 semi-colon, not comma.) A "new" server can detect cookies that have
1046 passed through an "old" client, because they lack a $Version
1049 10.1.2 Expires and Max-Age
1051 Netscape's original proposal defined an Expires header that took a
1052 date value in a fixed-length variant format in place of Max-Age:
1054 Wdy, DD-Mon-YY HH:MM:SS GMT
1056 Note that the Expires date format contains embedded spaces, and that
1057 "old" cookies did not have quotes around values. Clients that
1058 implement to this specification should be aware of "old" cookies and
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1068 RFC 2109 HTTP State Management Mechanism February 1997
1073 In Netscape's original proposal, the values in attribute-value pairs
1074 did not accept "-quoted strings. Origin servers should be cautious
1075 about sending values that require quotes unless they know the
1076 receiving user agent understands them (i.e., "new" cookies). A
1077 ("new") user agent should only use quotes around values in Cookie
1078 headers when the cookie's version(s) is (are) all compliant with this
1079 specification or later.
1081 In Netscape's original proposal, no whitespace was permitted around
1082 the = that separates attribute-value pairs. Therefore such
1083 whitespace should be used with caution in new implementations.
1085 10.2 Caching and HTTP/1.0
1087 Some caches, such as those conforming to HTTP/1.0, will inevitably
1088 cache the Set-Cookie header, because there was no mechanism to
1089 suppress caching of headers prior to HTTP/1.1. This caching can lead
1090 to security problems. Documents transmitted by an origin server
1091 along with Set-Cookie headers will usually either be uncachable, or
1092 will be "pre-expired". As long as caches obey instructions not to
1093 cache documents (following Expires: <a date in the past> or Pragma:
1094 no-cache (HTTP/1.0), or Cache-control: no-cache (HTTP/1.1))
1095 uncachable documents present no problem. However, pre-expired
1096 documents may be stored in caches. They require validation (a
1097 conditional GET) on each new request, but some cache operators loosen
1098 the rules for their caches, and sometimes serve expired documents
1099 without first validating them. This combination of factors can lead
1100 to cookies meant for one user later being sent to another user. The
1101 Set-Cookie header is stored in the cache, and, although the document
1102 is stale (expired), the cache returns the document in response to
1103 later requests, including cached headers.
1105 11. ACKNOWLEDGEMENTS
1107 This document really represents the collective efforts of the
1108 following people, in addition to the authors: Roy Fielding, Marc
1109 Hedlund, Ted Hardie, Koen Holtman, Shel Kaphan, Rohit Khare.
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1127 12. AUTHORS' ADDRESSES
1130 Bell Laboratories, Lucent Technologies
1131 600 Mountain Ave. Room 2A-227
1132 Murray Hill, NJ 07974
1134 Phone: (908) 582-2250
1136 EMail: dmk@bell-labs.com
1140 Netscape Communications Corp.
1141 501 E. Middlefield Rd.
1142 Mountain View, CA 94043
1144 Phone: (415) 528-2600
1145 EMail: montulli@netscape.com
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