RFC2109 - HTTP State Management Mechanism

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1.  ABSTRACT   This document specifies a way to create a stateful session with HTTP   requests and responses.  It describes two new headers, Cookie and   Set-Cookie, which carry state information between participating   origin servers and user agents.  The method described here differs   from Netscape's Cookie proposal, but it can interoperate with   HTTP/1.0 user agents that use Netscape's method.  (See the HISTORICAL   section.)2.  TERMINOLOGY   The terms user agent, client, server, proxy, and origin server have   the same meaning as in the HTTP/1.0 specification.   Fully-qualified host name (FQHN) means either the fully-qualified   domain name (FQDN) of a host (i.e., a completely specified domain   name ending in a top-level domain such as .com or .uk), or the   numeric Internet Protocol (IP) address of a host.  The fully   qualified domain name is preferred; use of numeric IP addresses is   strongly discouraged.   The terms request-host and request-URI refer to the values the client   would send to the server as, respectively, the host (but not port)   and abs_path portions of the absoluteURI (http_URL) of the HTTP   request line.  Note that request-host must be a FQHN.   Hosts names can be specified either as an IP address or a FQHN   string.  Sometimes we compare one host name with another.  Host A's   name domain-matches host B's if   * both host names are IP addresses and their host name strings match     exactly; or   * both host names are FQDN strings and their host name strings match     exactly; or   * A is a FQDN string and has the form. NB, where N is a non-empty name     string, B has the form. .B', and B' is a FQDN string.  (So, x.y.com     domain-matches .y.com but not y.com.)   Note that domain-match is not a commutative operation: a.b.c.com   domain-matches .c.com, but not the reverse.   Because it was used in Netscape's original implementation of state   management, we will use the term cookie to refer to the state   information that passes between an origin server and user agent, and   that gets stored by the user agent.3.  STATE AND SESSIONS   This document describes a way to create stateful sessions with HTTP   requests and responses.  Currently, HTTP servers respond to each   client request without relating that request to previous or   subsequent requests; the technique allows clients and servers that   wish to exchange state information to place HTTP requests and   responses within a larger context, which we term a "session".  This   context might be used to create, for example, a "shopping cart", in   which user selections can be aggregated before purchase, or a   magazine browsing system, in which a user's previous reading affects   which offerings are presented.   There are, of course, many different potential contexts and thus many   different potential types of session.  The designers' paradigm for   sessions created by the exchange of cookies has these key attributes:      1.  Each session has a beginning and an end.      2.  Each session is relatively short-lived.      3.  Either the user agent or the origin server may terminate a          session.      4.  The session is implicit in the exchange of state information.4.  OUTLINE   We outline here a way for an origin server to send state information   to the user agent, and for the user agent to return the state   information to the origin server.  The goal is to have a minimal   impact on HTTP and user agents.  Only origin servers that need to   maintain sessions would suffer any significant impact, and that   impact can largely be confined to Common Gateway Interface (CGI)   programs, unless the server provides more sophisticated state   management support.  (See Implementation Considerations, below.)4.1  Syntax:  General   The two state management headers, Set-Cookie and Cookie, have common   syntactic properties involving attribute-value pairs.  The following   grammar uses the notation, and tokens DIGIT (decimal digits) and   token (informally, a sequence of non-special, non-white space   characters) from the HTTP/1.1 specification [RFC 2068] to describe   their syntax.   av-pairs        =       av-pair *(";" av-pair)   av-pair         =       attr ["=" value]        ; optional value   attr            =       token   value           =       word   word            =       token | quoted-string   Attributes (names) (attr) are case-insensitive.  White space is   permitted between tokens.  Note that while the above syntax   description shows value as optional, most attrs require them.   NOTE: The syntax above allows whitespace between the attribute and   the = sign.4.2  Origin Server Role4.2.1  General   The origin server initiates a session, if it so desires.  (Note that   "session" here does not refer to a persistent network connection but   to a logical session created from HTTP requests and responses.  The   presence or absence of a persistent connection should have no effect   on the use of cookie-derived sessions).  To initiate a session, the   origin server returns an extra response header to the client, Set-   Cookie.  (The details follow later.)   A user agent returns a Cookie request header (see below) to the   origin server if it chooses to continue a session.  The origin server   may ignore it or use it to determine the current state of the   session.  It may send back to the client a Set-Cookie response header   with the same or different information, or it may send no Set-Cookie   header at all.  The origin server effectively ends a session by   sending the client a Set-Cookie header with Max-Age=0.   Servers may return a Set-Cookie response headers with any response.   User agents should send Cookie request headers, subject to other   rules detailed below, with every request.   An origin server may include multiple Set-Cookie headers in a   response.  Note that an intervening gateway could fold multiple such   headers into a single header.4.2.2  Set-Cookie Syntax   The syntax for the Set-Cookie response header is   set-cookie      =       "Set-Cookie:" cookies   cookies         =       1#cookie   cookie          =       NAME "=" VALUE *(";" cookie-av)   NAME            =       attr   VALUE           =       value   cookie-av       =       "Comment" "=" value                   |       "Domain" "=" value                   |       "Max-Age" "=" value                   |       "Path" "=" value                   |       "Secure"                   |       "Version" "=" 1*DIGIT   Informally, the Set-Cookie response header comprises the token Set-   Cookie:, followed by a comma-separated list of one or more cookies.   Each cookie begins with a NAME=VALUE pair, followed by zero or more   semi-colon-separated attribute-value pairs.  The syntax for   attribute-value pairs was shown earlier.  The specific attributes and   the semantics of their values follows.  The NAME=VALUE attribute-   value pair must come first in each cookie.  The others, if present,   can occur in any order.  If an attribute appears more than once in a   cookie, the behavior. is undefined.   NAME=VALUE      Required.  The name of the state information ("cookie") is NAME,      and its value is VALUE.  NAMEs that begin with $ are reserved for      other uses and must not be used by applications.      The VALUE is opaque to the user agent and may be anything the      origin server chooses to send, possibly in a server-selected      printable ASCII encoding. "Opaque" implies that the content is of      interest and relevance only to the origin server.  The content      may, in fact, be readable by anyone that examines the Set-Cookie      header.   Comment=comment      Optional.  Because cookies can contain private information about a      user, the Cookie attribute allows an origin server to document its      intended use of a cookie.  The user can inspect the information to      decide whether to initiate or continue a session with this cookie.   Domain=domain      Optional.  The Domain attribute specifies the domain for which the      cookie is valid.  An explicitly specified domain must always start      with a dot.   Max-Age=delta-seconds      Optional.  The Max-Age attribute defines the lifetime of the      cookie, in seconds.  The delta-seconds value is a decimal non-      negative integer.  After delta-seconds seconds elapse, the client      should discard the cookie.  A value of zero means the cookie      should be discarded immediately.   Path=path      Optional.  The Path attribute specifies the subset of URLs to      which this cookie applies.   Secure      Optional.  The Secure attribute (with no value) directs the user      agent to use only (unspecified) secure means to contact the origin      server whenever it sends back this cookie.      The user agent (possibly under the user's control) may determine      what level of security it considers appropriate for "secure"      cookies.  The Secure attribute should be considered security      advice from the server to the user agent, indicating that it is in      the session's interest to protect the cookie contents.   Version=version      Required.  The Version attribute, a decimal integer, identifies to      which version of the state management specification the cookie      conforms.  For this specification, Version=1 applies.4.2.3  Controlling Caching   An origin server must be cognizant of the effect of possible caching   of both the returned resource and the Set-Cookie header.  Caching   "public" documents is desirable.  For example, if the origin server   wants to use a public document such as a "front door" page as a   sentinel to indicate the beginning of a session for which a Set-   Cookie response header must be generated, the page should be stored   in caches "pre-expired" so that the origin server will see further   requests.  "Private documents", for example those that contain   information strictly private to a session, should not be cached in   shared caches.   If the cookie is intended for use by a single user, the Set-cookie   header should not be cached.  A Set-cookie header that is intended to   be shared by multiple users may be cached.   The origin server should send the following additional HTTP/1.1   response headers, depending on circumstances:   * To suppress caching of the Set-Cookie header: Cache-control: no-     cache="set-cookie".   and one of the following:   * To suppress caching of a private document in shared caches: Cache-     control: private.   * To allow caching of a document and require that it be validated     before returning it to the client: Cache-control: must-revalidate.   * To allow caching of a document, but to require that proxy caches     (not user agent caches) validate it before returning it to the     client: Cache-control: proxy-revalidate.   * To allow caching of a document and request that it be validated     before returning it to the client (by "pre-expiring" it):     Cache-control: max-age=0.  Not all caches will revalidate the     document in every case.   HTTP/1.1 servers must send Expires: old-date (where old-date is a   date long in the past) on responses containing Set-Cookie response   headers unless they know for certain (by out of band means) that   there are no downsteam HTTP/1.0 proxies.  HTTP/1.1 servers may send   other Cache-Control directives that permit caching by HTTP/1.1   proxies in addition to the Expires: old-date directive; the Cache-   Control directive will override the Expires: old-date for HTTP/1.1   proxies.4.3  User Agent Role4.3.1  Interpreting Set-Cookie   The user agent keeps separate track of state information that arrives   via Set-Cookie response headers from each origin server (as   distinguished by name or IP address and port).  The user agent   applies these defaults for optional attributes that are missing:   VersionDefaults to "old cookie" behavior. as originally specified by          Netscape.  See the HISTORICAL section.   Domain Defaults to the request-host.  (Note that there is no dot at          the beginning of request-host.)   Max-AgeThe default behavior. is to discard the cookie when the user          agent exits.   Path   Defaults to the path of the request URL that generated the          Set-Cookie response, up to, but not including, the          right-most /.   Secure If absent, the user agent may send the cookie over an          insecure channel.4.3.2  Rejecting Cookies   To prevent possible security or privacy violations, a user agent   rejects a cookie (shall not store its information) if any of the   following is true:   * The value for the Path attribute is not a prefix of the request-     URI.   * The value for the Domain attribute contains no embedded dots or     does not start with a dot.   * The value for the request-host does not domain-match the Domain     attribute.   * The request-host is a FQDN (not IP address) and has the form. HD,     where D is the value of the Domain attribute, and H is a string     that contains one or more dots.   Examples:   * A Set-Cookie from request-host y.x.foo.com for Domain=.foo.com     would be rejected, because H is y.x and contains a dot.   * A Set-Cookie from request-host x.foo.com for Domain=.foo.com would     be accepted.   * A Set-Cookie with Domain=.com or Domain=.com., will always be     rejected, because there is no embedded dot.   * A Set-Cookie with Domain=ajax.com will be rejected because the     value for Domain does not begin with a dot.4.3.3  Cookie Management   If a user agent receives a Set-Cookie response header whose NAME is   the same as a pre-existing cookie, and whose Domain and Path   attribute values exactly (string) match those of a pre-existing   cookie, the new cookie supersedes the old.  However, if the Set-   Cookie has a value for Max-Age of zero, the (old and new) cookie is   discarded.  Otherwise cookies accumulate until they expire (resources   permitting), at which time they are discarded.   Because user agents have finite space in which to store cookies, they   may also discard older cookies to make space for newer ones, using,   for example, a least-recently-used algorithm, along with constraints   on the maximum number of cookies that each origin server may set.   If a Set-Cookie response header includes a Comment attribute, the   user agent should store that information in a human-readable form. with the cookie and should display the comment text as part of a   cookie inspection user interface.   User agents should allow the user to control cookie destruction.  An   infrequently-used cookie may function as a "preferences file" for   network applications, and a user may wish to keep it even if it is   the least-recently-used cookie.  One possible implementation would be   an interface that allows the permanent storage of a cookie through a   checkbox (or, conversely, its immediate destruction).   Privacy considerations dictate that the user have considerable   control over cookie management.  The PRIVACY section contains more   information.4.3.4  Sending Cookies to the Origin Server   When it sends a request to an origin server, the user agent sends a   Cookie request header to the origin server if it has cookies that are   applicable to the request, based on   * the request-host;   * the request-URI;   * the cookie's age.   The syntax for the header is:   cookie          =       "Cookie:" cookie-version                           1*((";" | ",") cookie-value)   cookie-value    =       NAME "=" VALUE [";" path] [";" domain]   cookie-version  =       "$Version" "=" value   NAME            =       attr   VALUE           =       value   path            =       "$Path" "=" value   domain          =       "$Domain" "=" value   The value of the cookie-version attribute must be the value from the   Version attribute, if any, of the corresponding Set-Cookie response   header.  Otherwise the value for cookie-version is 0.  The value for   the path attribute must be the value from the Path attribute, if any,   of the corresponding Set-Cookie response header.  Otherwise the   attribute should be omitted from the Cookie request header.  The   value for the domain attribute must be the value from the Domain   attribute, if any, of the corresponding Set-Cookie response header.   Otherwise the attribute should be omitted from the Cookie request   header.   Note that there is no Comment attribute in the Cookie request header   corresponding to the one in the Set-Cookie response header.  The user   agent does not return the comment information to the origin server.   The following rules apply to choosing applicable cookie-values from   among all the cookies the user agent has.   Domain Selection        The origin server's fully-qualified host name must domain-match        the Domain attribute of the cookie.   Path Selection        The Path attribute of the cookie must match a prefix of the        request-URI.   Max-Age Selection        Cookies that have expired should have been discarded and thus        are not forwarded to an origin server.   If multiple cookies satisfy the criteria above, they are ordered in   the Cookie header such that those with more specific Path attributes   precede those with less specific.  Ordering with respect to other   attributes (e.g., Domain) is unspecified.   Note: For backward compatibility, the separator in the Cookie header   is semi-colon (;) everywhere.  A server should also accept comma (,)   as the separator between cookie-values for future compatibility.4.3.5  Sending Cookies in Unverifiable Transactions   Users must have control over sessions in order to ensure privacy.   (See PRIVACY section below.)  To simplify implementation and to   prevent an additional layer of complexity where adequate safeguards   exist, however, this document distinguishes between transactions that   are verifiable and those that are unverifiable.  A transaction is   verifiable if the user has the option to review the request-URI prior   to its use in the transaction.  A transaction is unverifiable if the   user does not have that option.  Unverifiable transactions typically   arise when a user agent automatically requests inlined or embedded   entities or when it resolves redirection (3xx) responses from an   origin server.  Typically the origin transaction, the transaction   that the user initiates, is verifiable, and that transaction may   directly or indirectly induce the user agent to make unverifiable   transactions.   When it makes an unverifiable transaction, a user agent must enable a   session only if a cookie with a domain attribute D was sent or   received in its origin transaction, such that the host name in the   Request-URI of the unverifiable transaction domain-matches D.   This restriction prevents a malicious service author from using   unverifiable transactions to induce a user agent to start or continue   a session with a server in a different domain.  The starting or   continuation of such sessions could be contrary to the privacy   expectations of the user, and could also be a security problem.   User agents may offer configurable options that allow the user agent,   or any autonomous programs that the user agent executes, to ignore   the above rule, so long as these override options default to "off".   Many current user agents already provide a review option that would   render many links verifiable.  For instance, some user agents display   the URL that would be referenced for a particular link when the mouse   pointer is placed over that link.  The user can therefore determine   whether to visit that site before causing the browser to do so.   (Though not implemented on current user agents, a similar technique   could be used for a button used to submit a form. -- the user agent   could display the action to be taken if the user were to select that   button.) However, even this would not make all links verifiable; for   example, links to automatically loaded images would not normally be   subject to "mouse pointer" verification.   Many user agents also provide the option for a user to view the HTML   source of a document, or to save the source to an external file where   it can be viewed by another application.  While such an option does   provide a crude review mechanism, some users might not consider it   acceptable for this purpose.4.4  How an Origin Server Interprets the Cookie Header   A user agent returns much of the information in the Set-Cookie header   to the origin server when the Path attribute matches that of a new   request.  When it receives a Cookie header, the origin server should   treat cookies with NAMEs whose prefix is $ specially, as an attribute   for the adjacent cookie.  The value for such a NAME is to be   interpreted as applying to the lexically (left-to-right) most recent   cookie whose name does not have the $ prefix.  If there is no   previous cookie, the value applies to the cookie mechanism as a   whole.  For example, consider the cookie   Cookie: $Version="1"; Customer="WILE_E_COYOTE";           $Path="/acme"   $Version applies to the cookie mechanism as a whole (and gives the   version number for the cookie mechanism).  $Path is an attribute   whose value (/acme) defines the Path attribute that was used when the   Customer cookie was defined in a Set-Cookie response header.4.5  Caching Proxy Role   One reason for separating state information from both a URL and   document content is to facilitate the scaling that caching permits.   To support cookies, a caching proxy must obey these rules already in   the HTTP specification:   * Honor requests from the cache, if possible, based on cache validity     rules.   * Pass along a Cookie request header in any request that the proxy     must make of another server.   * Return the response to the client.  Include any Set-Cookie response     header.   * Cache the received response subject to the control of the usual     headers, such as Expires, Cache-control: no-cache, and Cache-     control: private,   * Cache the Set-Cookie subject to the control of the usual header,     Cache-control: no-cache="set-cookie".  (The Set-Cookie header     should usually not be cached.)   Proxies must not introduce Set-Cookie (Cookie) headers of their own   in proxy responses (requests).5.  EXAMPLES5.1  Example 1   Most detail of request and response headers has been omitted.  Assume   the user agent has no stored cookies.     1.  User Agent -> Server         POST /acme/login HTTP/1.1         [form. data]         User identifies self via a form.     2.  Server -> User Agent         HTTP/1.1 200 OK         Set-Cookie: Customer="WILE_E_COYOTE"; Version="1"; Path="/acme"         Cookie reflects user's identity.     3.  User Agent -> Server         POST /acme/pickitem HTTP/1.1         Cookie: $Version="1"; Customer="WILE_E_COYOTE"; $Path="/acme"         [form. data]         User selects an item for "shopping basket."     4.  Server -> User Agent         HTTP/1.1 200 OK         Set-Cookie: Part_Number="Rocket_Launcher_0001"; Version="1";                 Path="/acme"         Shopping basket contains an item.     5.  User Agent -> Server         POST /acme/shipping HTTP/1.1         Cookie: $Version="1";                 Customer="WILE_E_COYOTE"; $Path="/acme";                 Part_Number="Rocket_Launcher_0001"; $Path="/acme"         [form. data]         User selects shipping method from form.     6.  Server -> User Agent         HTTP/1.1 200 OK         Set-Cookie: Shipping="FedEx"; Version="1"; Path="/acme"         New cookie reflects shipping method.     7.  User Agent -> Server         POST /acme/process HTTP/1.1         Cookie: $Version="1";                 Customer="WILE_E_COYOTE"; $Path="/acme";                 Part_Number="Rocket_Launcher_0001"; $Path="/acme";                 Shipping="FedEx"; $Path="/acme"         [form. data]         User chooses to process order.     8.  Server -> User Agent         HTTP/1.1 200 OK         Transaction is complete.   The user agent makes a series of requests on the origin server, after   each of which it receives a new cookie.  All the cookies have the   same Path attribute and (default) domain.  Because the request URLs   all have /acme as a prefix, and that matches the Path attribute, each   request contains all the cookies received so far.5.2  Example 2   This example illustrates the effect of the Path attribute.  All   detail of request and response headers has been omitted.  Assume the   user agent has no stored cookies.   Imagine the user agent has received, in response to earlier requests,   the response headers   Set-Cookie: Part_Number="Rocket_Launcher_0001"; Version="1";           Path="/acme"   and   Set-Cookie: Part_Number="Riding_Rocket_0023"; Version="1";           Path="/acme/ammo"   A subsequent request by the user agent to the (same) server for URLs   of the form. /acme/ammo/...  would include the following request   header:   Cookie: $Version="1";           Part_Number="Riding_Rocket_0023"; $Path="/acme/ammo";           Part_Number="Rocket_Launcher_0001"; $Path="/acme"   Note that the NAME=VALUE pair for the cookie with the more specific   Path attribute, /acme/ammo, comes before the one with the less   specific Path attribute, /acme.  Further note that the same cookie   name appears more than once.   A subsequent request by the user agent to the (same) server for a URL   of the form. /acme/parts/ would include the following request header:   Cookie: $Version="1"; Part_Number="Rocket_Launcher_0001"; $Path="/acme"   Here, the second cookie's Path attribute /acme/ammo is not a prefix   of the request URL, /acme/parts/, so the cookie does not get   forwarded to the server.6.  IMPLEMENTATION CONSIDERATIONS   Here we speculate on likely or desirable details for an origin server   that implements state management.6.1  Set-Cookie Content   An origin server's content should probably be divided into disjoint   application areas, some of which require the use of state   information.  The application areas can be distinguished by their   request URLs.  The Set-Cookie header can incorporate information   about the application areas by setting the Path attribute for each   one.   The session information can obviously be clear or encoded text that   describes state.  However, if it grows too large, it can become   unwieldy.  Therefore, an implementor might choose for the session   information to be a key to a server-side resource.  Of course, using   a database creates some problems that this state management   specification was meant to avoid, namely:     1.  keeping real state on the server side;     2.  how and when to garbage-collect the database entry, in case the         user agent terminates the session by, for example, exiting.6.2  Stateless Pages   Caching benefits the scalability of WWW.  Therefore it is important   to reduce the number of documents that have state embedded in them   inherently.  For example, if a shopping-basket-style. application   always displays a user's current basket contents on each page, those   pages cannot be cached, because each user's basket's contents would   be different.  On the other hand, if each page contains just a link   that allows the user to "Look at My Shopping Basket", the page can be   cached.6.3  Implementation Limits   Practical user agent implementations have limits on the number and   size of cookies that they can store.  In general, user agents' cookie   support should have no fixed limits.  They should strive to store as   many frequently-used cookies as possible.  Furthermore, general-use   user agents should provide each of the following minimum capabilities   individually, although not necessarily simultaneously:      * at least 300 cookies      * at least 4096 bytes per cookie (as measured by the size of the        characters that comprise the cookie non-terminal in the syntax        description of the Set-Cookie header)      * at least 20 cookies per unique host or domain name   User agents created for specific purposes or for limited-capacity   devices should provide at least 20 cookies of 4096 bytes, to ensure   that the user can interact with a session-based origin server.   The information in a Set-Cookie response header must be retained in   its entirety.  If for some reason there is inadequate space to store   the cookie, it must be discarded, not truncated.   Applications should use as few and as small cookies as possible, and   they should cope gracefully with the loss of a cookie.6.3.1  Denial of Service Attacks   User agents may choose to set an upper bound on the number of cookies   to be stored from a given host or domain name or on the size of the   cookie information.  Otherwise a malicious server could attempt to   flood a user agent with many cookies, or large cookies, on successive   responses, which would force out cookies the user agent had received   from other servers.  However, the minima specified above should still   be supported.7.  PRIVACY7.1  User Agent Control   An origin server could create a Set-Cookie header to track the path   of a user through the server.  Users may object to this behavior. as   an intrusive accumulation of information, even if their identity is   not evident.  (Identity might become evident if a user subsequently   fills out a form. that contains identifying information.)  This state   management specification therefore requires that a user agent give   the user control over such a possible intrusion, although the   interface through which the user is given this control is left   unspecified.  However, the control mechanisms provided shall at least   allow the user      * to completely disable the sending and saving of cookies.      * to determine whether a stateful session is in progress.      * to control the saving of a cookie on the basis of the cookie's        Domain attribute.   Such control could be provided by, for example, mechanisms      * to notify the user when the user agent is about to send a cookie        to the origin server, offering the option not to begin a session.      * to display a visual indication that a stateful session is in        progress.      * to let the user decide which cookies, if any, should be saved        when the user concludes a window or user agent session.      * to let the user examine the contents of a cookie at any time.   A user agent usually begins execution with no remembered state   information.  It should be possible to configure a user agent never   to send Cookie headers, in which case it can never sustain state with   an origin server.  (The user agent would then behave like one that is   unaware of how to handle Set-Cookie response headers.)   When the user agent terminates execution, it should let the user   discard all state information.  Alternatively, the user agent may ask   the user whether state information should be retained; the default   should be "no".  If the user chooses to retain state information, it   would be restored the next time the user agent runs.   NOTE: User agents should probably be cautious about using files to   store cookies long-term.  If a user runs more than one instance of   the user agent, the cookies could be commingled or otherwise messed   up.7.2  Protocol Design   The restrictions on the value of the Domain attribute, and the rules   concerning unverifiable transactions, are meant to reduce the ways   that cookies can "leak" to the "wrong" site.  The intent is to   restrict cookies to one, or a closely related set of hosts.   Therefore a request-host is limited as to what values it can set for   Domain.  We consider it acceptable for hosts host1.foo.com and   host2.foo.com to share cookies, but not a.com and b.com.   Similarly, a server can only set a Path for cookies that are related   to the request-URI.8.  SECURITY CONSIDERATIONS8.1  Clear Text   The information in the Set-Cookie and Cookie headers is unprotected.   Two consequences are:   1.  Any sensitive information that is conveyed in them is exposed       to intruders.   2.  A malicious intermediary could alter the headers as they travel       in either direction, with unpredictable results.   These facts imply that information of a personal and/or financial   nature should only be sent over a secure channel.  For less sensitive   information, or when the content of the header is a database key, an   origin server should be vigilant to prevent a bad Cookie value from   causing failures.8.2  Cookie Spoofing   Proper application design can avoid spoofing attacks from related   domains.  Consider:     1.  User agent makes request to victim.cracker.edu, gets back         cookie session_id="1234" and sets the default domain         victim.cracker.edu.     2.  User agent makes request to spoof.cracker.edu, gets back         cookie session-id="1111", with Domain=".cracker.edu".     3.  User agent makes request to victim.cracker.edu again, and         passes         Cookie: $Version="1";                         session_id="1234";                         session_id="1111"; $Domain=".cracker.edu"         The server at victim.cracker.edu should detect that the second         cookie was not one it originated by noticing that the Domain         attribute is not for itself and ignore it.8.3  Unexpected Cookie Sharing   A user agent should make every attempt to prevent the sharing of   session information between hosts that are in different domains.   Embedded or inlined objects may cause particularly severe privacy   problems if they can be used to share cookies between disparate   hosts.  For example, a malicious server could embed cookie   information for host a.com in a URI for a CGI on host b.com.  User   agent implementors are strongly encouraged to prevent this sort of   exchange whenever possible.9.  OTHER, SIMILAR, PROPOSALS   Three other proposals have been made to accomplish similar goals.   This specification is an amalgam of Kristol's State-Info proposal and   Netscape's Cookie proposal.   Brian Behlendorf proposed a Session-ID header that would be user-   agent-initiated and could be used by an origin server to track   "clicktrails".  It would not carry any origin-server-defined state,   however.  Phillip Hallam-Baker has proposed another client-defined   session ID mechanism for similar purposes.   While both session IDs and cookies can provide a way to sustain   stateful sessions, their intended purpose is different, and,   consequently, the privacy requirements for them are different.  A   user initiates session IDs to allow servers to track progress through   them, or to distinguish multiple users on a shared machine.  Cookies   are server-initiated, so the cookie mechanism described here gives   users control over something that would otherwise take place without   the users' awareness.  Furthermore, cookies convey rich, server-   selected information, whereas session IDs comprise user-selected,   simple information.