Cookies: HTTP State Management Mechanism

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("MUST", "SHOULD", "MAY", etc.) used in introducing the algorithm.¶

Conformance requirements phrased as algorithms or specific steps can be implemented in any manner, so long as the end result is equivalent. In particular, the algorithms defined in this specification are intended to be easy to understand and are not intended to be performant.¶

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234].¶

The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTLs (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), NUL (null octet), OCTET (any 8-bit sequence of data except NUL), SP (space), HTAB (horizontal tab), CHAR (any [USASCII] character), VCHAR (any visible [USASCII] character), and WSP (whitespace).¶

The OWS (optional whitespace) and BWS (bad whitespace) rules are defined in Section 5.6.3 of [HTTP].¶

The terms "user agent", "client", "server", "proxy", and "origin server" have the same meaning as in the HTTP/1.1 specification ([HTTP], Section 3).¶

The request-host is the name of the host, as known by the user agent, to which the user agent is sending an HTTP request or from which it is receiving an HTTP response (i.e., the name of the host to which it sent the corresponding HTTP request).¶

The term request-uri refers to "target URI" as defined in Section 7.1 of [HTTP].¶

Two sequences of octets are said to case-insensitively match each other if and only if they are equivalent under the i;ascii-casemap collation defined in [RFC4790].¶

The term string means a sequence of non-NUL octets.¶

The terms "active browsing context", "active document", "ancestor navigables", "container document", "content navigable", "dedicated worker", "Document", "inclusive ancestor navigables", "navigable", "opaque origin", "sandboxed origin browsing context flag", "shared worker", "the worker's Documents", "top-level traversable", and "WorkerGlobalScope" are defined in [HTML].¶

"Service Workers" are defined in the Service Workers specification [SERVICE-WORKERS].¶

The term "origin", the mechanism of deriving an origin from a URI, and the "the same" matching algorithm for origins are defined in [RFC6454].¶

"Safe" HTTP methods include GET, HEAD, OPTIONS, and TRACE, as defined in Section 9.2.1 of [HTTP].¶

A domain's "public suffix" is the portion of a domain that is controlled by a public registry, such as "com", "co.uk", and "pvt.k12.wy.us". A domain's "registrable domain" is the domain's public suffix plus the label to its left. That is, for https://www.site.example, the public suffix is example, and the registrable domain is site.example. Whenever possible, user agents SHOULD use an up-to-date public suffix list, such as the one maintained by the Mozilla project at [PSL].¶

The term "request", as well as a request's "client", "current url", "method", "target browsing context", and "url list", are defined in [FETCH].¶

The term "non-HTTP APIs" refers to non-HTTP mechanisms used to set and retrieve cookies, such as a web browser API that exposes cookies to scripts.¶

The term "top-level navigation" refers to a navigation of a top-level traversable.¶

Using the Set-Cookie header field, a server can send the user agent a short string in an HTTP response that the user agent will return in future HTTP requests that are within the scope of the cookie. For example, the server can send the user agent a "session identifier" named SID with the value 31d4d96e407aad42. The user agent then returns the session identifier in subsequent requests.¶

The server can alter the default scope of the cookie using the Path and Domain attributes. For example, the server can instruct the user agent to return the cookie to every path and every subdomain of site.example.¶

As shown in the next example, the server can store multiple cookies at the user agent. For example, the server can store a session identifier as well as the user's preferred language by returning two Set-Cookie header fields. Notice that the server uses the Secure and HttpOnly attributes to provide additional security protections for the more sensitive session identifier (see Section 4.1.2).¶

Notice that the Cookie header field above contains two cookies, one named SID and one named lang. If the server wishes the user agent to persist the cookie over multiple "sessions" (e.g., user agent restarts), the server can specify an expiration date in the Expires attribute. Note that the user agent might delete the cookie before the expiration date if the user agent's cookie store exceeds its quota or if the user manually deletes the server's cookie.¶

Finally, to remove a cookie, the server returns a Set-Cookie header field with an expiration date in the past. The server will be successful in removing the cookie only if the Path and the Domain attribute in the Set-Cookie header field match the values used when the cookie was created.¶

The upcoming two sections, Section 4 and Section 5, discuss the set of requirements for two distinct types of implementations. This section is meant to help guide implementers in determining which set of requirements best fits their goals. Choosing the wrong set of requirements could result in a lack of compatibility with other cookie implementations.¶

It's important to note that being compatible means different things depending on the implementer's goals. These differences have built up over time due to both intentional and unintentional spec changes, spec interpretations, and historical implementation differences.¶

This section roughly divides implementers of the cookie spec into two types, producers and consumers. These are not official terms and are only used here to help readers develop an intuitive understanding of the use cases.¶

The Set-Cookie HTTP response header field is used to send cookies from the server to the user agent.¶

This section defines some algorithms used by user agents to process specific subcomponents of the Cookie and Set-Cookie header fields.¶

1. The request is not the result of a reload navigation triggered through a user interface element (as defined by the user agent; e.g., a request triggered by the user clicking a refresh button on a toolbar).
2. The request's current url's origin is same-site with the request's client's "site for cookies" (which is an origin), or if the request has no client or the request's client is null.

Requests which are the result of a reload navigation triggered through a user interface element are same-site if the reloaded document was originally navigated to via a same-site request. A request that is not "same-site" is instead "cross-site".¶

User agents MAY ignore Set-Cookie header fields contained in responses with 100-level status codes or based on its cookie policy (see Section 7.2).¶

All other Set-Cookie header fields SHOULD be processed according to Section 5.6. That is, Set-Cookie header fields contained in responses with non-100-level status codes (including those in responses with 400- and 500-level status codes) SHOULD be processed unless ignored according to the user agent's cookie policy.¶

User agents' requirements for cookie name prefixes differ slightly from servers' (Section 4.1.3) in that UAs MUST match the prefix string case-insensitively.¶

The normative requirements for the prefixes are detailed in the storage model algorithm defined in Section 5.7.¶

This is because some servers will process cookies case-insensitively, resulting in them unintentionally miscapitalizing and accepting miscapitalized prefixes.¶

For example, if a server sends the following Set-Cookie header field¶

to a UA which checks prefixes case-sensitively it will accept this cookie and the server would incorrectly believe the cookie is subject the same guarantees as one spelled __Secure-.¶

Additionally the server is vulnerable to an attacker that purposefully miscapitalizes a cookie in order to impersonate a prefixed cookie. For example, a site already has a cookie __Secure-SID=12345 and by some means an attacker sends the following Set-Cookie header field for the site to a UA which checks prefixes case-sensitively.¶

The server, being case-insensitive, won't be able to tell the difference between the two cookies allowing the attacker to compromise the site.¶

To prevent these issues, UAs MUST match cookie name prefixes case-insensitive.¶

Note: Cookies with different names are still considered separate by UAs. So both __Secure-foo=bar and __secure-foo=baz can exist as distinct cookies simultaneously and both would have the requirements of the __Secure- prefix applied.¶

The following are examples of Set-Cookie header fields that would be rejected by a conformant user agent.¶

Whereas the following Set-Cookie header fields would be accepted if set from a secure origin.¶

When processing cookies with a specified lifetime, either with the Expires or with the Max-Age attribute, the user agent MUST limit the maximum age of the cookie. The limit SHOULD NOT be greater than 400 days (34560000 seconds) in the future. The RECOMMENDED limit is 400 days in the future, but the user agent MAY adjust the limit (see Section 7.2). Expires or Max-Age attributes that specify a lifetime longer than the limit MUST be reduced to the limit.¶

When a user agent receives a Set-Cookie header field in an HTTP response, the user agent MAY ignore the Set-Cookie header field in its entirety (see Section 5.3).¶

If the user agent does not ignore the Set-Cookie header field in its entirety, the user agent MUST parse the field-value of the Set-Cookie header field as a set-cookie-string (defined below).¶

NOTE: The algorithm below is more permissive than the grammar in Section 4.1. For example, the algorithm strips leading and trailing whitespace from the cookie name and value (but maintains internal whitespace), whereas the grammar in Section 4.1 forbids whitespace in these positions. In addition, the algorithm below accommodates some characters that are not cookie-octets according to the grammar in Section 4.1. User agents use this algorithm so as to interoperate with servers that do not follow the recommendations in Section 4.¶

NOTE: As set-cookie-string may originate from a non-HTTP API, it is not guaranteed to be free of CTL characters, so this algorithm handles them explicitly. Horizontal tab (%x09) is excluded from the CTL characters that lead to set-cookie-string rejection, as it is considered whitespace, which is handled separately.¶

NOTE: The set-cookie-string may contain octet sequences that appear percent-encoded as per Section 2.1 of [RFC3986]. However, a user agent MUST NOT decode these sequences and instead parse the individual octets as specified in this algorithm.¶

1. If the set-cookie-string contains a %x00-08 / %x0A-1F / %x7F character (CTL characters excluding HTAB): Abort these steps and ignore the set-cookie-string entirely.
2. If the set-cookie-string contains a %x3B (";") character:
   1. The name-value-pair string consists of the characters up to, but not including, the first %x3B (";"), and the unparsed-attributes consist of the remainder of the set-cookie-string (including the %x3B (";") in question).
   Otherwise:
   1. The name-value-pair string consists of all the characters contained in the set-cookie-string, and the unparsed-attributes is the empty string.
3. If the name-value-pair string lacks a %x3D ("=") character, then the name string is empty, and the value string is the value of name-value-pair.
   
   Otherwise, the name string consists of the characters up to, but not including, the first %x3D ("=") character, and the (possibly empty) value string consists of the characters after the first %x3D ("=") character.
4. Remove any leading or trailing WSP characters from the name string and the value string.
5. If the sum of the lengths of the name string and the value string is more than 4096 octets, abort these steps and ignore the set-cookie-string entirely.
6. The cookie-name is the name string, and the cookie-value is the value string.

1. If the unparsed-attributes string is empty, skip the rest of these steps.
2. Discard the first character of the unparsed-attributes (which will be a %x3B (";") character).
3. If the remaining unparsed-attributes contains a %x3B (";") character:
   1. Consume the characters of the unparsed-attributes up to, but not including, the first %x3B (";") character.
   Otherwise:
   1. Consume the remainder of the unparsed-attributes.
   Let the cookie-av string be the characters consumed in this step.
4. If the cookie-av string contains a %x3D ("=") character:
   1. The (possibly empty) attribute-name string consists of the characters up to, but not including, the first %x3D ("=") character, and the (possibly empty) attribute-value string consists of the characters after the first %x3D ("=") character.
   Otherwise:
   1. The attribute-name string consists of the entire cookie-av string, and the attribute-value string is empty.
5. Remove any leading or trailing WSP characters from the attribute-name string and the attribute-value string.
6. If the attribute-value is longer than 1024 octets, ignore the cookie-av string and return to Step 1 of this algorithm.
7. Process the attribute-name and attribute-value according to the requirements in the following subsections. (Notice that attributes with unrecognized attribute-names are ignored.)
8. Return to Step 1 of this algorithm.

When the user agent finishes parsing the set-cookie-string, the user agent is said to "receive a cookie" from the request-uri with name cookie-name, value cookie-value, and attributes cookie-attribute-list. (See Section 5.7 for additional requirements triggered by receiving a cookie.)¶

The user agent stores the following fields about each cookie: name, value, expiry-time, domain, path, creation-time, last-access-time, persistent-flag, host-only-flag, secure-only-flag, http-only-flag, and same-site-flag.¶

1. A user agent MAY ignore a received cookie in its entirety. See Section 5.3.
2. If cookie-name is empty and cookie-value is empty, abort these steps and ignore the cookie entirely.
3. If the cookie-name or the cookie-value contains a %x00-08 / %x0A-1F / %x7F character (CTL characters excluding HTAB), abort these steps and ignore the cookie entirely.
4. If the sum of the lengths of cookie-name and cookie-value is more than 4096 octets, abort these steps and ignore the cookie entirely.
5. Create a new cookie with name cookie-name, value cookie-value. Set the creation-time and the last-access-time to the current date and time.
6. If the cookie-attribute-list contains an attribute with an attribute-name of "Max-Age":
   1. Set the cookie's persistent-flag to true.
   2. Set the cookie's expiry-time to attribute-value of the last attribute in the cookie-attribute-list with an attribute-name of "Max-Age".
   Otherwise, if the cookie-attribute-list contains an attribute with an attribute-name of "Expires" (and does not contain an attribute with an attribute-name of "Max-Age"):
   1. Set the cookie's persistent-flag to true.
   2. Set the cookie's expiry-time to attribute-value of the last attribute in the cookie-attribute-list with an attribute-name of "Expires".
   Otherwise:
   1. Set the cookie's persistent-flag to false.
   2. Set the cookie's expiry-time to the latest representable date.
7. If the cookie-attribute-list contains an attribute with an attribute-name of "Domain":
   1. Let the domain-attribute be the attribute-value of the last attribute in the cookie-attribute-list with both an attribute-name of "Domain" and an attribute-value whose length is no more than 1024 octets. (Note that a leading %x2E ("."), if present, is ignored even though that character is not permitted.)
   Otherwise:
   1. Let the domain-attribute be the empty string.
8. If the domain-attribute contains a character that is not in the range of [USASCII] characters, abort these steps and ignore the cookie entirely.
9. If the user agent is configured to reject "public suffixes" and the domain-attribute is a public suffix:
   1. If the domain-attribute is identical to the canonicalized request-host:
      1. Let the domain-attribute be the empty string.
      Otherwise:
      1. Abort these steps and ignore the cookie entirely.
   NOTE: This step prevents attacker.example from disrupting the integrity of site.example by setting a cookie with a Domain attribute of "example".
10. If the domain-attribute is non-empty:
    1. If the canonicalized request-host does not domain-match the domain-attribute:
       1. Abort these steps and ignore the cookie entirely.
       Otherwise:
       1. Set the cookie's host-only-flag to false.
       2. Set the cookie's domain to the domain-attribute.
    Otherwise:
    1. Set the cookie's host-only-flag to true.
    2. Set the cookie's domain to the canonicalized request-host.
11. If the cookie-attribute-list contains an attribute with an attribute-name of "Path", set the cookie's path to attribute-value of the last attribute in the cookie-attribute-list with both an attribute-name of "Path" and an attribute-value whose length is no more than 1024 octets. Otherwise, set the cookie's path to the default-path of the request-uri.
12. If the cookie-attribute-list contains an attribute with an attribute-name of "Secure", set the cookie's secure-only-flag to true. Otherwise, set the cookie's secure-only-flag to false.
13. If the request-uri does not denote a "secure" connection (as defined by the user agent), and the cookie's secure-only-flag is true, then abort these steps and ignore the cookie entirely.
14. If the cookie-attribute-list contains an attribute with an attribute-name of "HttpOnly", set the cookie's http-only-flag to true. Otherwise, set the cookie's http-only-flag to false.
15. If the cookie was received from a "non-HTTP" API and the cookie's http-only-flag is true, abort these steps and ignore the cookie entirely.
16. If the cookie's secure-only-flag is false, and the request-uri does not denote a "secure" connection, then abort these steps and ignore the cookie entirely if the cookie store contains one or more cookies that meet all of the following criteria:
    1. Their name matches the name of the newly-created cookie.
    2. Their secure-only-flag is true.
    3. Their domain domain-matches the domain of the newly-created cookie, or vice-versa.
    4. The path of the newly-created cookie path-matches the path of the existing cookie.
    Note: The path comparison is not symmetric, ensuring only that a newly-created, non-secure cookie does not overlay an existing secure cookie, providing some mitigation against cookie-fixing attacks. That is, given an existing secure cookie named 'a' with a path of '/login', a non-secure cookie named 'a' could be set for a path of '/' or '/foo', but not for a path of '/login' or '/login/en'.
17. If the cookie-attribute-list contains an attribute with an attribute-name of "SameSite", and an attribute-value of "Strict", "Lax", or "None", set the cookie's same-site-flag to the attribute-value of the last attribute in the cookie-attribute-list with an attribute-name of "SameSite". Otherwise, set the cookie's same-site-flag to "Default".
18. If the cookie's same-site-flag is not "None":
    1. If the cookie was received from a "non-HTTP" API, and the API was called from a navigable's active document whose "site for cookies" is not same-site with the top-level origin, then abort these steps and ignore the newly created cookie entirely.
    2. If the cookie was received from a "same-site" request (as defined in Section 5.2), skip the remaining substeps and continue processing the cookie.
    3. If the cookie was received from a request which is navigating a top-level traversable [HTML] (e.g. if the request's "reserved client" is either null or an environment whose "target browsing context"'s navigable is a top-level traversable), skip the remaining substeps and continue processing the cookie.
       
       Note: Top-level navigations can create a cookie with any SameSite value, even if the new cookie wouldn't have been sent along with the request had it already existed prior to the navigation.
    4. Abort these steps and ignore the newly created cookie entirely.
19. If the cookie's "same-site-flag" is "None", abort these steps and ignore the cookie entirely unless the cookie's secure-only-flag is true.
20. If the cookie-name begins with a case-insensitive match for the string "__Secure-", abort these steps and ignore the cookie entirely unless the cookie's secure-only-flag is true.
21. If the cookie-name begins with a case-insensitive match for the string "__Host-", abort these steps and ignore the cookie entirely unless the cookie meets all the following criteria:
    1. The cookie's secure-only-flag is true.
    2. The cookie's host-only-flag is true.
    3. The cookie-attribute-list contains an attribute with an attribute-name of "Path", and the cookie's path is /.
22. If the cookie-name is empty and either of the following conditions are true, abort these steps and ignore the cookie entirely:
    • the cookie-value begins with a case-insensitive match for the string "__Secure-"
    • the cookie-value begins with a case-insensitive match for the string "__Host-"
23. If the cookie store contains a cookie with the same name, domain, host-only-flag, and path as the newly-created cookie:
    1. Let old-cookie be the existing cookie with the same name, domain, host-only-flag, and path as the newly-created cookie. (Notice that this algorithm maintains the invariant that there is at most one such cookie.)
    2. If the newly-created cookie was received from a "non-HTTP" API and the old-cookie's http-only-flag is true, abort these steps and ignore the newly created cookie entirely.
    3. Update the creation-time of the newly-created cookie to match the creation-time of the old-cookie.
    4. Remove the old-cookie from the cookie store.
24. Insert the newly-created cookie into the cookie store.

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).¶

1. Expired cookies.
2. Cookies whose secure-only-flag is false, and which share a domain field with more than a predetermined number of other cookies.
3. Cookies that share a domain field with more than a predetermined number of other cookies.
4. All cookies.

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

This section defines how cookies are retrieved from a cookie store in the form of a cookie-string. A "retrieval" is any event which requires generating a cookie-string. For example, a retrieval may occur in order to build a Cookie header field for an HTTP request, or may be required in order to return a cookie-string from a call to a "non-HTTP" API that provides access to cookies. A retrieval has an associated URI, same-site status, and type, which are defined below depending on the type of retrieval.¶

• At least 50 cookies per domain.
• At least 3000 cookies total.

User agents MAY limit the maximum number of cookies they store, and may evict any cookie at any time (whether at the request of the user or due to implementation limitations).¶

Note that a limit on the maximum number of cookies also limits the total size of the stored cookies, due to the length limits which MUST be enforced in Section 5.6.¶

Servers SHOULD use as few and as small cookies as possible to avoid reaching these implementation limits, minimize network bandwidth due to the Cookie header field being included in every request, and to avoid reaching server header field limits (See Section 4.2.1).¶

Servers SHOULD gracefully degrade if the user agent fails to return one or more cookies in the Cookie header field because the user agent might evict any cookie at any time.¶

One reason the Cookie and Set-Cookie header fields use such esoteric syntax is that many platforms (both in servers and user agents) provide a string-based application programming interface (API) to cookies, requiring application-layer programmers to generate and parse the syntax used by the Cookie and Set-Cookie header fields, which many programmers have done incorrectly, resulting in interoperability problems.¶

Instead of providing string-based APIs to cookies, platforms would be well-served by providing more semantic APIs. It is beyond the scope of this document to recommend specific API designs, but there are clear benefits to accepting an abstract "Date" object instead of a serialized date string.¶

IDNA2008 [RFC5890] supersedes IDNA2003 [RFC3490]. However, there are differences between the two specifications, and thus there can be differences in processing (e.g., converting) domain name labels that have been registered under one from those registered under the other. There will be a transition period of some time during which IDNA2003-based domain name labels will exist in the wild. User agents SHOULD implement IDNA2008 [RFC5890] and MAY implement [UTS46] or [RFC5895] in order to facilitate their IDNA transition. If a user agent does not implement IDNA2008, the user agent MUST implement IDNA2003 [RFC3490].¶

A "third-party" or cross-site cookie is one that is associated with embedded content (such as scripts, images, stylesheets, frames) that is obtained from a different server than the one that hosts the primary resource (usually, the Web page that the user is viewing). Third-party cookies are often used to correlate users' activity on different sites.¶

Because of their inherent privacy issues, most user agents now limit third-party cookies in a variety of ways. Some completely block third-party cookies by refusing to process third-party Set-Cookie header fields and refusing to send third-party Cookie header fields. Some partition cookies based upon the first-party context, so that different cookies are sent depending on the site being browsed. Some block cookies based upon user agent cookie policy and/or user controls.¶

While this document does not endorse or require a specific approach, it is RECOMMENDED that user agents adopt a policy for third-party cookies that is as restrictive as compatibility constraints permit. Consequently, resources cannot rely upon third-party cookies being treated consistently by user agents for the foreseeable future.¶

User agents MAY enforce a cookie policy consisting of restrictions on how cookies may be used or ignored (see Section 5.3).¶

A cookie policy may govern which domains or parties, as in first and third parties (see Section 7.1), for which the user agent will allow cookie access. The policy can also define limits on cookie size, cookie expiry (see Section 5.5), and the number of cookies per domain or in total.¶

The recommended cookie expiry upper limit is 400 days. User agents may set a lower limit to enforce shorter data retention timelines, or set the limit higher to support longer retention when appropriate (e.g., server-to-server communication over HTTPS).¶

The goal of a restrictive cookie policy is often to improve security or privacy. User agents often allow users to change the cookie policy (see Section 7.3).¶

User agents SHOULD provide users with a mechanism for managing the cookies stored in the cookie store. For example, a user agent might let users delete all cookies received during a specified time period or all the cookies related to a particular domain. In addition, many user agents include a user interface element that lets users examine the cookies stored in their cookie store.¶

User agents SHOULD provide users with a mechanism for disabling cookies. When cookies are disabled, the user agent MUST NOT include a Cookie header field in outbound HTTP requests and the user agent MUST NOT process Set-Cookie header fields in inbound HTTP responses.¶

User agents MAY offer a way to change the cookie policy (see Section 7.2).¶

User agents MAY provide users the option of preventing persistent storage of cookies across sessions. When configured thusly, user agents MUST treat all received cookies as if the persistent-flag were set to false. Some popular user agents expose this functionality via "private browsing" mode [Aggarwal2010].¶

Although servers can set the expiration date for cookies to the distant future, most user agents do not actually retain cookies for multiple decades. Rather than choosing gratuitously long expiration periods, servers SHOULD promote user privacy by selecting reasonable cookie expiration periods based on the purpose of the cookie. For example, a typical session identifier might reasonably be set to expire in two weeks.¶

Cookies have a number of security pitfalls. This section overviews a few of the more salient issues.¶

In particular, cookies encourage developers to rely on ambient authority for authentication, often becoming vulnerable to attacks such as cross-site request forgery [CSRF]. Also, when storing session identifiers in cookies, developers often create session fixation vulnerabilities.¶

Transport-layer encryption, such as that employed in HTTPS, is insufficient to prevent a network attacker from obtaining or altering a victim's cookies because the cookie protocol itself has various vulnerabilities (see "Weak Confidentiality" and "Weak Integrity", below). In addition, by default, cookies do not provide confidentiality or integrity from network attackers, even when used in conjunction with HTTPS.¶

A server that uses cookies to authenticate users can suffer security vulnerabilities because some user agents let remote parties issue HTTP requests from the user agent (e.g., via HTTP redirects or HTML forms). When issuing those requests, user agents attach cookies even if the remote party does not know the contents of the cookies, potentially letting the remote party exercise authority at an unwary server.¶

Although this security concern goes by a number of names (e.g., cross-site request forgery, confused deputy), the issue stems from cookies being a form of ambient authority. Cookies encourage server operators to separate designation (in the form of URLs) from authorization (in the form of cookies). Consequently, the user agent might supply the authorization for a resource designated by the attacker, possibly causing the server or its clients to undertake actions designated by the attacker as though they were authorized by the user.¶

Instead of using cookies for authorization, server operators might wish to consider entangling designation and authorization by treating URLs as capabilities. Instead of storing secrets in cookies, this approach stores secrets in URLs, requiring the remote entity to supply the secret itself. Although this approach is not a panacea, judicious application of these principles can lead to more robust security.¶

Unless sent over a secure channel (such as TLS), the information in the Cookie and Set-Cookie header fields is transmitted in the clear.¶

1. All sensitive information conveyed in these header fields is exposed to an eavesdropper.
2. A malicious intermediary could alter the header fields as they travel in either direction, with unpredictable results.
3. A malicious client could alter the Cookie header fields before transmission, with unpredictable results.

Servers SHOULD encrypt and sign the contents of cookies (using whatever format the server desires) when transmitting them to the user agent (even when sending the cookies over a secure channel). However, encrypting and signing cookie contents does not prevent an attacker from transplanting a cookie from one user agent to another or from replaying the cookie at a later time.¶

In addition to encrypting and signing the contents of every cookie, servers that require a higher level of security SHOULD use the Cookie and Set-Cookie header fields only over a secure channel. When using cookies over a secure channel, servers SHOULD set the Secure attribute (see Section 4.1.2.5) for every cookie. If a server does not set the Secure attribute, the protection provided by the secure channel will be largely moot.¶

For example, consider a webmail server that stores a session identifier in a cookie and is typically accessed over HTTPS. If the server does not set the Secure attribute on its cookies, an active network attacker can intercept any outbound HTTP request from the user agent and redirect that request to the webmail server over HTTP. Even if the webmail server is not listening for HTTP connections, the user agent will still include cookies in the request. The active network attacker can intercept these cookies, replay them against the server, and learn the contents of the user's email. If, instead, the server had set the Secure attribute on its cookies, the user agent would not have included the cookies in the clear-text request.¶

Instead of storing session information directly in a cookie (where it might be exposed to or replayed by an attacker), servers commonly store a nonce (or "session identifier") in a cookie. When the server receives an HTTP request with a nonce, the server can look up state information associated with the cookie using the nonce as a key.¶

Using session identifier cookies limits the damage an attacker can cause if the attacker learns the contents of a cookie because the nonce is useful only for interacting with the server (unlike non-nonce cookie content, which might itself be sensitive). Furthermore, using a single nonce prevents an attacker from "splicing" together cookie content from two interactions with the server, which could cause the server to behave unexpectedly.¶

Using session identifiers is not without risk. For example, the server SHOULD take care to avoid "session fixation" vulnerabilities. A session fixation attack proceeds in three steps. First, the attacker transplants a session identifier from his or her user agent to the victim's user agent. Second, the victim uses that session identifier to interact with the server, possibly imbuing the session identifier with the user's credentials or confidential information. Third, the attacker uses the session identifier to interact with server directly, possibly obtaining the user's authority or confidential information.¶

Cookies do not provide isolation by port. If a cookie is readable by a service running on one port, the cookie is also readable by a service running on another port of the same server. If a cookie is writable by a service on one port, the cookie is also writable by a service running on another port of the same server. For this reason, servers SHOULD NOT both run mutually distrusting services on different ports of the same host and use cookies to store security-sensitive information.¶

Cookies do not provide isolation by scheme. Although most commonly used with the http and https schemes, the cookies for a given host might also be available to other schemes, such as ftp and gopher. Although this lack of isolation by scheme is most apparent in non-HTTP APIs that permit access to cookies (e.g., HTML's document.cookie API), the lack of isolation by scheme is actually present in requirements for processing cookies themselves (e.g., consider retrieving a URI with the gopher scheme via HTTP).¶

Cookies do not always provide isolation by path. Although the network-level protocol does not send cookies stored for one path to another, some user agents expose cookies via non-HTTP APIs, such as HTML's document.cookie API. Because some of these user agents (e.g., web browsers) do not isolate resources received from different paths, a resource retrieved from one path might be able to access cookies stored for another path.¶

Cookies do not provide integrity guarantees for sibling domains (and their subdomains). For example, consider foo.site.example and bar.site.example. The foo.site.example server can set a cookie with a Domain attribute of "site.example" (possibly overwriting an existing "site.example" cookie set by bar.site.example), and the user agent will include that cookie in HTTP requests to bar.site.example. In the worst case, bar.site.example will be unable to distinguish this cookie from a cookie it set itself. The foo.site.example server might be able to leverage this ability to mount an attack against bar.site.example.¶

Even though the Set-Cookie header field supports the Path attribute, the Path attribute does not provide any integrity protection because the user agent will accept an arbitrary Path attribute in a Set-Cookie header field. For example, an HTTP response to a request for http://site.example/foo/bar can set a cookie with a Path attribute of "/qux". Consequently, servers SHOULD NOT both run mutually distrusting services on different paths of the same host and use cookies to store security-sensitive information.¶

An active network attacker can also inject cookies into the Cookie header field sent to https://site.example/ by impersonating a response from http://site.example/ and injecting a Set-Cookie header field. The HTTPS server at site.example will be unable to distinguish these cookies from cookies that it set itself in an HTTPS response. An active network attacker might be able to leverage this ability to mount an attack against site.example even if site.example uses HTTPS exclusively.¶

Servers can partially mitigate these attacks by encrypting and signing the contents of their cookies, or by naming the cookie with the __Secure- prefix. However, using cryptography does not mitigate the issue completely because an attacker can replay a cookie he or she received from the authentic site.example server in the user's session, with unpredictable results.¶

Finally, an attacker might be able to force the user agent to delete cookies by storing a large number of cookies. Once the user agent reaches its storage limit, the user agent will be forced to evict some cookies. Servers SHOULD NOT rely upon user agents retaining cookies.¶

Cookies rely upon the Domain Name System (DNS) for security. If the DNS is partially or fully compromised, the cookie protocol might fail to provide the security properties required by applications.¶

Header field name:

Cookie

Applicable protocol:

http

Status:

standard

Author/Change controller:

IETF

Specification document:

this specification (Section 5.8.1)

Header field name:

Set-Cookie

Applicable protocol:

http

Status:

standard

Author/Change controller:

IETF

Specification document:

this specification (Section 5.6)

IANA is requested to create the "Cookie Attribute" registry, defining the name space of attribute used to control cookies' behavior. The registry should be maintained at https://www.iana.org/assignments/cookie-attribute-names.¶

10.1. Normative References

[DOM-DOCUMENT-COOKIE]

WHATWG, “HTML - Living Standard”, May 2021, <https://html.spec.whatwg.org/#dom-document-cookie>.

[FETCH]

van Kesteren, A., “Fetch”, n.d., <https://fetch.spec.whatwg.org/>.

[HTML]

Hickson, I., Pieters, S., van Kesteren, A., Jägenstedt, P., and D. Denicola, “HTML”, n.d., <https://html.spec.whatwg.org/>.

[HTTP]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “HTTP Semantics”, STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/info/rfc9110>.

[RFC1034]

Mockapetris, P., “Domain names - concepts and facilities”, STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, <https://www.rfc-editor.org/info/rfc1034>.

[RFC1123]

Braden, R., Ed., “Requirements for Internet Hosts - Application and Support”, STD 3, RFC 1123, DOI 10.17487/RFC1123, October 1989, <https://www.rfc-editor.org/info/rfc1123>.

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC3490]

Costello, A., “Internationalizing Domain Names in Applications (IDNA)”, RFC 3490, DOI 10.17487/RFC3490, March 2003, <https://www.rfc-editor.org/info/rfc3490>.
See Section 6.3 for an explanation why the normative reference to an obsoleted specification is needed.

[RFC4790]

Newman, C., Duerst, M., and A. Gulbrandsen, “Internet Application Protocol Collation Registry”, RFC 4790, DOI 10.17487/RFC4790, March 2007, <https://www.rfc-editor.org/info/rfc4790>.

[RFC5234]

Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>.

[RFC5890]

Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework”, RFC 5890, DOI 10.17487/RFC5890, August 2010, <https://www.rfc-editor.org/info/rfc5890>.

[RFC6454]

Barth, A., “The Web Origin Concept”, RFC 6454, DOI 10.17487/RFC6454, December 2011, <https://www.rfc-editor.org/info/rfc6454>.

[RFC8126]

Cotton, M., Leiba, B., and T. Narten, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.

[SAMESITE]

WHATWG, “HTML - Living Standard”, January 2021, <https://html.spec.whatwg.org/#same-site>.

[SERVICE-WORKERS]

Russell, A., Song, J., and J. Archibald, “Service Workers”, n.d., <http://www.w3.org/TR/service-workers/>.

[USASCII]

American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI X3.4, 1986.

10.2. Informative References

[Aggarwal2010]

Aggarwal, G., Burzstein, E., Jackson, C., and D. Boneh, “An Analysis of Private Browsing Modes in Modern Browsers”, 2010, <http://www.usenix.org/events/sec10/tech/full_papers/Aggarwal.pdf>.

[app-isolation]

Chen, E., Bau, J., Reis, C., Barth, A., and C. Jackson, “App Isolation - Get the Security of Multiple Browsers with Just One”, 2011, <http://www.collinjackson.com/research/papers/appisolation.pdf>.

[CSRF]

Barth, A., Jackson, C., and J. Mitchell, “Robust Defenses for Cross-Site Request Forgery”, DOI 10.1145/1455770.1455782, ISBN 978-1-59593-810-7, ACM CCS '08: Proceedings of the 15th ACM conference on Computer and communications security (pages 75-88), October 2008, <http://portal.acm.org/citation.cfm?id=1455770.1455782>.

[HttpFieldNameRegistry]

“Hypertext Transfer Protocol (HTTP) Field Name Registry”, n.d., <https://www.iana.org/assignments/http-fields/>.

[I-D.ietf-httpbis-cookie-alone]

West, M., “Deprecate modification of 'secure' cookies from non-secure origins”, Internet-Draft draft-ietf-httpbis-cookie-alone-01 (work in progress), September 2016.

[I-D.ietf-httpbis-cookie-prefixes]

West, M., “Cookie Prefixes”, Internet-Draft draft-ietf-httpbis-cookie-prefixes-00 (work in progress), February 2016.

[I-D.ietf-httpbis-cookie-same-site]

West, M. and M. Goodwin, “Same-Site Cookies”, Internet-Draft draft-ietf-httpbis-cookie-same-site-00 (work in progress), June 2016.

[prerendering]

Bentzel, C., “Chrome Prerendering”, n.d., <https://www.chromium.org/developers/design-documents/prerender>.

[PSL]

“Public Suffix List”, n.d., <https://publicsuffix.org/list/>.

[RFC2109]

Kristol, D. and L. Montulli, “HTTP State Management Mechanism”, RFC 2109, DOI 10.17487/RFC2109, February 1997, <https://www.rfc-editor.org/info/rfc2109>.

[RFC2818]

Rescorla, E., “HTTP Over TLS”, RFC 2818, DOI 10.17487/RFC2818, May 2000, <https://www.rfc-editor.org/info/rfc2818>.

[RFC3986]

Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <https://www.rfc-editor.org/info/rfc3986>.

[RFC4648]

Josefsson, S., “The Base16, Base32, and Base64 Data Encodings”, RFC 4648, DOI 10.17487/RFC4648, October 2006, <https://www.rfc-editor.org/info/rfc4648>.

[RFC5895]

Resnick, P. and P. Hoffman, “Mapping Characters for Internationalized Domain Names in Applications (IDNA) 2008”, RFC 5895, DOI 10.17487/RFC5895, September 2010, <https://www.rfc-editor.org/info/rfc5895>.

[RFC6265]

Barth, A., “HTTP State Management Mechanism”, RFC 6265, DOI 10.17487/RFC6265, April 2011, <https://www.rfc-editor.org/info/rfc6265>.

[RFC7034]

Ross, D. and T. Gondrom, “HTTP Header Field X-Frame-Options”, RFC 7034, DOI 10.17487/RFC7034, October 2013, <https://www.rfc-editor.org/info/rfc7034>.

[RFC9113]

Thomson, M., Ed. and C. Benfield, Ed., “HTTP/2”, RFC 9113, DOI 10.17487/RFC9113, June 2022, <https://www.rfc-editor.org/info/rfc9113>.

[RFC9114]

Bishop, M., Ed., “HTTP/3”, RFC 9114, DOI 10.17487/RFC9114, June 2022, <https://www.rfc-editor.org/info/rfc9114>.

[UTS46]

Davis, M. and M. Suignard, “Unicode IDNA Compatibility Processing”, UNICODE Unicode Technical Standards # 46, June 2016, <http://unicode.org/reports/tr46/>.