Internet DRAFT - draft-west-http-state-tokens
draft-west-http-state-tokens
Network Working Group M. West
Internet-Draft Google
Intended status: Standards Track March 28, 2019
Expires: September 29, 2019
HTTP State Tokens
draft-west-http-state-tokens-00
Abstract
This document describes a mechanism which allows HTTP servers to
maintain stateful sessions with HTTP user agents. It aims to address
some of the security and privacy considerations which have been
identified in existing state management mechanisms, providing
developers with a well-lit path towards our current understanding of
best practice.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 29, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Wait. Don't we have cookies? . . . . . . . . . . . . . . 3
1.2. No. Really. We have cookies already. Why do we need this
new thing? . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Conformance . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Infrastructure . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. HTTP State Tokens . . . . . . . . . . . . . . . . . . . . 5
3.2. Requests and Responses . . . . . . . . . . . . . . . . . 6
3.3. Token Storage . . . . . . . . . . . . . . . . . . . . . . 7
4. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. The 'Sec-Http-State' HTTP Header Field . . . . . . . . . 8
4.2. The 'Sec-Http-State-Options' HTTP Header Field . . . . . 9
5. Delivering HTTP State Tokens . . . . . . . . . . . . . . . . 11
5.1. Attach HTTP State Tokens to a request . . . . . . . . . . 11
5.2. Generate a request's signature . . . . . . . . . . . . . 12
6. Configuring HTTP State Tokens . . . . . . . . . . . . . . . . 13
7. Security and Privacy Considerations . . . . . . . . . . . . . 15
7.1. Confidentiality and Integrity . . . . . . . . . . . . . . 15
7.2. Signed Sessions . . . . . . . . . . . . . . . . . . . . . 16
7.3. User Control . . . . . . . . . . . . . . . . . . . . . . 16
7.4. Lifetime . . . . . . . . . . . . . . . . . . . . . . . . 16
7.5. Ambient Authority and Cross-Site Delivery . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8.1. Header Field Registry . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 19
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 19
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 20
B.1. Since the beginning of time . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
This document defines a state-management mechanism for HTTP that
allows clients to create and persist origin-bound session identifiers
that can be delivered to servers in order to enable stateful
interaction. In a nutshell, each user agent will generate a single
token per secure origin, and will deliver it as a "Sec-Http-State"
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structured header along with requests to that origin (defined in
Section 4.1 and Section 5).
Servers can configure this token's characteristics via a "Sec-Http-
State-Options" response header (defined in Section 4.2 and
Section 6).
That's it.
1.1. Wait. Don't we have cookies?
Cookies [RFC6265] are indeed a pervasive HTTP state management
mechanism, and they enable practically everything interesting on the
web today. That said, cookies have some issues: they're hard to use
securely, they add substantial weight to users' outgoing requests,
and they enable tracking users' activity across the web in
potentially surprising ways.
The mechanism proposed in this document aims at a more minimal and
opinionated construct which takes inspiration from some of cookies'
optional characteristics. In particular:
1. The client controls the token's value, not the server.
2. The token will only be available to the network layer, not to
JavaScript (including network-like JavaScript, such as Service
Workers).
3. The user agent will generate only one token per origin, and will
only expose the token to the origin for which it was generated.
4. Tokens will not be generated for, or delivered to, non-secure
origins.
5. Tokens will be delivered only along with same-site requests by
default, and can only be created from same-site contexts.
6. Each token persists for one hour after generation by default.
This default expiration time can be overwritten by servers, and
tokens can be reset at any time by servers, users, or user
agents.
These distinctions might not be appropriate for all use cases, but
seem like a reasonable set of defaults. For folks for whom these
defaults aren't good enough, we'll provide developers with a few
control points that can be triggered via a "Sec-HTTP-State-Options"
HTTP response header, described in Section 4.2.
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1.2. No. Really. We have cookies already. Why do we need this new
thing?
We do have cookies. And we've defined a number of extensions to
cookies to blunt some of their sharper edges: the "HttpOnly"
attribute, the "Secure" attribute, "SameSite", prefixes like
"__Host-" and "__Secure-", and so on. Isn't that the right way
forward? Shouldn't we just push developers towards these existing
flags on the existing state management primitive?
This document's underlying assumption is that it's going to be easier
to teach developers about a crazy new thing that's secure by default
than it would be to convince them to change their "Set-Cookie"
headers to include "__Host-name=value; HttpOnly; Secure;
SameSite=Lax; Path=/". A new thing resets expectations in a way that
vastly exceeds the impact of explanations about the the four
attributes that must be used, the one attribute that must not be
used, and the weird naming convention that ought to be adopted.
Moreover, it appears that we're collectively pretty bad at helping
developers understand the risks that might lead them to adopt The
Good Cookie Syntax(tm) above. Adoption of these features has been
quite slow. Based on data gathered from Chrome's telemetry in March,
2019, cookies are set as follows:
o ~6.8% of cookies are set with "HttpOnly".
o ~5.5% are set with "Secure".
o ~3.1% are set with "HttpOnly; Secure".
o ~0.06% are set with "SameSite=*; Secure".
o ~0.05% are set with "SameSite=*".
o ~0.03% are set with "HttpOnly; Secure; SameSite=*".
o ~0.006% are set with "SameSite=*; HttpOnly".
o ~0.005% are set with a "__Secure-" prefix.
o ~0.01% are set with a "__Host-" prefix.
In total:
o ~9.9% of cookies are marked as "HttpOnly".
o ~8.8% of cookies are marked as "Secure".
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o ~0.1% of cookies are marked as "SameSite".
o ~84.2% of cookies use none of these features.
Given that "Secure" has been around since at least 1997 [RFC2109];
~9% adoption after more than two decades is not inspiring.
1.3. Examples
User agents can deliver HTTP state tokens to a server in a "Sec-Http-
State" header. For example, if a user agent has generated a token
bound to "https://example.com/" whose base64 encoding is
"hB2RfWaGyNk60sjHze5DzGYjSnL7tRF2HWSBx6J1o4k=" ([RFC4648],
Section 4), then it would generate the following header when
delivering the token along with requests to "https://example.com/":
Sec-Http-State: token=*hB2RfWa...GyNko4k=*
The server can control certain aspects of the token's delivery by
responding to requests with a "Sec-Http-State-Options" header:
Sec-Http-State-Options: max-age=3600, key=*b7kuUkp...lkRioC2=*
2. Conventions
2.1. Conformance
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. Syntax
This document defines two Structured Headers
[I-D.ietf-httpbis-header-structure]. In doing so it relies upon the
Augmented Backus-Naur Form (ABNF) notation of [RFC5234] and the OWS
rule from [RFC7230].
3. Infrastructure
3.1. HTTP State Tokens
An HTTP State Token holds a session identifier which allows a user
agent to maintain a stateful session with a specific origin, along
with associated metadata:
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o "creation" is a timestamp representing the point in time when the
token was created.
o "delivery" specifies the initiating contexts from which the token
can be delivered. It is an enum of either "same-origin", "same-
site", or "cross-site". Unless otherwise specified, its value is
"same-site".
o "key" is a server-provided key which can be used to sign requests
with which the token is delivered. It is either null, or contains
up to 256-bits of binary data. Unless otherwise specified, its
value is null.
o "max-age" is a timestamp representing the token's lifetime in
seconds. Unless otherwise specified, HTTP State Tokens have a
3600 second (1 hour) "max-age".
o "value" is the token's value (surprising, right?). It contains up
to 256-bits of binary data.
An HTTP State Token is said to be "expired" if its "creation"
timestamp plus "max-age" seconds is in the past.
3.2. Requests and Responses
This document relies upon the definitions of "request" and "response"
found in [Fetch].
A request's delivery scope can be obtained as follows:
1. Let "request-origin" be the request's "origin", and "target-
origin" be the request's "URL"'s "origin".
2. If the request was generated by the user agent as a response to
direct user interaction with the user agent (e.g. the user typed
an address into the agent's address bar, clicked a bookmark, or
etc.), return "same-origin".
3. If "request-origin" is same-origin with "target-origin", return
"same-origin".
4. If "request-origin"'s registrable domain is the same as "target-
origin"'s registrable domain, return "same-site".
5. Return "cross-site".
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3.3. Token Storage
User agents MUST keep a list of all the unexpired HTTP State Tokens
which have been created. For the purposes of this document, we'll
assume that user agents keep this list in the form of a map whose
keys are origins, and whose values are HTTP State Tokens.
This map exposes three functions:
o An HTTP State Token can be stored for a given origin. If the
origin already exists in the map, the entry's value will be
overwritten with the new HTTP State Token.
o An origin's HTTP State Token can be retrieved. If the origin does
not exist in the map, "null" will be returned instead.
o An origin (along with its HTTP State Token) can be deleted from
the map.
The map is initially empty.
3.3.1. Generate an HTTP State Token for an origin
The user agent can generate a new HTTP State Token for an origin
using an algorithm equivalent to the following:
1. Delete "origin" from the user agent's token store.
2. Let "token" be a newly created HTTP State Token with its
properties set as follows:
* "creation": The current time.
* "delivery": "same-site"
* "key": null
* "max-age": 3600
* "value": 256 cryptographically random bits.
3. Store "token" in the user agent's token store for "origin".
4. If the user agent has defined a "NotifyHostHTTPStateReset()"
algorithm, call it with "origin".
5. Return "token".
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Note: Step 4 recognizes that user agents may wish to notify an
origin's developers that HTTP state has been reset in order to enable
cleanup of state stored client-side. HTML might, for instance, wish
to post a message to a specially-named "BroadcastChannel" to enable
this kind of work. This could take something like the following
form:
let resetChannel = new BroadcastChannel('http-state-reset'));
resetChannel.onmessage = e => { /* Do exciting cleanup here. */ };
4. Syntax
4.1. The 'Sec-Http-State' HTTP Header Field
The "Sec-Http-State" HTTP header field allows user agents to deliver
HTTP state tokens to servers as part of an HTTP request.
"Sec-Http-State" is a Structured Header
[I-D.ietf-httpbis-header-structure]. Its value MUST be a dictionary
([I-D.ietf-httpbis-header-structure], Section 3.1). Its ABNF is:
Sec-Http-State = sh-dictionary
The dictionary MUST contain:
o Exactly one member whose key is "token", and whose value is binary
content ([I-D.ietf-httpbis-header-structure], Section 3.9) that
encodes the HTTP state token's value for the origin to which the
header is delivered.
If the "token" member contains more than 256 bits of binary
content, the member MUST be ignored.
The dictionary MAY contain:
o Exactly one member whose key is "sig", and whose value is binary
content ([I-D.ietf-httpbis-header-structure], Section 3.9) that
encodes a signature over the token and the request which contains
it, using a key previously delivered by the server. This
mechanism is described in Section 5.2.
If the "sig" member contains more than 256 bits of binary content,
the member MUST be ignored.
The "Sec-Http-State" header is parsed per the algorithm in
Section 4.2 of [I-D.ietf-httpbis-header-structure]. Servers MUST
ignore the header if parsing fails, or if the parsed header does not
contain a member whose key is "token".
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User agents will attach a "Sec-Http-State" header to outgoing
requests according to the processing rules described in Section 5.
4.2. The 'Sec-Http-State-Options' HTTP Header Field
The "Sec-Http-State-Options" HTTP header field allows servers to
deliver configuration information to user agents as part of an HTTP
response.
"Sec-Http-State-Options" is a Structured Header
[I-D.ietf-httpbis-header-structure]. Its value MUST be a dictionary
([I-D.ietf-httpbis-header-structure], Section 3.1). Its ABNF is:
Sec-Http-State-Options = sh-dictionary
The "Sec-Http-State-Options" header is parsed per the algorithm in
Section 4.2 of [I-D.ietf-httpbis-header-structure]. User agents MUST
ignore the header if parsing fails.
The dictionary MAY contain:
o Exactly one member whose key is "key", and whose value is binary
content ([I-D.ietf-httpbis-header-structure], Section 3.10) that
encodes an key which can be used to generate a signature over
outgoing requests.
o Exactly one member whose key is "delivery", and whose value is one
of the following tokens ([I-D.ietf-httpbis-header-structure],
Section 3.9): "same-origin", "same-site", or "cross-site".
If the "delivery" member contains an unknown identifier, the
member MUST be ignored.
o Exactly one member whose key is "max-age", and whose value is an
integer ([I-D.ietf-httpbis-header-structure], Section 3.6)
representing the server's desired lifetime for its HTTP State
Token.
If the "max-age" member contains anything other than a positive
integer, the member MUST be ignored.
User agents will process the "Sec-Http-State-Options" header on
incoming responses according to the processing rules described in
Section 6.
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4.2.1. Examples
4.2.1.1. Cross-Site Delivery
Some servers will require access to their tokens from cross-site
contexts (perhaps to support authenticated activity or single-sign
on, etc). These servers can request a "cross-site" delivery option
by delivering the following header:
Sec-Http-State-Options: delivery=cross-site, ...
4.2.1.2. Token Lifetime
Other servers might want their sessions to persist for more than an
hour. These servers can request a more reasonable token lifetime
lifetime by by delivering the following header:
Sec-Http-State-Options: max-age=2592000, ...
Servers may also wish to explicitly trigger the token's expiration
(upon signout, for instance). Setting a "max-age" of "0" does the
trick:
Sec-Http-State-Options: max-age=0, ...
4.2.1.3. Token Provenance
For some servers, the client-generated token will be enough to
maintain state. They can treat it as an opaque session identifier,
and bind the user's state to it server-side. Other servers will
require additional assurance that they can trust the token's
provenance. To that end, servers can generate a unique key,
associate it with the session identifier on the server, and deliver
it to the client via an HTTP response header:
Sec-Http-State-Options: key=*ZH0GxtBMWA...nJudhZ8dtz*, ...
Clients will store that key, and use it to generate a signature over
some set of data that mitigates the risk of token capture:
Sec-HTTP-State:
token=*J6BRKa...MonM*,
sig=*(HMAC-SHA265(key, token+metadata))*
Note: This part in particular is not fully baked, and we need to do
some more work to flesh out the threat model (see also Token
Binding). Look at it as an area to explore, not a solidly thought-
out solution.
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5. Delivering HTTP State Tokens
User agents deliver HTTP state tokens to servers by appending a "Sec-
Http-State" header field to outgoing requests.
This specification provides algorithms which are called at the
appropriate points in [Fetch] in order to attach "Sec-Http-State"
headers to outgoing requests, and to ensure that "Sec-Http-State-
Options" headers are correctly processed.
5.1. Attach HTTP State Tokens to a request
The user agent can attach HTTP State Tokens to a given request using
an algorithm equivalent to the following. This algorithm is intended
to execute as the request is being sent out over the network (after
Service Worker processing), perhaps after the "Cookie" header is
handled in step 5.17.1 of Section 4.5 of [Fetch], describing the
"HTTP-network-or-cache fetch" algorithm:
1. If the user agent is configured to suppress explicit identifiers
for the request, or if the request's URL is not _a priori_
authenticated [Mixed-Content], then skip the remaining steps in
this algorithm, and return without modifying the request.
2. Let "target-origin" be the origin of "request"'s current URL.
3. Let "request-token" be the result of retrieving origin's token
from the user agent's token store, or "null" if no such token
exists.
4. If "request-token" is expired, clear the user agent's token
store for "target-origin", and set "request-token" to "null".
5. If "request-token" is "null", then:
1. If "request"'s delivery scope is "cross-site", return
without modifying the request.
Note: As the default "delivery" for HTTP State Tokens is
"same-site", we return early rather than generating a token
for a cross-site request.
2. Set "request-token" to the result of generating an HTTP
State Token for "target-origin", as defined in
Section 3.3.1.
6. Return without modifying the request if either of the following
statements are true:
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* "request-token"'s "delivery" is "same-origin", and
"request"'s delivery scope is not "same-origin".
* "request-token"'s "delivery" is "same-site", and "request"'s
delivery scope is neither "same-origin" nor "same-site".
7. Let "serialized-value" be the base64 encoding ([RFC4648],
Section 4) of "request-token"'s value.
8. Insert a member into "header-value" whose key is "token" and
whose value is "serialized-value".
9. If "request-token"'s "key" is not null, then insert a member
into "header-value" whose key is "sig", and whose value is the
result of executing Section 5.2 on request, "serialized-value",
and "request-token"'s "key".
10. Append a header to "request"'s header list whose name is "Sec-
Http-State", and whose value is the result of serializing
"header-value" ([I-D.ietf-httpbis-header-structure],
Section 4.1).
5.2. Generate a request's signature
If the origin server provides a "key", the user agent will use it to
sign any outgoing requests which target that origin and include an
HTTP State Token. Note that the signature is produced before adding
the "Sec-Http-State" header to the request.
Given a request, a base64-encoded token value, and a key:
1. Let "cbor-request" be the result of building a CBOR
representation [RFC7409] of the given request, as specified in
the first element of the array described in Section 3.2 of
[I-D.yasskin-http-origin-signed-responses].
2. Add an item to "cbor-request" which maps the byte string ':token'
to the byte string containing the given base64-encoded token
value.
3. Return the result of computing HMAC-SHA256 [RFC2104] over the
canonical CBOR serialization of "cbor-request" (Section 3.4 of
[I-D.yasskin-http-origin-signed-responses]), using the given
"key".
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5.2.1. Example
The following request:
GET / HTTP/1.1
Host: example.com
Accept: */*
results in the following CBOR representation (represented using the
extended diagnostic notation from Appendix G of
[I-D.ietf-cbor-cddl]):
{
':method': 'GET',
':token': 'hB2RfWaGyNk60sjHze5DzGYjSnL7tRF2HWSBx6J1o4k='
':url': 'https://example.com/',
'accept': '*/*',
}
6. Configuring HTTP State Tokens
Servers configure the HTTP State Token representing a given users'
state by appending a "Sec-Http-State-Options" header field to
outgoing responses.
User agents MUST process this header on a given response as per the
following algorithm, which is intended to be called after the "Set-
Cookie" header is handled in step 11.4 of Section 4.6 of [Fetch],
which defines the "HTTP-network fetch" algorithm.
1. Let "response-origin" be the origin of response's URL.
2. If the response's URL is not _a priori_ authenticated
[Mixed-Content], return without altering "response-origin"'s HTTP
State Token.
3. Let "token" be the result of retrieving "response-origin"'s token
from the user agent's token store, or "null" if no such token
exists.
4. If "token" is expired, clear the user agent's token store for
"response-origin", and set "token" to "null".
5. If "token" is "null", then:
1. If "request"'s delivery scope is "cross-site", return without
modifying the request.
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Note: As the default "delivery" for HTTP State Tokens is
"same-site", we return early rather than generating a token
for a cross-site request.
2. Set "token" to the result of generating an HTTP State Token
for "target-origin", as defined in Section 3.3.1.
6. If the response's header list contains "Sec-Http-State-Options",
then:
1. Let "header" be the result of getting response's "Sec-Http-
State-Options" header, and parsing parsing it per the
algorithm in Section 4.2 of
[I-D.ietf-httpbis-header-structure].
2. Return without altering "response-origin"'s HTTP State Token
if any of the following conditions hold:
+ Parsing the header results in failure.
+ "header" has a member named "key" whose value is not a
byte sequence (Section 3.10 of
[I-D.ietf-httpbis-header-structure])
+ "header" has a member named "delivery" whose value is not
one of the following tokens (Section 3.9 of
[I-D.ietf-httpbis-header-structure]): "same-origin",
"same-site", and "cross-site".
+ "header" has a member named "max-age" whose value is not a
positive integer (Section 3.6 of
[I-D.ietf-httpbis-header-structure]).
3. If "header" has a member named "key", set "token"'s "key" to
the member's value.
4. If "header" has a member named "delivery", set "token"'s
"delivery" to the member's value.
5. If "header" has a member named "max-age":
1. If the member's value is "0", generate a new HTTP State
Token for "response-origin" as defined in Section 3.3.1.
Otherwise, set "token"'s "max-age" to the member's value.
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Note that "max-age" is processed last, meaning that any other
options specified alongside "max-age=0" will be de facto
ignored as a new token is generated, replacing the old.
7. Security and Privacy Considerations
HTTP State Tokens aim to mitigate some of the security and privacy
drawbacks that decades of implementation experience with cookies have
laid bare. It would be worthwhile to skim through the privacy
considerations (Section 7 of [RFC6265]) and security considerations
(Section 8 of [RFC6265]) of that existing state management mechanism,
as it forms a foundation upon which this document builds.
7.1. Confidentiality and Integrity
HTTP State Tokens improve upon cookies' weak confidentiality/
integrity guarantees (see Sections 8.3, 8.5, 8.6, and 8.7 of
[RFC6265]) in several ways:
1. User agents MUST require secure channels (such as TLS) for
delivery and configuration of HTTP State Tokens. User agents
cannot be induced to deliver an origin's tokens across channels
visible to (and modifiable by) network attackers, nor can an
attack on DNS cause tokens to be revealed (as any server to which
the user could be directed will also need to authenticate itself,
which is presumably difficult).
2. HTTP State Tokens are mapped to origins, matching developers
expectations for client-side data generally. This ensures that
tokens are isolated by host and port: code running on
"https://bar.example.com/" cannot alter state on
"https://foo.example.com/" without the latter's cooperation, and
that the same applies to "https://example.com:8000/" and
"https://example.com:80/".
Note that this origin binding means that there are no path
restrictions for tokens. Servers relying upon these tokens for
state management SHOULD NOT run mutually distrusting services on
different paths of the same origin.
3. User agents MUST NOT expose HTTP State Tokens to non-HTTP APIs
which are web-accessible, thereby reducing the risk of accidental
exposure via cross-site scripting attack.
Further, the "Sec-" prefix on both "Sec-HTTP-State" and "Sec-
HTTP-State-Options" ensures that both are considered "forbidden
header names" by [Fetch]. The latter should also be treated as a
"forbidden response header".
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7.2. Signed Sessions
HTTP State Tokens embrace the session identifier pattern discussed in
Section 8.4 of [RFC6265] by requiring that the client control the
token's value, setting it to a fixed-length, random byte sequence.
The client's control mitigates the risk of sensitive information
being stored in the token directly, and the token's length makes it
unlikely to be easily guessed.
Some servers will be interested in proving the token's provenance
over time, which they do today by storing cookies with signed values.
Since storing a signed value directly is impossible in a client-
controlled world, servers can instead store a "key", which is used to
sign outgoing requests. Since this key is never exposed directly to
the web, it provides a reasonable guarantee of client stability over
time which a server can rely upon when making risk judgements.
7.3. User Control
User agents MUST provide users with the ability to control the
creation and distribution of HTTP State Tokens, just as they do for
cookies today. This certainly means providing controls over first-
vs third-party distribution, control over the origins which can store
state, control over the state presented to origins, visibility into
the state of the user agent's token store, and etc.
Further, this document grants user agents wide latitude to experiment
with various distribution policies and limitations. The capabilities
offered by "delivery" and "max-age" should be considered upper bounds
on distribution, within which user agents are free to roam.
7.4. Lifetime
By default, HTTP State Tokens live for an hour, which is a compromise
between the reasonable desire of servers to maintain state across a
given user's session, and the privacy risks associated with long-
lived tokens stored on a user's disk.
Servers that desire a longer session lifetime can explicitly request
an extension, which the browser can choose to act on.
7.5. Ambient Authority and Cross-Site Delivery
HTTP State Tokens, like cookies, provide a form of ambient authority
(see Section 8.2 of [RFC6265]). By default, this authority is
limited to requests initiated by same-site actors, which serves as a
reasonable mitigation against some classes of attack (e.g.
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"https://evil.com/" making authenticated requests to
"https://example.com/").
Servers that desire to interact in an authenticated manner in cross-
site contexts are required to opt-into doing so by delivering an
appropriate "delivery" value in a "Sec-HTTP-State-Options" response
header. Servers which choose to do so SHOULD take reasonable
precautions, implementing CSRF tokens for sensitive actions, and
taking stock of the context from which a given request is initiated
(by examining incoming "Referrer", "Origin", and "Sec-Fetch-Site"
headers).
Further, tokens can only be created in same-origin or same-site
contexts, which means that cross-site identifier would only be
available after the relevant origin was visited in a same-site
context, and explicitly declared its tokens as being deliverable
cross-site (at which point the user agent is empowered to make some
decisions about how to handle that declaration).
8. IANA Considerations
8.1. Header Field Registry
This document registers the "Sec-Http-State" and "Sec-Http-State-
Options" header fields in the "Permanent Message Header Field Names"
registry located at https://www.iana.org/assignments/message-headers
[1].
8.1.1. Sec-Http-State Header Field
Header field name: Sec-Http-State
Applicable protocol: http
Status: experimental
Author/Change controller: IETF
Specification document(s): This document (see Section 4.1)
Related information: (empty)
8.1.2. Sec-Http-State-Options Header Field
Header field name: Sec-Http-State-Options
Applicable protocol: http
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Status: experimental
Author/Change controller: IETF
Specification document(s): This document (see Section 4.2)
Related information: (empty)
9. References
9.1. Normative References
[Fetch] van Kesteren, A., "Fetch", n.d.,
<https://fetch.spec.whatwg.org/>.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-09 (work in progress),
December 2018.
[I-D.yasskin-http-origin-signed-responses]
Yasskin, J., "Signed HTTP Exchanges", draft-yasskin-http-
origin-signed-responses-05 (work in progress), January
2019.
[Mixed-Content]
West, M., "Mixed Content", n.d.,
<https://w3c.github.io/webappsec-mixed-content/>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[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>.
[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>.
[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>.
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[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>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7409] Haleplidis, E. and J. Halpern, "Forwarding and Control
Element Separation (ForCES) Packet Parallelization",
RFC 7409, DOI 10.17487/RFC7409, November 2014,
<https://www.rfc-editor.org/info/rfc7409>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[I-D.abarth-cake]
Barth, A., "Origin Cookies", draft-abarth-cake-01 (work in
progress), March 2011.
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR and JSON data structures", draft-ietf-cbor-
cddl-08 (work in progress), March 2019.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
9.3. URIs
[1] https://www.iana.org/assignments/message-headers
Appendix A. Acknowledgements
This document owes much to Adam Barth's [I-D.abarth-cake] and
[RFC6265].
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Appendix B. Changes
_RFC Editor: Please remove this section before publication._
B.1. Since the beginning of time
o This document was created.
Author's Address
Mike West
Google
Email: mkwst@google.com
URI: https://www.mikewest.org/
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