Network Working Group J. Hodges
Internet-Draft PayPal
Intended status: Standards Track C. Jackson
Expires: July 8, 2011 Carnegie Mellon University
A. Barth
Google, Inc.
Jan 4, 2011
HTTP Strict Transport Security (HSTS)
draft-ietf-websec-strict-transport-sec-00
Abstract
This specification defines a mechanism enabling Web sites to declare
themselves accessible only via secure connections, and/or for users
to be able to direct their user agent(s) to interact with given sites
only over secure connections. This overall policy is referred to as
HTTP Strict Transport Security (HSTS). The policy is declared by Web
sites via the Strict-Transport-Security HTTP Response Header Field,
and/or by other means, e.g. user agent configuration.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 8, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Strict Transport Security Policy Effects . . . . . . . . . 5
2.3. Threat Model . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Threats Addressed . . . . . . . . . . . . . . . . . . 5
2.3.1.1. Passive Network Attackers . . . . . . . . . . . . 6
2.3.1.2. Active Network Attackers . . . . . . . . . . . . . 6
2.3.1.3. Web Site Development and Deployment Bugs . . . . . 6
2.3.2. Threats Not Addressed . . . . . . . . . . . . . . . . 7
2.3.2.1. Phishing . . . . . . . . . . . . . . . . . . . . . 7
2.3.2.2. Malware and Browser Vulnerabilities . . . . . . . 7
2.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7
2.4.1. Overall Requirement . . . . . . . . . . . . . . . . . 7
2.4.1.1. Detailed Core Requirements . . . . . . . . . . . . 8
2.4.1.2. Detailed Ancillary Requirements . . . . . . . . . 9
3. Conformance Criteria . . . . . . . . . . . . . . . . . . . . . 9
3.1. Document Conventions . . . . . . . . . . . . . . . . . . . 9
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Strict-Transport-Security HTTP Response Header Field . . . 12
5.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Server Processing Model . . . . . . . . . . . . . . . . . . . 14
6.1. HTTP-over-Secure-Transport Request Type . . . . . . . . . 15
6.2. HTTP Request Type . . . . . . . . . . . . . . . . . . . . 15
7. User Agent Processing Model . . . . . . . . . . . . . . . . . 16
7.1. Strict-Transport-Security Response Header Field
Processing . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1.1. Noting a HSTS Server . . . . . . . . . . . . . . . . . 17
7.1.2. Known HSTS Server Domain Name Matching . . . . . . . . 17
7.2. URI Loading . . . . . . . . . . . . . . . . . . . . . . . 18
7.3. Errors in Secure Transport Establishment . . . . . . . . . 18
7.4. HTTP-Equiv <Meta> Element Attribute . . . . . . . . . . . 19
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8. Domain Name ToASCII Conversion Operation . . . . . . . . . . . 19
9. Server Implementation Advice . . . . . . . . . . . . . . . . . 19
10. UA Implementation Advice . . . . . . . . . . . . . . . . . . . 20
11. Constructing an Effective Request URI . . . . . . . . . . . . 21
12. Security Considerations . . . . . . . . . . . . . . . . . . . 23
12.1. Denial of Service (DoS) . . . . . . . . . . . . . . . . . 23
12.2. Bootstrap MITM Vulnerability . . . . . . . . . . . . . . . 23
12.3. Network Time Attacks . . . . . . . . . . . . . . . . . . . 23
12.4. Bogus Root CA Certificate Phish plus DNS Cache
Poisoning Attack . . . . . . . . . . . . . . . . . . . . . 23
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
14. Design Decision Notes . . . . . . . . . . . . . . . . . . . . 24
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
15.1. Normative References . . . . . . . . . . . . . . . . . . . 25
15.2. Informative References . . . . . . . . . . . . . . . . . . 26
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 27
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
[ Please disscuss this draft on the hasmat@ietf.org mailing list
[HASMAT]. ]
The HTTP protocol [RFC2616] may be used over various transports,
typically the Transmission Control Protocol (TCP) [RFC0793].
However, TCP does not provide channel integrity protection,
confidentiality, nor secure server identification. Thus the Secure
Sockets Layer (SSL) protocol [I-D.ietf-tls-ssl-version3] and its
successor Transport Layer Security (TLS) [RFC4346], were developed in
order to provide channel-oriented security, and are typically layered
between application protocols and TCP. [RFC2818] specifies how HTTP
is layered onto TLS, and defines the Universal Resource Identifier
(URI) scheme of "https" (in practice however, HTTP user agents (UAs)
typically offer their users choices among SSL2, SSL3, and TLS for
secure transport). URIs themselves are specified in [RFC3986].
UAs employ various local security policies with respect to the
characteristics of their interactions with web resources depending on
(in part) whether they are communicating with a given web resource
using HTTP or HTTP-over-a-Secure-Transport. For example, cookies
([RFC2109] and [RFC2965]) may be flagged as Secure. UAs are to send
such Secure cookies to their addressed server only over a secure
transport. This is in contrast to non-Secure cookies, which are
returned to the server regardless of transport (although modulo other
rules).
UAs typically annunciate to their users any issues with secure
connection establishment, such as being unable to validate a server
certificate trust chain, or if a server certificate is expired, or if
a server's domain name appears incorrectly in the server certificate
(see section 3.1 of [RFC2818]). Often, UAs enable users to elect to
continue to interact with a web resource in the face of such issues.
This behavior is sometimes referred to as "click(ing) through"
security [GoodDhamijaEtAl05] [SunshineEgelmanEtAl09], and thus can be
described as "click-through insecurity" .
Jackson and Barth proposed an approach, in [ForceHTTPS], to enable
web sites and/or users to declare that such issues are to be treated
as fatal and without direct user recourse. The aim is to prevent
users from unintentionally downgrading their security.
This specification embodies and refines the approach proposed in
[ForceHTTPS], e.g. a HTTP response header field is used to convey
site policy to the UA rather than a cookie.
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2. Overview
This section discusses the use cases, summarizes the HTTP Strict
Transport Security (HSTS) policy, and continues with a discussion of
the threat model, non-addressed threats, and derived requirements.
2.1. Use Cases
The high-level use case is a combination of:
o Web browser user wishes to discover, or be introduced to, and/or
utilize various web sites (some arbitrary, some known) in a secure
fashion.
o Web site deployer wishes to offer their site in an explicitly
secure fashion for both their own, as well as their users',
benefit.
2.2. Strict Transport Security Policy Effects
The characteristics of the HTTP Strict Transport Security policy, as
applied by a UA in its interactions with a web site wielding HSTS
Policy, known as a HSTS Server, is summarized as follows:
1. All insecure ("http") connections to a HSTS Server are redirected
by the HSTS Server to be secure connections ("https").
2. The UA terminates, without user recourse, any secure transport
connection attempts upon any and all secure transport errors or
warnings, including those caused by a site presenting self-signed
certificates.
3. UAs transform insecure URI references to a HSTS Server into
secure URI references before dereferencing them.
2.3. Threat Model
HSTS is concerned with three threat classes: passive network
attackers, active network attackers, and imperfect web developers.
However, it is explicitly not a remedy for two other classes of
threats: phishing and malware. Addressed and not addressed threats
are briefly discussed below. Readers may wish refer to [ForceHTTPS]
for details as well as relevant citations.
2.3.1. Threats Addressed
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2.3.1.1. Passive Network Attackers
When a user browses the web on a local wireless network (e.g. an
802.11-based wireless local area network) a nearby attacker can
possibly eavesdrop on the user's unencrypted Internet Protocol-based
connections, such as HTTP, regardless of whether or not the local
wireless network itself is secured [BeckTews09]. Freely available
wireless sniffing toolkits, e.g. [Aircrack-ng], enable such passive
eavesdropping attacks. A passive network attacker using such tools
can steal session identifiers and hijack the user's web session(s),
by obtaining cookies containing authentication credentials for
example [ForceHTTPS].
To mitigate such threats, some Web sites support, but usually do not
force, access using end-to-end secure transport -- e.g. signaled
through URIs constructed with the "https" scheme [RFC2818]. This can
lead users to believe that accessing such services using secure
transport protects them from passive network attackers.
Unfortunately, this is often not the case in real-world deployments
as session identifiers are often stored in non-Secure cookies to
permit interoperability with versions of the service offered over
insecure transport ("Secure cookes" are those cookies containing the
"Secure" attribute [RFC2109]). For example, if the session
identifier for a web site (an email service, say) is stored in a non-
Secure cookie, it permits an attacker to hijack the user's session if
the user's UA makes a single insecure HTTP request to the site.
2.3.1.2. Active Network Attackers
A determined attacker can mount an active attack, either by
impersonating a user's DNS server or, in a wireless network, by
spoofing network frames or offering a similarly-named evil twin
access point. If the user is behind a wireless home router, an
attacker can attempt to reconfigure the router using default
passwords and other vulnerabilities. Some sites, such as banks, rely
on end-to-end secure transport to protect themselves and their users
from such active attackers. Unfortunately, browsers allow their
users to easily opt-out of these protections in order to be usable
for sites that incorrectly deploy secure transport, for example by
generating and self-signing their own certificates (without also
distributing their CA certificate to their users' browsers).
2.3.1.3. Web Site Development and Deployment Bugs
The security of an otherwise uniformly secure site (i.e. all of its
content is materialized via "https" URIs), can be compromised
completely by an active attacker exploiting a simple mistake, such as
the loading of a cascading style sheet or a SWF movie over an
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insecure connection (both cascading style sheets and SWF movies can
script the embedding page, to the surprise of many web developers --
most browsers do not issue mixed content warnings when insecure SWF
files are embedded). Even if the site's developers carefully
scrutinize their login page for mixed content, a single insecure
embedding anywhere on the site compromises the security of their
login page because an attacker can script (control) the login page by
injecting script into the page with mixed content.
Note: "Mixed content" here refers to the same notion referred to as
"mixed security context" later elsewhere in this
specification.
2.3.2. Threats Not Addressed
2.3.2.1. Phishing
Phishing attacks occur when an attacker solicits authentication
credentials from the user by hosting a fake site located on a
different domain than the real site, perhaps driving traffic to the
fake site by sending a link in an email message. Phishing attacks
can be very effective because users find it difficult to distinguish
the real site from a fake site. HSTS is not a defense against
phishing per se; rather, it complements many existing phishing
defenses by instructing the browser to protect session integrity and
long-lived authentication tokens [ForceHTTPS].
2.3.2.2. Malware and Browser Vulnerabilities
Because HSTS is implemented as a browser security mechanism, it
relies on the trustworthiness of the user's system to protect the
session. Malicious code executing on the user's system can
compromise a browser session, regardless of whether HSTS is used.
2.4. Requirements
This section identifies and enumerates various requirements derived
from the use cases and the threats discussed above, and lists the
detailed core requirements HTTP Strict Transport Security addresses,
as well as ancillary requirements that are not directly addressed.
2.4.1. Overall Requirement
o Minimize the risks to web browser users and web site deployers
that are derived from passive and active network attackers, web
site development and deployment bugs, as well as insecure user
actions.
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2.4.1.1. Detailed Core Requirements
These core requirements are derived from the overall requirement, and
are addressed by this specification.
1. Web sites need to be able to declare to UAs that they should be
interacted with using a strict security policy.
2. Web sites need to be able to instruct UAs that contact them
insecurely to do so securely.
3. UAs need to note web sites that signal strict security policy
enablement, for a web site declared time span.
4. UAs need to re-write all insecure UA "http" URI loads to use the
"https" secure scheme for those web sites for which secure policy
is enabled.
5. Web site administrators need to be able to signal strict security
policy application to subdomains of higher-level domains for
which strict security policy is enabled, and UAs need to enforce
such policy.
6. For example, both example.com and foo.example.com could set
policy for bar.foo.example.com.
7. UAs need to disallow security policy application to peer domains,
and/or higher-level domains, by domains for which strict security
policy is enabled.
8. For example, neither bar.foo.example.com nor foo.example.com can
set policy for example.com, nor can bar.foo.example.com set
policy for foo.example.com. Also, foo.example.com cannot set
policy for sibling.example.com.
9. UAs need to prevent users from clicking-through security
warnings. Halting connection attempts in the face of secure
transport exceptions is acceptable.
Note: A means for uniformly securely meeting the first core
requirement above is not specifically addressed by this
specification (see Section 12.2 "Bootstrap MITM
Vulnerability"). It may be addressed by a future revision of
this specification or some other specification. Note also
that there are means by which UA implementations may more
fully meet the first core requirement, see Section 10 "UA
Implementation Advice".
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2.4.1.2. Detailed Ancillary Requirements
These ancillary requirements are also derived from the overall
requirement. They are not normatively addressed in this
specification, but could be met by UA implementations at their
implementor's discretion, although meeting these requirements may be
complex.
1. Disallow "mixed security context" (also known as "mixed-content")
loads (see section 5.3 "Mixed Content" in
[W3C.WD-wsc-ui-20100309]).
2. Facilitate user declaration of web sites for which strict
security policy is enabled, regardless of whether the sites
signal HSTS Policy.
3. Conformance Criteria
This specification is written for servers and user agents (UAs).
In this specification, the words MUST, MUST NOT, MAY, and SHOULD are
to be interpreted as described in [RFC2119].
A conformant server is one that implements all the requirements
listed in this specification that are applicable to servers.
A conformant user agent is one that implements all the requirements
listed in this specification that are applicable to user agents.
3.1. Document Conventions
Note: ..is a note to the reader. These are points that should be
expressly kept in mind and/or considered.
Warning: This is how a warning is shown. These are things that can
have suboptimal downside risks if not heeded.
[[XXXn: Some of the more major known issues are marked like this
(where "n" in "XXXn" is a number). --JeffH]]
[[TODOn: Things to fix (where "n" in "TODOn" is a number). --JeffH]]
4. Terminology
Terminology is defined in this section.
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ASCII case-insensitive comparison
means comparing two strings exactly, codepoint for
codepoint, except that the characters in the range
U+0041 .. U+005A (i.e. LATIN CAPITAL LETTER A to
LATIN CAPITAL LETTER Z) and the corresponding
characters in the range U+0061 .. U+007A (i.e.
LATIN SMALL LETTER A to LATIN SMALL LETTER Z) are
considered to also match. See [Unicode5] for
details.
codepoint is a colloquial contraction of Code Point, which is
any value in the Unicode codespace; that is, the
range of integers from 0 to 10FFFF(hex) [Unicode5].
Domain Name Domain Names, also referred to as DNS Names, are
defined in [RFC1035] to be represented outside of
the DNS protocol itself (and implementations
thereof) as a series of labels separated by dots,
e.g. "example.com" or "yet.another.example.org".
In the context of this specification, Domain Names
appear in that portion of a URI satisfying the reg-
name production in "Appendix A. Collected ABNF for
URI" in [RFC3986], and the host component from the
Host HTTP header field production in section 14.23
of [RFC2616].
Note: The Domain Names appearing in actual URI
instances and matching the aforementioned
production components may or may not be
FQDNs.
Domain Name Label is that portion of a Domain Name appearing "between
the dots", i.e. consider "foo.example.com": "foo",
"example", and "com" are all domain name labels.
Effective Request URI
is a URI that can be constructed by an HTTP server
for any given HTTP request sent to it. Some HTTP
requests do not contain a contiguous representation
of the URI identifying the resource being addressed
by the HTTP request. Rather, different portions of
a resource's URI may be mapped to both the Request-
Line header field and the Host header field in an
HTTP request message
[I-D.ietf-httpbis-p1-messaging]. The HTTP server
coalesces these URI fragments and constructs an
equivalent of the Request-URI that was used by the
UA to generate the received HTTP request message.
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See Section 11 "Constructing an Effective Request
URI", below.
FQDN is an acronym for Fully-qualified Domain Name. A
FQDN is a Domain Name that includes all higher
level domains relevant to the named entity
(typically a HSTS Server in the context of this
specification). If one thinks of the DNS as a
tree-structure with each node having its own Domain
Name Label, a FQDN for a specific node would be its
label followed by the labels of all the other nodes
between it and the root of the tree. For example,
for a host, a FQDN would include the label that
identifies the particular host, plus all domains of
which the host is a part, up to and including the
top-level domain (the root domain is always null)
[RFC1594].
HTTP Strict Transport Security
is the overall name for the combined UA- and
server-side security policy defined by this
specification.
HTTP Strict Transport Security Server
is a HTTP server implementing the server aspects of
the HSTS policy.
HTTP Strict Transport Security Policy
is the name of the combined overall UA- and server-
side facets of the behavior specified in this
specification.
HSTS See HTTP Strict Transport Security.
HSTS Policy See HTTP Strict Transport Security Policy.
HSTS Server See HTTP Strict Transport Security Server.
Known HSTS Server is a HSTS Server for which the UA has an HSTS
Policy in effect.
Local policy is comprised of policy rules deployers specify and
which are often manifested as "configuration
settings".
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MITM is an acronym for man-in-the-middle. See "man-in-
the-middle attack" in [RFC4949].
Request URI is the URI used to cause a UA to issue an HTTP
request message.
UA is a an acronym for user agent. For the purposes
of this specification, a UA is an HTTP client
application typically actively manipulated by a
user [RFC2616] .
5. Syntax
This section defines the syntax of the new header this specification
introduces. It also provides a short description of the function the
header.
The Section 6 "Server Processing Model" section details how servers
are to use this header. Likewise, the Section 7 "User Agent
Processing Model" section details how user agents are to use this
header.
5.1. Strict-Transport-Security HTTP Response Header Field
The Strict-Transport-Security HTTP response header field indicates to
a UA that it MUST enforce the HSTS Policy in regards to the server
emitting the response message containing this header field.
The ABNF syntax for the Strict-Transport-Security HTTP Response
Header field is:
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Strict-Transport-Security =
"Strict-Transport-Security" ":" OWS STS-v OWS
; value
STS-v = STS-d
/ STS-d *( OWS ";" OWS STS-d OWS )
; STS directive
STS-d = STS-d-cur / STS-d-ext
; defined STS directives
STS-d-cur = maxAge / includeSubDomains
maxAge = "max-age" OWS "=" OWS delta-seconds v-ext
includeSubDomains = [ "includeSubDomains" ] v-ext
; extension points
STS-d-ext = name ; STS extension directive
v-ext = value ; STS extension value
name = token
value = OWS / %x21-3A / %x3C-7E ; i.e. optional white space, or
; [ ! .. : ] [ < .. ~ ] any visible chars other than ";"
; productions imported from [ID.ietf-httpbis-p1-messaging]:
token
OWS ; Optional White Space
Note: [I-D.ietf-httpbis-p1-messaging] is used as the ABNF basis in
order to ensure that the new header has equivalent parsing
rules to the header fields defined in that same specification.
Also:
1. Quoted-string literals in the above ABNF stanza are
case-insensitive.
2. In order to correctly match the grammar above, the
Strict-Transport-Security HTTP Response Header MUST
include at least a max-age directive with at least a
single-digit value for delta-seconds.
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max-age specifies the number of seconds, after the recption of the
Strict-Transport-Security HTTP Response Header, during which
the UA regards the host the message was received from as a
Known HSTS Server (see also Section 7.1.1 "Noting a HSTS
Server", below). The delta-seconds production is specified
in [RFC2616].
[[TODO1: The above para wrt max-age may need further refinement.
--JeffH]]
includeSubDomains is a flag which, if present, signals to the UA that
the HSTS Policy applies to this HSTS Server as well
as any subdomains of the server's FQDN.
5.2. Examples
The below HSTS header field stipulates that the HSTS policy is to
remain in effect for one year (there are approximately 31 536 000
seconds in a year), and the policy applies only to the domain of the
HSTS Server issuing it:
Strict-Transport-Security: max-age=31536000
The below HSTS header field is stipulates that the HSTS policy is to
remain in effect for approximately six months and the policy applies
only to the domain of the issuing HSTS Server and all of its
subdomains:
Strict-Transport-Security: max-age=15768000 ; includeSubDomains
6. Server Processing Model
This section describes the processing model that HSTS Servers
implement. The model is comprised of two facets: the first being the
processing rules for HTTP request messages received over a secure
transport (e.g. TLS [RFC4346], SSL [I-D.ietf-tls-ssl-version3], or
perhaps others, the second being the processing rules for HTTP
request messages received over non-secure transports, i.e. over
TCP/IP [RFC0793].
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6.1. HTTP-over-Secure-Transport Request Type
When replying to an HTTP request that was conveyed over a secure
transport, a HSTS Server SHOULD include in its response message a
Strict-Transport-Security HTTP Response Header that MUST satisfy the
grammar specified above in Section 5.1 "Strict-Transport-Security
HTTP Response Header Field". If a Strict-Transport-Security HTTP
Response Header is included, the HSTS Server MUST include only one
such header.
Note: Including the Strict-Transport-Security HTTP Response Header
is stipulated as a "SHOULD" in order to accomodate various
server- and network-side caches and load-balancing
configurations where it may be difficult to uniformly emit
Strict-Transport-Security HTTP Response Headers on behalf of a
given HSTS Server.
Establishing a given host as a Known HSTS Server, in the
context of a given UA, MAY be accomplished over the HTTP
protocol by correctly returning, per this specification, at
least one valid Strict-Transport-Security HTTP Response Header
to the UA. Other mechanisms, such as a client-side pre-loaded
Known HSTS Server list MAY also be used. E.g. see Section 10
"UA Implementation Advice".
6.2. HTTP Request Type
If a HSTS Server receives a HTTP request message over a non-secure
transport, it SHOULD send a HTTP response message containing a
Status-Code of 301 and a Location header field value containing
either the HTTP request's original Effective Request URI (see
Section 11 "Constructing an Effective Request URI", below) altered as
necessary to have a URI scheme of "https", or a URI generated
according to local policy (which SHOULD employ a URI scheme of
"https").
Note: The above behavior is a "SHOULD" rather than a "MUST&"
because:
There are risks in server-side non-secure-to-secure
redirects [owaspTLSGuide].
Site deployment characteristics -- e.g. a site that
incorporates third-party components may not behave
correctly when doing server-side non-secure-to-secure
redirects in the case of being accessed over non-secure
transport, but does behave correctly when accessed
uniformly over secure transport. The latter is the
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case given a HSTS-capapble UA that has already noted
the site as a Known HSTS Server (by whatever means,
e.g. prior interaction or UA configuration).
[[XXX1: perhaps the "SHOULD" in the above behavior should be a "MAY"
given the reasons it's presently not a "MUST". --JeffH]]
A HSTS Server MUST NOT include the Strict-Transport-Security HTTP
Response Header in HTTP responses conveyed over a non-secure
transport.
7. User Agent Processing Model
This section describes the HTTP Strict Transport Security processing
model for UAs. There are several facets to the model, enumerated by
the following subsections.
Also, this processing model assumes that all Domain Names manipulated
in this specification's context are already in ASCII Compatible
Encoding (ACE) format as specified in [RFC3490]. If this is not the
case in some situation, use the operation given in Section 8 "Domain
Name ToASCII Conversion Operation" to convert any encountered
internationalized Domain Names to ACE format before processing them.
7.1. Strict-Transport-Security Response Header Field Processing
If an HTTP response, received over a secure transport, includes a
Strict-Transport-Security HTTP Response Header field, conforming to
the grammar specified in Section 5.1 "Strict-Transport-Security HTTP
Response Header Field" (above), and there are no underlying secure
transport errors or warnings, the UA MUST either:
o Note the server as a Known HSTS Server if it is not already so
noted (see Section 7.1.1 "Noting a HSTS Server", below),
or,
o Update its cached information for the Known HSTS Server if the
max-age and/or includeSubDomains header field value tokens are
conveying information different than that already maintained by
the UA.
Note: The max-age value is essentially a "time to live" value
relative to the reception time of the Strict-Transport-
Security HTTP Response Header.
[[TODO2: Decide UA behavior in face of encountering multiple HSTS
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headers in a message. Use first header? Last? --JeffH]]
Otherwise:
o If an HTTP response is received over insecure transport, the UA
MUST ignore any present Strict-Transport-Security HTTP Response
Header(s).
o The UA MUST ignore any Strict-Transport-Security HTTP Response
Headers not conforming to the grammar specified in Section 5.1
"Strict-Transport-Security HTTP Response Header Field" (above).
7.1.1. Noting a HSTS Server
If the substring matching the host production from the Request-URI,
that the server responded to, syntactically matches the IP-literal or
IPv4address productions from section 3.2.2 of [RFC3986], then the UA
MUST NOT note this server as a Known HSTS Server.
Otherwise, if the substring does not congruently match a presently
known HSTS Server, per the matching procedure specified in
Section 7.1.2 "Known HSTS Server Domain Name Matching" below, then
the UA MUST note this server as a Known HSTS Server, caching the HSTS
Server's Domain Name and noting along with it the expiry time of this
information, as effectively stipulated per the given max-age value,
as well as whether the includeSubDomains flag is asserted or not.
7.1.2. Known HSTS Server Domain Name Matching
A UA determines whether a Domain Name represents a Known HSTS Server
by looking for a match between the query Domain Name and the UA's set
of Known HSTS Servers.
1. Compare the query Domain Name string with the Domain Names of the
UA's set of Known HSTS Servers. For each Known HSTS Server's
Domain Name, the comparison is done with the query Domain Name
label-by-label using an ASCII case-insensitive comparison
beginning with the rightmost label, and continuing right-to-left,
and ignoring separator characters (see clause 3.1(4) of
[RFC3986].
* If a label-for-label match between an entire Known HSTS
Server's Domain Name and a right-hand portion of the query
Domain Name is found, then the Known HSTS Server's Domain Name
is a superdomain match for the query Domain Name.
For example:
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Query Domain Name: bar.foo.example.com
Superdomain matched
Known HSTS Server DN: foo.example.com
At this point, the query Domain Name is ascertained to
effectively represent a Known HSTS Server. There may also be
additional matches further down the Domain Name Label tree, up
to and including a congruent match.
* If a label-for-label match between a Known HSTS Server's
Domain Name and the query domain name is found, i.e. there are
no further labels to compare, then the query Domain Name
congruently matches this Known HSTS Server.
For example:
Query Domain Name: foo.example.com
Congruently matched
Known HSTS Server DN: foo.example.com
The query Domain Name is ascertained to represent a Known HSTS
Server. However, if there are also superdomain matches, the
one highest in the tree asserts the HSTS Policy for this Known
HSTS Server.
* Otherwise, if no matches are found, the query Domain Name does
not represent a Known HSTS Server.
7.2. URI Loading
Whenever the UA prepares to "load", also known as "dereference", any
URI where the host production of the URI [RFC3986] matches that of a
Known HSTS Server -- either as a congruent match or as a superdomain
match where the superdomain Known HSTS Server has includeSubDomains
asserted -- and the URI's scheme is "http", then the UA "MUST"
replace the URI scheme with "https" before proceeding with the load.
7.3. Errors in Secure Transport Establishment
When connecting to a Known HSTS Server, the UA MUST terminate the
connection with no user recourse if there are any errors (e.g.
certificate errors), whether "warning" or "fatal" or any other error
level, with the underlying secure transport.
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7.4. HTTP-Equiv <Meta> Element Attribute
UAs MUST NOT heed http-equiv="Strict-Transport-Security" attribute
settings on <meta> elements in received content.
8. Domain Name ToASCII Conversion Operation
This operation converts a string-serialized Domain Name possibly
containing arbitrary Unicode characters [Unicode5] into a string-
serialized Domain Name in ASCII Compatible Encoding (ACE) format as
specified in [RFC3490].
The operation is:
o Apply the IDNA conversion operation (section 4 of [RFC3490]) to
the string, selecting the ToASCII operation and setting both the
AllowUnassigned and UseSTD3ASCIIRules flags.
9. Server Implementation Advice
HSTS Policy expiration time considerations:
o Server implementations and deploying web sites need to consider
whether they are setting an expiry time that is a constant value
into the future, e.g. by constantly sending the same max-age value
to UAs. For exmple:
Strict-Transport-Security: max-age=778000
A max-age value of 778000 is 90 days. Note that each receipt of
this header by a UA will require the UA to update its notion of
when it must delete its knowledge of this Known HSTS Server. The
specifics of how this is accomplished is out of the scope of this
specification.
o Or, whether they are setting an expiry time that is a fixed point
in time, e.g. by sending max-age values that represent the
remaining time until the expiry time.
o A consideration here is whether a deployer wishes to have signaled
HSTS Policy expiry time match that for the web site's domain
certificate.
Considerations for using HTTP Strict Transport Security in
conjunction with self-signed public-key certificates:
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o If a web site/organization/enterprise is generating their own
secure transport public-key certificates for web sites, and that
organization's root certificate authority (CA) certificate is not
typically embedded by default in browser CA certificate stores,
and if HSTS Policy is enabled on a site identifying itself using a
self-signed certificate, then secure connections to that site will
fail without user recourse, per the HSTS design. This is to
protect against various active attacks, as discussed above.
o However, if said organization strongly wishes to employ self-
signed certificates, and their own CA in concert with HSTS, they
can do so by deploying their root CA certificate to their users'
browsers. There are various ways in which this can be
accomplished (details are out of scope for this specification).
Once their root CA cert is installed in the browsers, they may
employ HSTS Policy on their site(s).
Note: Interactively distributing root CA certs to users, e.g. via
email, and having the users install them, is arguably
training the users to be susceptible to a possible form of
phishing attack, see Section 12.4 "Bogus Root CA
Certificate Phish plus DNS Cache Poisoning Attack".
10. UA Implementation Advice
In order to provide users and web sites more effective protection, UA
implementors should consider including features such as:
o Disallowing "mixed security context" (also known as "mixed-
content") loads (see section 5.3 "Mixed Content" in
[W3C.WD-wsc-ui-20100309]).
Note: In order to provide behavioral uniformity across UA
implementations, the notion of mixed security context aka
mixed-content will require (further) standardization work,
e.g. to more clearly define the term(s) and to define
specific behaviors with respect to it.
In order to provide users effective controls for managing their UA's
caching of HSTS Policy, UA implementors should consider including
features such as:
o Ability to delete UA's cached HSTS Policy on a per HSTS Server
basis.
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Note: Adding such a feature should be done very carefully in both
the user interface and security senses. Deleting a cache
entry for a Known HSTS Server should be a very deliberate
and well-considered act -- it shouldn't be something users
get used to just "clicking through" in order to get work
done. Also, it shouldn't be possible for an attacker to
inject script into the UA that silently and
programmatically removes entries from the UA's cache of
Known HSTS Servers.
In order to provide users and web sites more complete protection, UAs
could offer advanced features such as these:
o Ability for users to explicitly declare a given Domain Name as
representing a HSTS Server, thus seeding it as a Known HSTS Server
before any actual interaction with it. This would help protect
against the Section 12.2 "Bootstrap MITM Vulnerability".
Note: Such a feature is difficult to get right on a per-site
basis -- see the discussion of "rewrite rules" in section
5.5 of [ForceHTTPS]. For example, arbitrary web sites may
not materialize all their URIs using the "https" scheme,
and thus could "break" if a UA were to attempt to access
the site exclusively using such URIs. Also note that this
feature would complement, but is independent of the
following described facility.
o Facility whereby web site administrators can have UAs pre-
configured with HSTS Policy for their site(s) by the UA vendor(s)
-- in a manner similar to how root CA certificates are embedded in
browsers "at the factory". This would help protect against the
Section 12.2 "Bootstrap MITM Vulnerability".
Note: Such a facility complements the preceding described
feature.
[[XXX2: These latter items beg the question of having some means of
secure web site metadata and policy discovery and acquisition. There
is extant work that may be of interest, e.g. the W3C POWDER work,
OASIS XRI/XRD work (as well as XRDS-Simple), and "Link-based Resource
Descriptor Discovery" (draft-hammer-discovery). --JeffH]]
11. Constructing an Effective Request URI
This section specifies how an HSTS Server must construct the
Effective Request URI for a received HTTP request.
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The first line of an HTTP request message is specified by the
following ABNF ([I-D.ietf-httpbis-p1-messaging] section 4.1):
Request-Line = Method SP request-target SP HTTP-Version CRLF
The request-target is following ABNF ([I-D.ietf-httpbis-p1-messaging]
section 4.1.2):
request-target = "*"
/ absolute-URI
/ ( path-absolute [ "?" query ] )
/ authority
Additionally, many HTTP requests contain an additional Host request
header field. It is specified by the following ABNF
([I-D.ietf-httpbis-p1-messaging] section 4.1.2):
Host = "Host:" OWS Host-v
Host-v = uri-host [ ":" port ]
Thus an example HTTP message containing the above header fields is:
GET /hello.txt HTTP/1.1
Host: www.example.com
Another example is:
GET HTTP://www.example.com/hello.txt HTTP/1.1
An HSTS Server constructs the Effective Request URI using the
following ABNF grammar (which imports some productions from the above
ABNF for Request-Line, request-target, and Host:
Effective-Request-URI = absolute-URI-present / path-absolute-form
absolute-URI-present = absolute-URI
path-absolute-form = scheme "://" Host-v path-absolute [ "?" query ]
where:
scheme is "http" if the request was received over
insecure transport, or scheme is "https" if the
request was received over secure transport.
For example, if the request message contains a request-target
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component that matches the grammar of absolute-URI, then the
Effective-Request-URI is simply the value of the absolute-URI
component. Otherwise, the Effective-Request-URI is a combination,
per the path-absolute-form production, of the Host-v, path-absolute,
and query components from the request-target and Host components of
the request message.
[[TODO3: This is a first SWAG at this section. Fix/add prose as
appropriate, fix ABNF as needed per review. --JeffH]]
12. Security Considerations
12.1. Denial of Service (DoS)
HSTS could be used to mount certain forms of DoS attacks, where
attackers set fake HSTS headers on legitimate sites available only
insecurely (e.g. social network service sites, wikis, etc.).
12.2. Bootstrap MITM Vulnerability
The bootstrap MITM (Man-In-The-Middle) vulnerability is a
vulnerability users and HSTS Servers encounter in the situation where
the user manually enters, or follows a link, to a HSTS Server using a
"http" URI rather than a "https" URI. Because the UA uses an
insecure channel in the initial attempt to interact with the
specified serve, such an initial interaction is vulnerable to various
attacks [ForceHTTPS] .
Note: There are various features/facilities that UA implementations
may employ in order to mitigate this vulnerability. Please
see Section 10 UA Implementation Advice.
12.3. Network Time Attacks
Active network attacks can subvert network time protocols (like NTP)
- making this header less effective against clients that trust NTP
and/or lack a real time clock. Network time attacks are therefore
beyond the scope of the defense. Note that modern operating systems
use NTP by default.
12.4. Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack
If an attacker can convince users of, say, https://bank.example.com
(which is protected by HSTS Policy), to install their own version of
a root CA certificate purporting to be bank.example.com's CA, e.g.
via a phishing email message with a link to such a certificate --
then, if they can perform an attack on the users' DNS, e.g. via cache
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poisoning, and turn on HSTS Policy for their fake bank.example.com
site, then they have themselves some new users.
13. IANA Considerations
Below is the Internet Assigned Numbers Authority (IANA) Provisional
Message Header Field registration information per [RFC3864].
Header field name: Strict-Transport-Security
Applicable protocol: HTTP
Status: provisional
Author/Change controller: TBD
Specification document(s): this one
14. Design Decision Notes
This appendix documents various design decisions.
1. Cookies aren't appropriate for HSTS Policy expression as they are
potentially mutable (while stored in the UA), therefore an HTTP
header field is employed.
2. We chose to not attempt to specify how "mixed security context
loads" (aka "mixed-content loads") are handled due to UA
implementation considerations as well as classification
difficulties.
3. A HSTS Server may update UA notions of HSTS Policy via new HSTS
header field values. We chose to have UAs honor the "freshest"
information received from a server because there is the chance of
a web site sending out an errornous HSTS Policy, such as a multi-
year max-age value, and/or an incorrect includeSubDomains flag.
If the HSTS Server couldn't correct such errors over protocol, it
would require some form of annunciation to users and manual
intervention on their part, which could be a non-trivial problem.
4. HSTS Servers are identified only via Domain Names -- explicit IP
address identification of all forms is excluded. This is for
simplification and also is in recognition of various issues with
using direct IP address identification in concert with PKI-based
security.
15. References
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15.1. Normative References
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-09 (work in
progress), March 2010.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1594] Marine, A., Reynolds, J., and G. Malkin, "FYI on Questions
and Answers - Answers to Commonly asked "New Internet
User" Questions", RFC 1594, March 1994.
[RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983,
August 1996.
[RFC2109] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2109, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2965] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2965, October 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
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September 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[Unicode5]
The Unicode Consortium, "The Unicode Standard, Version
5.0", Boston, MA, Addison-Wesley ISBN 0-321-48091-0, 2007.
[W3C.WD-html5-20100304]
Hyatt, D. and I. Hickson, "HTML5", World Wide Web
Consortium WD WD-html5-20100304, March 2010,
<http://www.w3.org/TR/2010/WD-html5-20100304>.
15.2. Informative References
[Aircrack-ng]
d'Otreppe, T., "Aircrack-ng", Accessed: 11-Jul-2010,
<http://www.aircrack-ng.org/>.
[BeckTews09]
Beck, M. and E. Tews, "Practical Attacks Against WEP and
WPA", Second ACM Conference on Wireless Network
Security Zurich, Switzerland, 2009, <http://
wirelesscenter.dk/Crypt/wifi-security-attacks/
Practical%20Attacks%20Against%20WEP%20and%20WPA.pdf>.
[ForceHTTPS]
Jackson, C. and A. Barth, "ForceHTTPS: Protecting High-
Security Web Sites from Network Attacks", In Proceedings
of the 17th International World Wide Web Conference
(WWW2008) , 2008,
<https://crypto.stanford.edu/forcehttps/>.
[GoodDhamijaEtAl05]
Good, N., Dhamija, R., Grossklags, J., Thaw, D.,
Aronowitz, S., Mulligan, D., and J. Konstan, "Stopping
Spyware at the Gate: A User Study of Privacy, Notice and
Spyware", In Proceedings of Symposium On Usable Privacy
and Security (SOUPS) Pittsburgh, PA, USA, July 2005, <http
://people.ischool.berkeley.edu/~rachna/papers/
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spyware_study.pdf>.
[HASMAT] "HASMAT -- HTTP Application Security Minus Authentication
and Transport",
<https://www.ietf.org/mailman/listinfo/hasmat>.
[I-D.ietf-tls-ssl-version3]
Freier, A., Karlton, P., and P. Kocher, "The SSL Protocol
Version 3.0", draft-ietf-tls-ssl-version3 (work in
progress), November 1996, <http://tools.ietf.org/html/
draft-ietf-tls-ssl-version3-00>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[SunshineEgelmanEtAl09]
Sunshine, J., Egelman, S., Almuhimedi, H., Atri, N., and
L. Cranor, "Crying Wolf: An Empirical Study of SSL Warning
Effectiveness", In Proceedings of 18th USENIX Security
Symposium Montreal, Canada, Augus 2009, <http://
www.usenix.org/events/sec09/tech/full_papers/
sunshine.pdf>.
[W3C.WD-wsc-ui-20100309]
Saldhana, A. and T. Roessler, "Web Security Context: User
Interface Guidelines", World Wide Web Consortium
LastCall WD-wsc-ui-20100309, March 2010,
<http://www.w3.org/TR/2010/WD-wsc-ui-20100309>.
[owaspTLSGuide]
Coates, M., Wichers, d., Boberski, M., and T. Reguly,
"Transport Layer Protection Cheat Sheet", Accessed: 11-
Jul-2010, <http://www.owasp.org/index.php/
Transport_Layer_Protection_Cheat_Sheet>.
Appendix A. Acknowledgments
The authors thank Michael Barrett, Sid Stamm, Maciej Stachowiak, Andy
Steingrubl, Brandon Sterne, Daniel Veditz for their review and
contributions.
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Appendix B. Change Log
Changes from -01 to -02:
1. updated abstract such that means for expressing HSTS Policy
other than via HSTS header field is noted.
2. Changed spec title to "HTTP Strict Transport Security (HSTS)"
from "Strict Transport Security". Updated use of "STS"
acronym throughout spec to HSTS (except for when specifically
discussing syntax of Strict-Transport-Security HTTP Response
Header field), updated "Terminology" appropriately.
3. Updated the discussion of "Passive Network Attackers" to be
more precise and offered references.
4. Removed para on nomative/non-normative from "Conformance
Criteria" pending polishing said section to IETF RFC norms.
5. Added examples subsection to "Syntax" section.
6. Added OWS to maxAge production in Strict-Transport-Security
ABNF.
7. Cleaned up explanation in the "Note:" in the "HTTP-over-
Secure-Transport Request Type" section, folded 3d para into
"Note:", added conformance clauses to the latter.
8. Added exaplanatory "Note:" and reference to "HTTP Request
Type" section. Added "XXX1" issue.
9. Added conformance clause to "URI Loading".
10. Moved "Notes for STS Server implementors:" from "UA
Implementation dvice " to "HSTS Policy expiration time
considerations:" in "Server Implementation Advice", and also
noted another option.
11. Added cautionary "Note:" to "Ability to delete UA's cached
HSTS Policy on a per HSTS Server basis".
12. Added some informative references.
13. Various minor editorial fixes.
Changes from -00 to -01:
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1. Added reference to HASMAT mailing list and request that this
spec be discussed there.
Authors' Addresses
Jeff Hodges
PayPal
2211 North First Street
San Jose, California 95131
US
Email: Jeff.Hodges@PayPal.com
Collin Jackson
Carnegie Mellon University
Email: collin.jackson@sv.cmu.edu
Adam Barth
Google, Inc.
Email: ietf@adambarth.com
URI: http://www.adambarth.com/
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