One document matched: draft-hodges-strict-transport-sec-01.xml
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<rfc category="std" ipr="trust200902"
docName="draft-hodges-strict-transport-sec-01">
<front>
<title>HTTP Strict Transport Security</title>
<author initials="J." surname="Hodges" fullname="Jeff Hodges">
<organization>PayPal</organization>
<address>
<email>Jeff.Hodges@PayPal.com</email>
</address>
</author>
<author initials="C." surname="Jackson" fullname="Collin Jackson" >
<organization>Carnegie Mellon University</organization>
<address>
<email>collin.jackson@sv.cmu.edu</email>
</address>
</author>
<author initials="A." surname="Barth" fullname="Adam Barth" >
<organization>University of
California Berkeley</organization>
<address>
<email>abarth@eecs.berkeley.edu</email>
</address>
</author>
<date month="June" year="2010"/>
<area>Applications</area>
<keyword>Internet-Draft</keyword>
<abstract>
<t>
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 Strict Transport Security
(STS). The policy is declared by Web sites via the
Strict-Transport-Security HTTP Response Header Field.
</t>
</abstract>
</front>
<middle>
<section title="Introduction" anchor="sec-intro">
<!-- <t>This section is non-normative.</t> -->
<t>
[ Please disscuss this draft on the hasmat@ietf.org
mailing list <xref target="HASMAT"/>. ]
</t>
<t>
The HTTP protocol <xref target="RFC2616" /> may be used over
various transports, typically the Transmission Control
Protocol (TCP) <xref target="RFC0793" />. However, TCP does
not provide channel integrity protection, confidentiality, nor
secure server identification. Thus the Secure Sockets Layer
(SSL) protocol <xref target="I-D.ietf-tls-ssl-version3" /> and
its successor Transport Layer Security (TLS) <xref
target="RFC4346" />, were developed in order to provide
channel-oriented security, and are typically layered between
application protocols and TCP. <xref target="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 <xref
target="RFC3986" />.
</t>
<t>
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 (<xref
target="RFC2109" /> and <xref target="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).
</t>
<t>
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 <xref
target="RFC2818" />). Often, UAs provide for users to be able
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 <xref
target="GoodDhamijaEtAl05" /> <xref
target="SunshineEgelmanEtAl09" />, and thus can be described
as "click-through insecurity" .
</t>
<t>
Jackson and Barth proposed an approach, in <xref
target="ForceHTTPS" />, to enable web sites and/or users to be
able 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.
</t>
<t>
This specification embodies and refines the approach proposed
in <xref target="ForceHTTPS" />, e.g. a HTTP response header
field is used to convey site policy to the UA rather than a
cookie.
</t>
</section>
<section anchor="sctn-overview" title="Overview">
<!-- <t> This section is non-normative. </t> -->
<t>
This section discusses the use cases, summarizes the Strict
Transport Security (STS) policy, and continues with a
discussion of the threat model, non-addressed threats, and
derived requirements.
</t>
<section anchor="sctn-use-cases" title="Use Cases">
<t>
The overall applicable use case here is a combination of
these two use cases:
</t>
<t>
<list style="symbols">
<t> Web browser user wishes to discover, or be introduced
to, and/or utilize various web sites (some arbitrary,
some known) in a secure fashion.
</t>
<t> Web site deployer wishes to offer their site in an
explicitly secure fashion for both their own, as well as
their users', benefit.
</t>
</list>
</t>
</section> <!-- sctn-use-cases -->
<section anchor="sctn-sts-policy-summary"
title="Strict Transport Security Policy Effects">
<t>
The characteristics of the Strict Transport Security policy,
as applied by a UA in its interactions with a web site
wielding STS Policy, known as a STS Server, is summarized as
follows:
</t>
<t>
<list style="numbers">
<t> Insecure ("http") connections to a STS Server
are redirected by the STS Server to be secure connections
("https").
</t>
<t>
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 wielding self-signed certificates.
</t>
<t> UAs transform insecure URI references to a STS Server
into secure URI references before dereferencing them.
</t>
</list>
</t>
</section> <!-- sctn-sts-policy-summary -->
<section anchor="sctn-threat-model" title="Threat Model">
<t>
STS 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 <xref target="ForceHTTPS" /> for
details as well as relevant citations.
</t>
<section anchor="sctn-threats-addr" title="Threats Addressed">
<section anchor="sctn-psv-net-atkr" title="Passive Network Attackers">
<t>
When a user browses the web on a wireless network, a
nearby attacker can eavesdrop on unencrypted
connections, such as HTTP requests. Such a passive
network attacker can steal session identifiers and
hijack the user's session, by obtaining cookies
containing authentication credentials for example. Such
passive eavesdropping attacks are easily performed using
wireless sniffing toolkits.
</t>
<t>
To mitigate this threat, some sites permit, but usually
do not force, access using secure transport -- e.g. by
employing "https" URIs. 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.
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 makes a single insecure HTTP request
to the site.
</t>
</section> <!-- sctn-psv-net-atkr -->
<section anchor="sctn-actv-net-atkr" title="Active Network Attackers">
<t>
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 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).
</t>
</section> <!-- sctn-actv-net-atkr -->
<section anchor="sctn-web-dvlp" title="Web Site Development and Deployment Bugs">
<t>
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 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.
</t>
<t>
<list style="hanging" hangIndent="7">
<t hangText="Note:">
"Mixed content" here refers to the same notion
referred to as "mixed security context" later
elsewhere in this specification.
</t>
</list>
</t>
</section> <!-- sctn-web-dvlp -->
</section> <!-- sctn-threats-addr -->
<section anchor="sctn-threats-not-addressed" title="Threats Not Addressed">
<section anchor="sctn-phishing" title="Phishing">
<t>
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. STS 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 <xref target="ForceHTTPS" />.
</t>
</section> <!-- sctn-phishing -->
<section anchor="sctn-malware" title="Malware and Browser Vulnerabilities">
<t>
Because STS 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 STS is used.
</t>
</section> <!-- sctn-malware -->
</section> <!-- sctn-threats-not-addressed -->
</section> <!-- sctn-threat-model -->
<section anchor="sctn-reqs" title="Requirements">
<t>
This section identifies and enumerates various
requirements derived from the use cases and the threats
discussed above, and lists the detailed core requirements
Strict Transport Security addresses, as well as ancillary
requirements that are not directly addressed.
</t>
<section anchor="sctn-reqs-ovrl-req" title="Overall Requirement">
<t>
<list style="symbols">
<t> 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.
</t>
</list>
</t>
<section anchor="sctn-reqs-core" title="Detailed Core Requirements">
<t>
These core requirements are derived from the overall
requirement, and are addressed by this specification.
</t>
<t>
<list style="numbers">
<!-- 1 -->
<t> Web sites need to be able to declare to UAs that
they should be interacted with using a strict security
policy.
</t>
<!-- 2 -->
<t> Web sites need to be able to instruct UAs that
contact them insecurely to do so securely.
</t>
<!-- 3 -->
<t> UAs need to note web sites that signal strict
security policy enablement, for a web site declared
time span.
</t>
<!-- 4 -->
<t> 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.
</t>
<!-- 5 -->
<t> 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.
</t>
<t>
For example, both example.com and foo.example.com
could set policy for bar.foo.example.com.
</t>
<!-- 6 -->
<t> UAs need to disallow security policy application to
peer domains, and/or higher-level domains, by domains
for which strict security policy is enabled.
</t>
<t>
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.
</t>
<!-- 7 -->
<t> UAs need to prevent users from clicking-through
security warnings. Halting connection attempts in the
face of secure transport exceptions is acceptable.
</t>
</list>
</t>
<t>
<list style="hanging" hangIndent="7">
<t hangText="Note:">
A means for uniformly securely meeting the first core
requirement above is not specifically addressed by
this specification (see <xref target="sctn-sec-cons-boot"/>
"<xref target="sctn-sec-cons-boot" format="title"/>").
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 <xref target="ua-impl-advice"/>
"<xref target="ua-impl-advice" format="title"/>".
</t>
</list>
</t>
</section> <!-- sctn-reqs-core -->
<section anchor="sctn-reqs-ancillary" title="Detailed Ancillary Requirements">
<t>
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.
</t>
<t>
<list style="numbers">
<t> Disallow "mixed security context" (also
known as "mixed-content") loads (see section
5.3 "Mixed Content" in <xref
target="W3C.WD-wsc-ui-20100309" />).
</t>
<t>
Facilitate user declaration of web sites for which
strict security policy is enabled, regardless of whether
the sites signal STS Policy.
</t>
</list>
</t>
</section> <!-- sctn-reqs-ancillary -->
</section> <!-- sctn-reqs-ovrl-req -->
</section> <!-- Requirements -->
</section>
<section anchor="sctn-conformance" title="Conformance Criteria">
<t>This specification is written for servers and user agents
(UAs).
</t>
<t>As well as sections and appendices marked as non-normative,
all diagrams, examples, and notes in this specification are
non-normative. Everything else in this specification is
normative.</t>
<t>In this specification, the words MUST, MUST NOT, MAY, and SHOULD
are to be interpreted as described
in <xref target="RFC2119" />.
</t>
<t>A conformant server is one that implements all the
requirements listed in this specification that are
applicable to servers.
</t>
<t>A conformant user agent is one that implements all the
requirements listed in this specification that are
applicable to user agents.
</t>
<section title="Document Conventions">
<t>
<list style="hanging" hangIndent="7">
<t hangText="Note:">
..is a note to the reader. These are points that should be
expressly kept in mind and/or considered.
</t>
</list>
<list style="hanging" hangIndent="10">
<t hangText="Warning:">
This is how a warning is shown.
These are things that can have suboptimal
downside risks if not heeded.
</t>
</list>
<cref anchor="XXXn" source="JeffH">
Some of the
more major known issues are marked like this
(where "n" in "XXXn" is a number).
</cref>
</t>
<t>
<cref anchor="TODOn" source="JeffH">
Things to fix
(where "n" in "TODOn" is a number).
</cref>
</t>
</section> <!-- Document Conventions -->
</section> <!-- sctn-conformance -->
<section anchor="sctn-terminology" title="Terminology">
<t>Terminology is defined in this section.</t>
<t><list style="hanging" hangIndent="18">
<t hangText="ASCII case-insensitive comparison">
<vspace/>
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 <xref target="Unicode5" /> for details.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="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) <xref target="Unicode5" />.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="Domain Name">
Domain Names, also
referred to as DNS Names, are defined in <xref
target="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 <xref target="RFC3986" />,
and the host component from the Host HTTP header field
production in section 14.23 of <xref target="RFC2616"
/>.
<list style="hanging" hangIndent="7">
<t hangText="Note:">
The Domain Names appearing in actual URI instances and
matching the aforementioned production components may or
may not be FQDNs.
</t>
</list>
</t>
</list>
</t>
<t><list style="hanging" hangIndent="18">
<t hangText="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.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="Effective Request URI">
<vspace/>
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 <xref
target="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. See
<xref target="sctn-svrproc-httpreq-efi"/>
"<xref target="sctn-svrproc-httpreq-efi" format="title"/>", below.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="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 STS 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) <xref target="RFC1594" />.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="Known STS Server">
is a STS
Server for which the UA has an STS Policy in effect.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="Local policy">
is comprised
of policy rules deployers specify and which are often
manifested as "configuration settings".
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="MITM">
is an acronym for
man-in-the-middle. See "man-in-the-middle
attack" in <xref target="RFC4949" />.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="Request URI">
is the URI used to
cause a UA to issue an HTTP request message.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="Strict Transport Security">
<vspace/>
is the
overall name for the combined UA- and server-side security
policy defined by this specification.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="Strict Transport Security Server">
<vspace/>
is a
HTTP server implementing the server aspects of the STS
policy.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="Strict Transport Security Policy">
<vspace/>
is the name of the combined overall
UA- and server-side facets of the behavior specified by
this specification.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="STS">
See Strict Transport
Security.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="STS Policy">
See Strict Transport Security Policy.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="STS Server">
See Strict
Transport Security Server.
</t></list></t>
<t><list style="hanging" hangIndent="18">
<t hangText="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 <xref
target="RFC2616" /> .
</t></list></t>
</section> <!-- sctn-terminology -->
<section anchor="syntax" title="Syntax">
<t>
This section defines the syntax of the new header this
specification introduces. It also provides a short
description of the function the header.
</t>
<t>
The <xref target="server-processing-model"/>
"<xref target="server-processing-model" format="title"/>"
section details how servers are to
use this header. Likewise, the
<xref target="user-agent-processing-model"/>
"<xref target="user-agent-processing-model" format="title"/>"
section details how user agents are to use this
header.
</t>
<section anchor="sctn-syntax" title="Strict-Transport-Security
HTTP Response Header Field">
<t>
The Strict-Transport-Security HTTP response header field
indicates to a UA that it MUST enforce the STS Policy in
regards to the server emitting the response message
containing this header field.
</t>
<t>
The ABNF syntax for the Strict-Transport-Security HTTP
Response Header field is:
</t>
<t>
<figure>
<artwork>
Strict-Transport-Security =
"Strict-Transport-Security" ":" OWS STS-v OWS
; STS 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" "=" 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
</artwork>
</figure>
<list style="hanging" hangIndent="7">
<t hangText="Note:">
<xref
target="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:
<list style="numbers">
<t>
Quoted-string literals in the above ABNF stanza
are case-insensitive.
</t>
<t>
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.
</t>
</list>
</t>
</list>
</t>
<t>
<list style="hanging" hangIndent="9">
<t hangText="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 STS Server (see
also <xref target="sctn-uaproc-stshf-note"/>
"<xref target="sctn-uaproc-stshf-note" format="title"/>",
below). The delta-seconds production is
specified in <xref target="RFC2616" />.
</t>
</list>
</t>
<t>
<cref anchor="TODO1" source="JeffH">
The above para wrt max-age may need
further refinement.
</cref>
</t>
<t>
<list style="hanging" hangIndent="18">
<t hangText="includeSubDomains">
is a flag which, if
present, signals to the UA that the STS Policy applies
to this STS Server as well as any subdomains of the
server's FQDN.
</t>
</list>
</t>
</section>
</section>
<section anchor="server-processing-model" title="Server Processing Model">
<t>
This section describes the processing model that STS 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 <xref
target="RFC4346" />, SSL <xref
target="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 <xref target="RFC0793" />.
</t>
<section title="HTTP-over-Secure-Transport Request Type">
<t>
When replying to an HTTP request that was conveyed over a
secure transport, a STS Server SHOULD include
in its response message a Strict-Transport-Security HTTP
Response Header that MUST satisfy the grammar
specified above in <xref target="sctn-syntax"/> "<xref
target="sctn-syntax" format="title"/>".
If a Strict-Transport-Sec HTTP Response Header is
included, the STS Server MUST include
only one such header.
</t>
<t>
<list style="hanging" hangIndent="7">
<t hangText="Note:">
Including the Strict-Transport-Sec 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 STS Server.
</t>
</list>
</t>
<t>
In order to establish a given host as a Known STS Server in
the context of a given UA, the host must correctly return,
per this specification, at least one valid
Strict-Transport-Security HTTP Response Header to the UA.
</t>
</section> <!-- HTTP-over-Secure-Transport Request Type -->
<section title="HTTP Request Type">
<t>
If a STS 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 <xref
target="sctn-svrproc-httpreq-efi"/> <xref
target="sctn-svrproc-httpreq-efi" format="title"/>, 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").
</t>
<t>
A STS Server
MUST NOT include the
Strict-Transport-Security HTTP Response Header in
HTTP responses conveyed over a non-secure
transport.
</t>
</section> <!-- HTTP Request Type -->
</section> <!-- server-processing-model -->
<section anchor="user-agent-processing-model" title="User Agent Processing Model">
<t>
This section describes the Strict Transport
Security processing model for UAs.
There are several facets to the model, enumerated by the
following subsections.
</t>
<t>
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
<xref target="RFC3490" />. If this is not the case in some situation, use the
operation given in
<xref target="sctn-force-tls-dns-name-toascii"/>
"<xref target="sctn-force-tls-dns-name-toascii" format="title"/>"
to convert any encountered internationalized Domain Names to
ACE format before processing them.
</t>
<section anchor="sctn-resp-hdr-proc" title="Strict-Transport-Security Response Header Field Processing">
<t>
If an HTTP response, received over a secure transport,
includes a Strict-Transport-Security
HTTP Response Header field,
conforming to the grammar specified in
<xref target="sctn-syntax"/>
"<xref target="sctn-syntax" format="title"/>"
(above),
and there are no underlying secure transport
errors or warnings, the UA MUST either:
</t>
<t>
<list style="symbols">
<t>
Note the server as a Known STS Server if it is not already
so noted (see <xref target="sctn-uaproc-stshf-note"/>
"<xref target="sctn-uaproc-stshf-note" format="title"/>", below),
</t>
</list>
</t>
<t>
or,
</t>
<t>
<list style="symbols">
<t>
Update its cached information for the Known STS
Server if the max-age and/or
includeSubDomains header field
value tokens are conveying information different than that
already maintained by the UA.
</t>
</list>
</t>
<t>
<list style="hanging" hangIndent="7">
<t hangText="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.
</t>
</list>
</t>
<t>
<cref anchor="TODO2" source="=JeffH">
Decide UA behavior in face of encountering multiple STS headers in a message. Use first header? Last?
</cref>
</t>
<t>
Otherwise:
</t>
<t>
<list style="symbols">
<t>
If an HTTP response is received over insecure
transport, the UA MUST ignore
any present Strict-Transport-Security HTTP Response
Header(s).
</t>
<t>
The UA MUST ignore any
Strict-Transport-Security HTTP Response Headers not
conforming to the grammar specified in
<xref target="sctn-syntax"/>
"<xref target="sctn-syntax" format="title"/>"
(above).
</t>
</list>
</t>
<section anchor="sctn-uaproc-stshf-note" title="Noting a STS Server">
<t>
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 <xref target="RFC3986" />, then
the UA MUST NOT note
this server as a Known STS Server.
</t>
<t>
Otherwise, if the substring does not congruently match a presently known STS Server,
per the matching procedure specified in
<xref target="sctn-ksts-dn-match"/>
"<xref target="sctn-ksts-dn-match" format="title"/>"
below, then
the UA MUST
note this server as a Known STS Server, caching the STS 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.
</t>
</section> <!-- h4 sctn-uaproc-stshf-note -->
<section anchor="sctn-ksts-dn-match" title="Known STS Server Domain Name Matching">
<t>
A UA determines whether a Domain Name represents a Known
STS Server by looking for a match between the query Domain
Name and the UA's set of Known STS Servers.
</t>
<t>
<list style="numbers">
<t>
Compare the query Domain Name string with the Domain
Names of the UA's set of Known STS Servers.
For each Known STS 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 <xref
target="RFC3986" />.
<list style="symbols">
<t>
If a label-for-label match between an entire
Known STS Server's Domain Name and a right-hand
portion of the query Domain Name is found, then the
Known STS Server's Domain Name is a superdomain
match for the query Domain Name.
<vspace blankLines="1"/>
For example:
<figure>
<artwork>
Query Domain Name: bar.foo.example.com
Superdomain matched
Known STS Server DN: foo.example.com
</artwork>
</figure>
<vspace blankLines="1"/>
At this point, the query Domain Name is
ascertained to effectively represent a Known STS
Server. There may also be additional matches
further down the Domain Name Label tree, up to and
including a congruent match.
</t>
<t>
If a label-for-label match between a Known STS
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 STS
Server.
<vspace blankLines="1"/>
For example:
<figure>
<artwork>
Query Domain Name: foo.example.com
Congruently matched
Known STS Server DN: foo.example.com
</artwork>
</figure>
<vspace blankLines="1"/>
The query Domain Name is ascertained to
represent
a Known STS Server. However, if there are also
superdomain matches, the one highest in the tree asserts
the STS Policy for this Known STS Server.
</t>
<t>
Otherwise, if no matches are found, the query Domain Name does not represent a
Known STS Server.
</t>
</list>
</t>
</list>
</t>
</section> <!-- h4 sctn-ksts-dn-match -->
</section> <!-- h3 sctn-resp-hdr-proc -->
<section title="URI Loading">
<t>
Whenever the UA prepares to "load",
also known as "dereference",
any URI where the host production of the URI
<xref target="RFC3986" />
matches that of a Known STS Server --
either as a congruent match or as a superdomain match where the
superdomain Known STS Server has
includeSubDomains asserted --
and the URI's scheme is
"http",
then replace the URI scheme with "https"
before proceeding with the load.
</t>
</section><!-- h3 URI Loading -->
<section title="Errors in Secure Transport Establishment">
<t>
When connecting to a Known STS 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.
</t>
</section> <!-- h3 Errors in Secure Transport Establishment -->
<section title="HTTP-Equiv <Meta> Element Attribute">
<t>
UAs MUST NOT heed
http-equiv="Strict-Transport-Security"
attribute settings on <meta> elements in
received content.
</t>
</section> <!-- h3 HTTP-Equiv <Meta> Element Attribute -->
</section> <!-- h2 user-agent-processing-model -->
<section anchor="sctn-force-tls-dns-name-toascii"
title="Domain Name ToASCII Conversion Operation">
<t>
This operation converts a string-serialized Domain Name
possibly containing arbitrary Unicode
characters <xref target="Unicode5" /> into
a string-serialized Domain Name in ASCII
Compatible Encoding (ACE) format as specified in
<xref target="RFC3490" />.
</t>
<t>
The operation is:
</t>
<t>
<list style="symbols">
<t>
Apply the IDNA conversion operation
(section 4 of <xref target="RFC3490" />) to the string,
selecting the ToASCII operation
and setting both the AllowUnassigned and UseSTD3ASCIIRules flags.
</t>
</list>
</t>
</section> <!-- h2 sctn-force-tls-dns-name-toascii -->
<section anchor="sctn-hosting-spec-advice" title="Server Implementation Advice">
<!-- <t>This section is non-normative.</t> -->
<t>
STS Policy expiration time considerations:
</t>
<t>
<list style="symbols">
<t>
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. 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.
</t>
<t>
A consideration here is whether a deployer wishes to have
signaled STS Policy expiry time match that
for the web site's domain certificate.
</t>
</list>
</t>
<t>
Considerations for using Strict Transport Security in conjunction with
self-signed public-key certificates:
</t>
<t>
<list style="symbols">
<t>
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 STS Policy is
enabled on a site wielding that organization's
certificates, then secure connections to that site will
fail without user recourse, per the STS design. This is to
protect against various active attacks, as discussed
above.
</t>
<t>
However, if said organization strongly wishes to employ
self-signed certificates, and their own CA in concert with
STS, 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 STS Policy on their
site(s).
<list style="hanging" hangIndent="7">
<t hangText="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
<xref target="sctn-sec-cons-bogus-ca"/>
"<xref target="sctn-sec-cons-bogus-ca" format="title"/>".
</t>
</list>
</t>
</list>
</t>
</section> <!-- h2 sctn-hosting-spec-advice -->
<section anchor="ua-impl-advice" title="UA Implementation Advice">
<!-- <t>This section is non-normative.</t> -->
<t>
Notes for STS Server implementors:
</t>
<t>
<list style="symbols">
<t>
A simplistic approach to enabling STS policy for
one's web site is to configure one's web server
to return a Strict-Transport-Security HTTP Response Header
with a constant max-age value. For exmple:
<figure>
<artwork>
Strict-Transport-Security: max-age=778000
</artwork>
</figure>
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 STS Server. The specifics of how this is
accomplished is out of the scope of this specification.
</t>
</list>
</t>
<t>
In order to provide users and web sites more effective
protection, UA implementors should consider including features
such as:
</t>
<t>
<list style="symbols">
<t>
Disallowing "mixed security context"
(also known as "mixed-content") loads (see section 5.3 "Mixed Content" in
<xref target="W3C.WD-wsc-ui-20100309" />).
<list style="hanging" hangIndent="7">
<t hangText="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.
</t>
</list>
</t>
</list>
</t>
<t>
In order to provide users effective controls for
managing their UA's caching of STS Policy,
UA implementors should consider including features such
as:
<list style="symbols">
<t>
Ability to delete UA's cached STS Policy
on a per STS Server basis.
</t>
</list>
</t>
<t>
In order to provide users and web sites more complete
protection, UAs could offer advanced
features such as these:
<list style="symbols">
<t>
Ability for users to explicitly declare a given Domain
Name as representing a STS Server, thus seeding it as a
Known STS Server before any actual interaction with it. This
would help protect against the <xref
target="sctn-sec-cons-boot"/> "<xref
target="sctn-sec-cons-boot" format="title"/>".
<list style="hanging" hangIndent="7">
<t hangText="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 <xref
target="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.
</t>
</list>
</t>
<t>
Facility whereby web site administrators can have UAs pre-configured
with STS 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
<xref target="sctn-sec-cons-boot"/>
"<xref target="sctn-sec-cons-boot" format="title"/>".
<list style="hanging" hangIndent="7">
<t hangText="Note:">
Such a facility complements the preceding described feature.
</t>
</list>
</t>
</list>
</t>
<t>
<cref anchor="XXX2" source="JeffH">
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).
</cref>
</t>
</section> <!-- h2 ua-impl-advice -->
<section anchor="sctn-svrproc-httpreq-efi" title="Constructing an Effective Request URI">
<t>
This section specifies how an STS Server must
construct the Effective
Request URI for a received HTTP request.
</t>
<t>
The first line of an HTTP request message is specified by the following ABNF
(<xref target="I-D.ietf-httpbis-p1-messaging" />
section 4.1):
<figure>
<artwork>
Request-Line = Method SP request-target SP HTTP-Version CRLF
</artwork>
</figure>
The request-target is following ABNF
(<xref target="I-D.ietf-httpbis-p1-messaging" />
section 4.1.2):
<figure>
<artwork>
request-target = "*"
/ absolute-URI
/ ( path-absolute [ "?" query ] )
/ authority
</artwork>
</figure>
Additionally, many HTTP requests contain an additional Host
request header field. It is specified by the following ABNF
(<xref target="I-D.ietf-httpbis-p1-messaging" />
section 4.1.2):
<figure>
<artwork>
Host = "Host:" OWS Host-v
Host-v = uri-host [ ":" port ]
</artwork>
</figure>
Thus an example HTTP message containing the above header fields is:
<figure>
<artwork>
GET /hello.txt HTTP/1.1
Host: www.example.com
</artwork>
</figure>
Another example is:
<figure>
<artwork>
GET HTTP://www.example.com/hello.txt HTTP/1.1
</artwork>
</figure>
An STS 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:
<figure>
<artwork>
<![CDATA[
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.
]]>
</artwork>
</figure>
For example, if the request message contains a request-target
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.
</t>
<t>
<cref anchor="TODO3" source="JeffH">
This is a first SWAG at this section. Fix/add prose as appropriate, fix ABNF as needed per review.
</cref>
</t>
</section> <!-- h2 sctn-svrproc-httpreq-efi -->
<section anchor="sctn-sec-cons" title="Security Considerations">
<!-- <t>This section is non-normative.</t> -->
<section title="Denial of Service (DoS)">
<t>
STS could be used to mount certain forms of DoS attacks, where
attackers set fake STS headers on legitimate sites
available only insecurely (e.g. social network service sites, wikis, etc.).
</t>
</section> <!-- h3 Denial of Service (DoS) -->
<section anchor="sctn-sec-cons-boot" title="Bootstrap MITM Vulnerability">
<t>
The bootstrap MITM (Man-In-The-Middle) vulnerability is a
vulnerability users and STS Servers encounter in the
situation where the user manually enters, or follows a link,
to a STS 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 <xref target="ForceHTTPS" /> .
</t>
<t>
<list style="hanging" hangIndent="7">
<t hangText="Note:">
There are various features/facilities that UA
implementations may employ in order to mitigate this
vulnerability. Please see <xref
target="ua-impl-advice"/> <xref target="ua-impl-advice"
format="title"/>. </t>
</list>
</t>
</section> <!-- h3 sctn-sec-cons-boot -->
<section title="Network Time Attacks">
<t>
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.
</t>
</section> <!-- h3 Network Time Attacks -->
<section anchor="sctn-sec-cons-bogus-ca" title="Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack">
<t>
If an attacker can convince users of, say,
https://bank.example.com (which is protected by STS 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
poisoning, and turn on STS Policy for their fake
bank.example.com site, then they have themselves some new
users.
</t>
</section> <!-- h3 sctn-sec-cons-bogus-ca -->
</section> <!-- h2 sctn-sec-cons -->
<section anchor="sec-iana-consid" title="IANA Considerations">
<t>
Below is the Internet Assigned Numbers Authority (IANA)
Provisional Message Header Field registration
information per <xref target="RFC3864" />.
</t>
<figure>
<artwork>
Header field name: Strict-Transport-Security
Applicable protocol: HTTP
Status: provisional
Author/Change controller: TBD
Specification document(s): this one
</artwork>
</figure>
</section> <!-- h2 sec-iana-consid -->
<section anchor="design-decision-faq" title="Design Decision Notes">
<!-- <t>This appendix is non-normative.</t> -->
<t>This appendix documents various design decisions.</t>
<t>
<list style="numbers">
<t>
Cookies aren't appropriate for STS Policy
expression as they are potentially
mutable (while stored in the UA),
therefore an HTTP header field is employed.
</t>
<t>
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.
</t>
<t>
A STS Server may update UA notions
of STS Policy via new STS 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 STS Policy, such as a multi-year
max-age value, and/or an incorrect
includeSubDomains flag. If the
STS 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.
</t>
<t>
STS 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.
</t>
</list>
</t>
</section> <!-- h2 design-decision-faq -->
</middle>
<back>
<references title="Normative References">
<!-- <xref target="I-D.draft-ietf-httpbis-p1-messaging" /> -->
&I-D.draft-ietf-httpbis-p1-messaging-09;
<!-- <xref target="W3C.WD-html5-20100304" /> -->
&W3C.WD-html5-20100304;
&RFC.1035; <!-- <xref target="RFC1035" /> -->
&RFC.1594; <!-- <xref target="RFC1594" /> -->
&RFC.1983; <!-- <xref target="RFC1983" /> -->
&RFC.2109; <!-- <xref target="RFC2109" /> -->
&RFC.2119; <!-- <xref target="RFC2119" /> -->
&RFC.2616; <!-- <xref target="RFC2616" /> -->
&RFC.2818; <!-- <xref target="RFC2818" /> -->
&RFC.2965; <!-- <xref target="RFC2965" /> -->
&RFC.3454; <!-- <xref target="RFC3454" /> -->
&RFC.3490; <!-- <xref target="RFC3490" /> -->
&RFC.3492; <!-- <xref target="RFC3492" /> -->
&RFC.3864; <!-- <xref target="RFC3864" /> -->
&RFC.3986; <!-- <xref target="RFC3986" /> -->
&RFC.4346; <!-- <xref target="RFC4346" /> -->
&RFC.4949; <!-- <xref target="RFC4949" /> -->
<!-- <xref target="Unicode5" /> -->
<reference anchor="Unicode5">
<front>
<title>The Unicode Standard, Version 5.0</title>
<author>
<organization>The Unicode Consortium</organization>
</author>
<date year="2007"/>
</front>
<seriesInfo name="Boston, MA, Addison-Wesley" value="ISBN 0-321-48091-0"/>
</reference>
</references>
<references title="Informative References">
<reference anchor="HASMAT" target="https://www.ietf.org/mailman/listinfo/hasmat">
<front>
<title>HASMAT -- HTTP Application Security Minus Authentication and Transport</title>
<author/>
</front>
</reference>
<!-- <xref target="ForceHTTPS" /> -->
<reference anchor="ForceHTTPS" target="https://crypto.stanford.edu/forcehttps/">
<front>
<title>
ForceHTTPS:
Protecting High-Security Web Sites from Network
Attacks
</title>
<author initials="C" surname="Jackson" fullname="Collin Jackson">
<organization />
</author>
<author initials="A" surname="Barth" fullname="Adam Barth">
<organization />
</author>
<date month="" year="2008" />
</front>
<seriesInfo name="In Proceedings of
the 17th International World Wide Web Conference (WWW2008)" value="" />
</reference>
<!-- <xref target="GoodDhamijaEtAl05" /> -->
<reference anchor="GoodDhamijaEtAl05" target="http://people.ischool.berkeley.edu/~rachna/papers/spyware_study.pdf">
<front>
<title>
Stopping
Spyware at the Gate: A User Study of Privacy, Notice and
Spyware
</title>
<author initials="N" surname="Good" fullname="">
<organization />
</author>
<author initials="R" surname="Dhamija" fullname="">
<organization />
</author>
<author initials="J" surname="Grossklags" fullname="">
<organization />
</author>
<author initials="D" surname="Thaw" fullname="">
<organization />
</author>
<author initials="S" surname="Aronowitz" fullname="">
<organization />
</author>
<author initials="D" surname="Mulligan" fullname="">
<organization />
</author>
<author initials="J" surname="Konstan" fullname="">
<organization />
</author>
<date month="July" year="2005" />
</front>
<seriesInfo name="In Proceedings of
Symposium On Usable Privacy and Security (SOUPS)" value="Pittsburgh, PA, USA" />
</reference>
<!-- <xref target="RFC0793" /> -->
&RFC.793;
<!-- <xref target="RFC2396" /> -->
&RFC.2396;
<!-- <xref target="I-D.ietf-tls-ssl-version3" /> -->
<reference anchor="I-D.ietf-tls-ssl-version3" target="http://tools.ietf.org/html/draft-ietf-tls-ssl-version3-00">
<front>
<title>
The SSL Protocol Version 3.0
</title>
<author initials="A" surname="Freier" fullname="">
<organization />
</author>
<author initials="P" surname="Karlton" fullname="">
<organization />
</author>
<author initials="P" surname="Kocher" fullname="">
<organization />
</author>
<date month="November" year="1996" />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-tls-ssl-version3" />
</reference>
<!-- <xref target="SunshineEgelmanEtAl09" /> -->
<reference anchor="SunshineEgelmanEtAl09" target="http://www.usenix.org/events/sec09/tech/full_papers/sunshine.pdf">
<front>
<title>
Crying Wolf: An Empirical Study of SSL Warning Effectiveness
</title>
<author initials="J" surname="Sunshine" fullname="">
<organization />
</author>
<author initials="S" surname="Egelman" fullname="">
<organization />
</author>
<author initials="H" surname="Almuhimedi" fullname="">
<organization />
</author>
<author initials="N" surname="Atri" fullname="">
<organization />
</author>
<author initials="L" surname="Cranor" fullname="">
<organization />
</author>
<date month="Augus" year="2009" />
</front>
<seriesInfo name="In Proceedings of
18th USENIX Security Symposium" value="Montreal, Canada" />
</reference>
<!-- <xref target="W3C.WD-wsc-ui-20100309" /> -->
&W3C.WD-wsc-ui-20100309;
</references>
<section anchor="acknowledgments" title="Acknowledgments">
<t>This appendix is non-normative.</t>
<t>The authors thank Michael Barrett, Sid Stamm, Maciej
Stachowiak, Andy Steingrubl, Brandon Sterne, Daniel Veditz for
their review and contributions. </t>
<!--
<t>Special thanks to ...</t>
-->
</section> <!-- h2 acknowledgments -->
</back>
</rfc>
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| PAFTECH AB 2003-2026 | 2026-04-23 20:44:20 |