Strict Transport Security

18 December 2009


Jeff Hodges (PayPal, Inc.) <> <>
Collin Jackson (Carnegie Mellon University) <>
Adam Barth (University of California Berkeley) <>

Creative Commons License
Strict Transport Security by PayPal, Inc., Collin Jackson, Adam Barth is licensed under a Creative Commons Attribution 3.0 United States License.



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.

Table of Contents

  1. 1 Introduction
  2. 2 Overview
    1. 2.1 Use Cases
    2. 2.2 Strict Transport Security Policy Effects
    3. 2.3 Threat Model
      1. 2.3.1 Threats Addressed
        1. Passive Network Attackers
        2. Active Network Attackers
        3. Web Site Development and Deployment Bugs
      2. 2.3.2 Threats Not Addressed
        1. Phishing
        2. Malware and Browser Vulnerabilities
    4. 2.4 Requirements
      1. 2.4.1 Overall Requirement
      2. 2.4.2 Detailed Core Requirements
      3. 2.4.3 Detailed Ancillary Requirements
  3. 3 Conformance Criteria
    1. 3.1 Document Conventions
  4. 4 Terminology
  5. 5 Syntax
    1. 5.1 Strict-Transport-Security HTTP Response Header Field
  6. 6 Server Processing Model
    1. 6.1 HTTP-over-Secure-Transport Request Type
    2. 6.2 HTTP Request Type
  7. 7 User Agent Processing Model
    1. 7.1 Strict-Transport-Security Response Header Field Processing
      1. 7.1.1 Noting a STS Server
      2. 7.1.2 Known STS Server Domain Name Matching
    2. 7.2 URI Loading
    3. 7.3 Errors in Secure Transport Establishment
    4. 7.4 HTTP-Equiv <Meta> Element Attribute
  8. 8 Domain Name ToASCII Conversion Operation
  9. 9 Server Implementation Advice
  10. 10 UA Implementation Advice
  11. 11 Constructing an Effective Request URI
  12. 12 Security Considerations
    1. 12.1 Denial of Service (DoS)
    2. 12.2 Bootstrap MITM Vulnerability
    3. 12.3 Network Time Attacks
    4. 12.4 Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack
  13. 13 IANA Considerations
  14. Design Decision Notes
  15. Acknowledgments
  16. References
    1. Normative
    2. Informative

1 Introduction

This section is non-normative.

The HTTP protocol [RFC2616] may be used over various transports, typically the Transmission Control Protocol (TCP) [RFC793]. However, TCP does not provide channel integrity protection, confidentiality, nor secure server identification. Thus the Secure Sockets Layer (SSL) protocol [SSL3], 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 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 [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 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.

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.

2 Overview

This section is non-normative.

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.

2.1 Use Cases

The overall applicable use case here is a combination of these two use cases:

2.2 Strict Transport Security Policy Effects

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:

  1. Insecure ("http") connections to a STS Server are redirected by the STS 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 wielding self-signed certificates.

  3. UAs transform insecure URI references to a STS Server into secure URI references before dereferencing them.

2.3 Threat Model

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 [ForceHTTPS] for details as well as relevant citations.

2.3.1 Threats Addressed Passive Network Attackers

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.

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. 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 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). 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 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.

"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 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. 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 [ForceHTTPS]. Malware and Browser Vulnerabilities

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.

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 Strict Transport Security addresses, as well as ancillary requirements that are not directly addressed.

2.4.1 Overall Requirement

2.4.2 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.

    For example, both and could set policy for

  6. UAs need to disallow security policy application to peer domains, and/or higher-level domains, by domains for which strict security policy is enabled.

    For example, neither nor can set policy for, nor can set policy for Also, cannot set policy for

  7. UAs need to prevent users from clicking-through security warnings. Halting connection attempts in the face of secure transport exceptions is acceptable.

A means for uniformly securely meeting the first core requirement above is not specifically addressed by this specification (see 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 UA Implementation Advice.

2.4.3 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-WebSecGuide-UI]).

  2. Facilitate user declaration of web sites for which strict security policy is enabled, regardless of whether the sites signal STS Policy.

3 Conformance Criteria

This specification is written for servers and user agents (UAs).

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.

In this specification, the words MUST, must not, and 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 a note.

Some of the more major known issues are marked like this.

this is how a warning is shown.

4 Terminology

Terminology is defined in this section.

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 [UNICODEv5.0] 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 10FFFF16 [UNICODEv5.0].

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. "" or "". 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].

The Domain Names appearing in actual URI instances and matching the aforementioned production components may or may not be FQDNs.

Domain Name Label: A domain name label is that portion of a Domain Name appearing "between the dots", i.e. consider "": "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 [ID.draft-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 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 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) [RFC1594].

Known STS Server is a STS Server for which the UA has an STS Policy in effect.

Local policy is comprised of policy rules deployers specify and which are often manifested as "configuration settings".

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.

Strict Transport Security is the overall name for the combined UA- and server-side security policy defined by this specification.

Strict Transport Security Server is a HTTP server implementing the server aspects of the STS policy.

Strict Transport Security Policy is the name of the combined overall UA- and server-side facets of the behavior specified by this specification.

STS: See Strict Transport Security.

STS Policy: See Strict Transport Security Policy.

STS Server: See Strict Transport Security Server.

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 Server Processing Model section details how servers are to use this header. Likewise, the 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 STS 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:

   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.draft-ietf-httpbis-p1-messaging]

   OWS        ; Optional White Space

[ID.draft-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.

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 Noting a STS Server, below). The delta-seconds production is specified in [RFC2616].

The above para wrt max-age may need further refinement.

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.

6 Server Processing Model

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 [RFC4346], SSL [SSL3], or perhaps others, the second being the processing rules for HTTP request messages received over non-secure transports, i.e. over TCP/IP [RFC793].

6.1 HTTP-over-Secure-Transport Request Type

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 Syntax. If a Strict-Transport-Sec HTTP Response Header is included, the STS Server must include only one such header.

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.

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.

6.2 HTTP Request Type

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 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").

A STS 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 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 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 Syntax (above), and there are no underlying secure transport errors or warnings, the UA must:


Decide UA behavior in face of encountering multiple STS headers in a message. Use first header? Last?


  1. If an HTTP response is received over insecure transport, the UA must ignore any present Strict-Transport-Security HTTP Response Header(s).

  2. The UA must ignore any Strict-Transport-Security HTTP Response Headers not conforming to the grammar specified in Syntax (above).

7.1.1 Noting a STS 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 STS Server.

Otherwise, if the substring does not congruently match a presently known STS Server, per the matching procedure specified in Known STS Server Domain Name Matching, 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.

7.1.2 Known STS Server Domain Name Matching

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.

  1. 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 [RFC3986]).

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 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.

7.3 Errors in Secure Transport Establishment

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.

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 [UNICODEv5.0] into a string-serialized Domain Name in ASCII Compatible Encoding (ACE) format as specified in [RFC3490].

The operation is:

9 Server Implementation Advice

This section is non-normative.

STS Policy expiration time considerations:

Considerations for using Strict Transport Security in conjunction with self-signed public-key certificates:

10 UA Implementation Advice

This section is non-normative.

Notes for STS Server implementors:

In order to provide users and web sites more effective protection, UA implementors should consider including features such as:

In order to provide users effective controls for managing their UA's caching of STS Policy, UA implementors should consider including features such as:

In order to provide users and web sites more complete protection, UAs could offer advanced features such as these:

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-03).

11 Constructing an Effective Request URI

This section specifies how an STS Server must construct the Effective Request URI for a received HTTP request.

The first line of an HTTP request message is specified by the following ABNF ([ID.draft-ietf-httpbis-p1-messaging] section 4.1):

    Request-Line   = Method SP request-target SP HTTP-Version CRLF

The request-target is following ABNF ([ID.draft-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 ([ID.draft-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

Another example is:

     GET HTTP://  HTTP/1.1

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:

   Effective-Request-URI = absolute-URI-present / path-absolute-form

   absolute-URI-present = absolute-URI

   path-absolute-form   = scheme "://" Host-v path-absolute [ "?" query ] 

   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 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.

This is a first SWAG at this section. Fix/add prose as appropriate, fix ABNF as needed per review.

12 Security Considerations

This section is non-normative.

12.1 Denial of Service (DoS)

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.).

12.2 Bootstrap MITM Vulnerability

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 [ForceHTTPS].

There are various features/facilities that UA implementations may employ in order to mitigate this vulnerability. Please see 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, (which is protected by STS Policy), to install their own version of a root CA certificate purporting to be'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 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

Design Decision Notes

This appendix is non-normative.

This appendix documents various design decisions.

  1. 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.

  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 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.

  4. 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.


This appendix is non-normative.

The authors thank Michael Barrett, Sid Stamm, Maciej Stachowiak, Andy Steingrubl, Brandon Sterne, Daniel Veditz for their review and contributions.



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