12. User Agent Implementation Advice
This section is non-normative.
In order to provide users and web sites more effective protection, as
well as controls for managing their UA's caching of HSTS Policy, UA
implementers should consider including features such as the
12.1. No User Recourse
Failing secure connection establishment on any warnings or errors
(per Section 8.4 ("Errors in Secure Transport Establishment")) should
be done with "no user recourse". This means that the user should not
be presented with a dialog giving her the option to proceed. Rather,
it should be treated similarly to a server error where there is
nothing further the user can do with respect to interacting with the
target web application, other than wait and retry.
Essentially, "any warnings or errors" means anything that would cause
the UA implementation to announce to the user that something is not
entirely correct with the connection establishment.
Not doing this, i.e., allowing user recourse such as "clicking
through warning/error dialogs", is a recipe for a man-in-the-middle
attack. If a web application issues an HSTS Policy, then it is
implicitly opting into the "no user recourse" approach, whereby all
certificate errors or warnings cause a connection termination, with
no chance to "fool" users into making the wrong decision and
12.2. User-Declared HSTS Policy
A user-declared HSTS Policy is the ability for users to explicitly
declare a given domain name as representing an HSTS Host, thus
seeding it as a Known HSTS Host before any actual interaction with
it. This would help protect against the bootstrap MITM vulnerability
as discussed in Section 14.6 ("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, an "HSTS pre-loaded
list" feature (see Section 12.3).
12.3. HSTS Pre-Loaded List
An HSTS pre-loaded list is a facility whereby web site administrators
can have UAs pre-configured with HSTS Policy for their site(s) by the
UA vendor(s) -- a so-called "pre-loaded list" -- in a manner similar
to how root CA certificates are embedded in browsers "at the
factory". This would help protect against the bootstrap MITM
vulnerability (Section 14.6).
NOTE: Such a facility would complement a "user-declared HSTS Policy"
feature (Section 12.2).
12.4. Disallow Mixed Security Context Loads
"Mixed security context" loads happen when a web application
resource, fetched by the UA over a secure transport, subsequently
causes the fetching of one or more other resources without using
secure transport. This is also generally referred to as "mixed
content" loads (see Section 5.3 ("Mixed Content") in
[W3C.REC-wsc-ui-20100812]) but should not be confused with the same
"mixed content" term that is also used in the context of markup
languages such as XML and HTML.
NOTE: In order to provide behavioral uniformity across UA
implementations, the notion of mixed security context will
require further standardization work, e.g., to define the
term(s) more clearly and to define specific behaviors with
respect to it.
12.5. HSTS Policy Deletion
HSTS Policy deletion is the ability to delete a UA's cached HSTS
Policy on a per-HSTS Host basis.
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 Host should be a very deliberate and
well-considered act -- it shouldn't be something that users
get used to doing as a matter of course: e.g., just "clicking
through" in order to get work done. Also, implementations
need to guard against allowing an attacker to inject code,
e.g., ECMAscript, into the UA that silently and
programmatically removes entries from the UA's cache of Known
13. Internationalized Domain Names for Applications (IDNA): Dependency
Textual domain names on the modern Internet may contain one or more
"internationalized" domain name labels. Such domain names are
referred to as "internationalized domain names" (IDNs). The
specification suites defining IDNs and the protocols for their use
are named "Internationalized Domain Names for Applications (IDNA)".
At this time, there are two such specification suites: IDNA2008
[RFC5890] and its predecessor IDNA2003 [RFC3490].
IDNA2008 obsoletes IDNA2003, but there are differences between the
two specifications, and thus there can be differences in processing
(e.g., converting) domain name labels that have been registered under
one from those registered under the other. There will be a
transition period of some time during which IDNA2003-based domain
name labels will exist in the wild. In order to facilitate their
IDNA transition, user agents SHOULD implement IDNA2008 [RFC5890] and
MAY implement [RFC5895] (see also Section 7 of [RFC5894]) or [UTS46].
If a user agent does not implement IDNA2008, the user agent MUST
14. Security Considerations
This specification concerns the expression, conveyance, and
enforcement of the HSTS Policy. The overall HSTS Policy threat
model, including addressed and unaddressed threats, is given in
Section 2.3 ("Threat Model").
Additionally, the sections below discuss operational ramifications of
the HSTS Policy, provide feature rationale, discuss potential HSTS
Policy misuse, and highlight some known vulnerabilities in the HSTS
14.1. Underlying Secure Transport Considerations
This specification is fashioned to be independent of the secure
transport underlying HTTP. However, the threat analysis and
requirements in Section 2 ("Overview") in fact presume TLS or SSL as
the underlying secure transport. Thus, employment of HSTS in the
context of HTTP running over some other secure transport protocol
would require assessment of that secure transport protocol's security
model in conjunction with the specifics of how HTTP is layered over
it in order to assess HSTS's subsequent security properties in that
14.2. Non-Conformant User Agent Implications
Non-conformant user agents ignore the Strict-Transport-Security
header field; thus, non-conformant user agents do not address the
threats described in Section 2.3.1 ("Threats Addressed").
This means that the web application and its users wielding
non-conformant UAs will be vulnerable to both of the following:
o Passive network attacks due to web site development and deployment
For example, if the web application contains any insecure
references (e.g., "http") to the web application server, and if
not all of its cookies are flagged as "Secure", then its
cookies will be vulnerable to passive network sniffing and,
potentially, subsequent misuse of user credentials.
o Active network attacks:
For example, if an attacker is able to place a "man in the
middle", secure transport connection attempts will likely yield
warnings to the user, but without HSTS Policy being enforced,
the present common practice is to allow the user to "click
through" and proceed. This renders the user and possibly the
web application open to abuse by such an attacker.
This is essentially the status quo for all web applications and their
users in the absence of HSTS Policy. Since web application providers
typically do not control the type or version of UAs their web
applications interact with, the implication is that HSTS Host
deployers must generally exercise the same level of care to avoid web
site development and deployment bugs (see Section 18.104.22.168) as they
would if they were not asserting HSTS Policy.
14.3. Ramifications of HSTS Policy Establishment Only over Error-Free
The user agent processing model defined in Section 8 ("User Agent
Processing Model") stipulates that a host is initially noted as a
Known HSTS Host, or that updates are made to a Known HSTS Host's
cached information, only if the UA receives the STS header field over
a secure transport connection having no underlying secure transport
errors or warnings.
The rationale behind this is that if there is a "man in the middle"
(MITM) -- whether a legitimately deployed proxy or an illegitimate
entity -- it could cause various mischief (see also Appendix A
("Design Decision Notes") item 3, as well as Section 14.6 ("Bootstrap
MITM Vulnerability")); for example:
o Unauthorized notation of the host as a Known HSTS Host,
potentially leading to a denial-of-service situation if the host
does not uniformly offer its services over secure transport (see
also Section 14.5 ("Denial of Service")).
o Resetting the time to live for the host's designation as a Known
HSTS Host by manipulating the max-age header field parameter value
that is returned to the UA. If max-age is returned as zero, this
will cause the host to cease being regarded as a Known HSTS Host
by the UA, leading to either insecure connections to the host or
possibly denial of service if the host delivers its services only
over secure transport.
However, this means that if a UA is "behind" a MITM non-transparent
TLS proxy -- within a corporate intranet, for example -- and
interacts with an unknown HSTS Host beyond the proxy, the user could
possibly be presented with the legacy secure connection error
dialogs. Even if the risk is accepted and the user "clicks through",
the host will not be noted as an HSTS Host. Thus, as long as the UA
is behind such a proxy, the user will be vulnerable and will possibly
be presented with the legacy secure connection error dialogs for
as-yet unknown HSTS Hosts.
Once the UA successfully connects to an unknown HSTS Host over error-
free secure transport, the host will be noted as a Known HSTS Host.
This will result in the failure of subsequent connection attempts
from behind interfering proxies.
The above discussion relates to the recommendation in Section 12
("User Agent Implementation Advice") that the secure connection be
terminated with "no user recourse" whenever there are warnings and
errors and the host is a Known HSTS Host. Such a posture protects
users from "clicking through" security warnings and putting
themselves at risk.
14.4. The Need for includeSubDomains
Without the includeSubDomains directive, a web application would not
be able to adequately protect so-called "domain cookies" (even if
these cookies have their "Secure" flag set and thus are conveyed only
on secure channels). These are cookies the web application expects
UAs to return to any and all subdomains of the web application.
For example, suppose example.com represents the top-level DNS name
for a web application. Further suppose that this cookie is set for
the entire example.com domain, i.e., it is a "domain cookie", and it
has its Secure flag set. Suppose example.com is a Known HSTS Host
for this UA, but the includeSubDomains directive is not set.
Now, if an attacker causes the UA to request a subdomain name that is
unlikely to already exist in the web application, such as
"https://uxdhbpahpdsf.example.com/", but that the attacker has
managed to register in the DNS and point at an HTTP server under the
attacker's control, then:
1. The UA is unlikely to already have an HSTS Policy established for
2. The HTTP request sent to uxdhbpahpdsf.example.com will include
the Secure-flagged domain cookie.
3. If "uxdhbpahpdsf.example.com" returns a certificate during TLS
establishment, and the user "clicks through" any warning that
might be presented (it is possible, but not certain, that one may
obtain a requisite certificate for such a domain name such that a
warning may or may not appear), then the attacker can obtain the
Secure-flagged domain cookie that's ostensibly being protected.
Without the "includeSubDomains" directive, HSTS is unable to protect
such Secure-flagged domain cookies.
14.5. Denial of Service
HSTS could be used to mount certain forms of Denial-of-Service (DoS)
attacks against web sites. A DoS attack is an attack in which one or
more network entities target a victim entity and attempt to prevent
the victim from doing useful work. This section discusses such
scenarios in terms of HSTS, though this list is not exhaustive. See
also [RFC4732] for a discussion of overall Internet DoS
o Web applications available over HTTP
There is an opportunity for perpetrating DoS attacks with web
applications (or critical portions of them) that are available
only over HTTP without secure transport, if attackers can cause
UAs to set HSTS Policy for such web applications' host(s).
This is because once the HSTS Policy is set for a web
application's host in a UA, the UA will only use secure transport
to communicate with the host. If the host is not using secure
transport or is not using it for critical portions of its web
application, then the web application will be rendered unusable
for the UA's user.
NOTE: This is a use case for UAs to offer an "HSTS Policy
deletion" feature as noted in Section 12.5 ("HSTS Policy
An HSTS Policy can be set for a victim host in various ways:
* If the web application has an HTTP response splitting
vulnerability [CWE-113] (which can be abused in order to
facilitate "HTTP header injection").
* If an attacker can spoof a redirect from an insecure victim
site, e.g., <http://example.com/> to <https://example.com/>,
where the latter is attacker-controlled and has an apparently
valid certificate. In this situation, the attacker can then
set an HSTS Policy for example.com and also for all subdomains
* If an attacker can convince users to manually configure HSTS
Policy for a victim host. This assumes that their UAs offer
such a capability (see Section 12 ("User Agent Implementation
Advice")). Alternatively, if such UA configuration is
scriptable, then an attacker can cause UAs to execute his
script and set HSTS Policies for whichever desired domains.
o Inadvertent use of includeSubDomains
The includeSubDomains directive instructs UAs to automatically
regard all subdomains of the given HSTS Host as Known HSTS Hosts.
If any such subdomains do not support properly configured secure
transport, then they will be rendered unreachable from such UAs.
14.6. Bootstrap MITM Vulnerability
Bootstrap MITM (man-in-the-middle) vulnerability is a vulnerability
that users and HSTS Hosts encounter in the situation where the user
manually enters, or follows a link, to an unknown HSTS Host using an
"http" URI rather than an "https" URI. Because the UA uses an
insecure channel in the initial attempt to interact with the
specified server, such an initial interaction is vulnerable to
various attacks (see Section 5.3 of [ForceHTTPS]).
NOTE: There are various features/facilities that UA implementations
may employ in order to mitigate this vulnerability. Please
see Section 12 ("User Agent Implementation Advice").
14.7. Network Time Attacks
Active network attacks can subvert network time protocols (such as
the Network Time Protocol (NTP) [RFC5905]) -- making HSTS less
effective against clients that trust NTP or lack a real time clock.
Network time attacks are beyond the scope of this specification.
Note that modern operating systems use NTP by default. See also
Section 2.10 of [RFC4732].
14.8. Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack
An attacker could conceivably obtain users' login credentials
belonging to a victim HSTS-protected web application via a bogus root
CA certificate phish plus DNS cache poisoning attack.
For example, the attacker could first convince users of a victim web
application (which is protected by HSTS Policy) to install the
attacker's version of a root CA certificate purporting (falsely) to
represent the CA of the victim web application. This might be
accomplished by sending the users a phishing email message with a
link to such a certificate, which their browsers may offer to install
if clicked on.
Then, if the attacker can perform an attack on the users' DNS
servers, (e.g., via cache poisoning) and turn on HSTS Policy for
their fake web application, the affected users' browsers would access
the attacker's web application rather than the legitimate web
This type of attack leverages vectors that are outside of the scope
of HSTS. However, the feasibility of such threats can be mitigated
by including in a web application's overall deployment approach
appropriate employment, in addition to HSTS, of security facilities
such as DNS Security Extensions [RFC4033], plus techniques to block
email phishing and fake certificate injection.
14.9. Creative Manipulation of HSTS Policy Store
Since an HSTS Host may select its own host name and subdomains
thereof, and this information is cached in the HSTS Policy store of
conforming UAs, it is possible for those who control one or more HSTS
Hosts to encode information into domain names they control and cause
such UAs to cache this information as a matter of course in the
process of noting the HSTS Host. This information can be retrieved
by other hosts through cleverly constructed and loaded web resources,
causing the UA to send queries to (variations of) the encoded domain
names. Such queries can reveal whether the UA had previously visited
the original HSTS Host (and subdomains).
Such a technique could potentially be abused as yet another form of
"web tracking" [WebTracking].
14.10. Internationalized Domain Names
Internet security relies in part on the DNS and the domain names it
hosts. Domain names are used by users to identify and connect to
Internet hosts and other network resources. For example, Internet
security is compromised if a user entering an internationalized
domain name (IDN) is connected to different hosts based on different
interpretations of the IDN.
The processing models specified in this specification assume that the
domain names they manipulate are IDNA-canonicalized, and that the
canonicalization process correctly performed all appropriate IDNA and
Unicode validations and character list testing per the requisite
specifications (e.g., as noted in Section 10 ("Domain Name IDNA-
Canonicalization")). These steps are necessary in order to avoid
various potentially compromising situations.
In brief, examples of issues that could stem from lack of careful and
consistent Unicode and IDNA validations include unexpected processing
exceptions, truncation errors, and buffer overflows, as well as
false-positive and/or false-negative domain name matching results.
Any of the foregoing issues could possibly be leveraged by attackers
in various ways.
Additionally, IDNA2008 [RFC5890] differs from IDNA2003 [RFC3490] in
terms of disallowed characters and character mapping conventions.
This situation can also lead to false-positive and/or false-negative
domain name matching results, resulting in, for example, users
possibly communicating with unintended hosts or not being able to
reach intended hosts.
For details, refer to the Security Considerations sections of
[RFC5890], [RFC5891], and [RFC3490], as well as the specifications
they normatively reference. Additionally, [RFC5894] provides
detailed background and rationale for IDNA2008 in particular, as well
as IDNA and its issues in general, and should be consulted in
conjunction with the former specifications.
15. IANA Considerations
Below is the Internet Assigned Numbers Authority (IANA) Permanent
Message Header Field registration information per [RFC3864].
Header field name: Strict-Transport-Security
Applicable protocol: http
Author/Change controller: IETF
Specification document(s): this one
16.1. Normative References
[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.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
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, August 2009, <http://
Appendix A. 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" (also known as "mixed content loads") are handled, due to
UA implementation considerations as well as classification
3. An HSTS Host may update UA notions of HSTS Policy via new HSTS
header field parameter 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 erroneous HSTS Policy,
such as a multi-year max-age value, and/or an incorrect
includeSubDomains directive. If the HSTS Host couldn't correct
such errors over protocol, it would require some form of
annunciation to users and manual intervention on the users' part,
which could be a non-trivial problem for both web application
providers and their users.
4. HSTS Hosts 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
5. The max-age approach of having the HSTS Host provide a simple
integer number of seconds for a cached HSTS Policy time-to-live
value, as opposed to an approach of stating an expiration time in
the future, was chosen for various reasons. Amongst the reasons
are no need for clock synchronization, no need to define date and
time value syntaxes (specification simplicity), and
6. In determining whether port mapping was to be employed, the
option of merely refusing to dereference any URL with an explicit
port was considered. It was felt, though, that the possibility
that the URI to be dereferenced is incorrect (and there is indeed
a valid HTTPS server at that port) is worth the small cost of
possibly wasted HTTPS fetches to HTTP servers.
Appendix B. Differences between HSTS Policy and Same-Origin Policy
HSTS Policy has the following primary characteristics:
HSTS Policy stipulates requirements for the security
characteristics of UA-to-host connection establishment, on a
Hosts explicitly declare HSTS Policy to UAs. Conformant UAs are
obliged to implement hosts' declared HSTS Policies.
HSTS Policy is conveyed over protocol from the host to the UA.
The UA maintains a cache of Known HSTS Hosts.
UAs apply HSTS Policy whenever making an HTTP connection to a
Known HSTS Host, regardless of host port number; i.e., it applies
to all ports on a Known HSTS Host. Hosts are unable to affect
this aspect of HSTS Policy.
Hosts may optionally declare that their HSTS Policy applies to all
subdomains of their host domain name.
In contrast, the Same-Origin Policy (SOP) [RFC6454] has the following
An origin is the scheme, host, and port of a URI identifying a
A UA may dereference a URI, thus loading a representation of the
resource the URI identifies. UAs label resource representations
with their origins, which are derived from their URIs.
The SOP refers to a collection of principles, implemented within
UAs, governing the isolation of and communication between resource
representations within the UA, as well as resource
representations' access to network resources.
In summary, although both HSTS Policy and SOP are enforced by UAs,
HSTS Policy is optionally declared by hosts and is not origin-based,
while the SOP applies to all resource representations loaded from all
hosts by conformant UAs.
Appendix C. Acknowledgments
The authors thank Devdatta Akhawe, Michael Barrett, Ben Campbell,
Tobias Gondrom, Paul Hoffman, Murray Kucherawy, Barry Leiba, James
Manger, Alexey Melnikov, Haevard Molland, Yoav Nir, Yngve N.
Pettersen, Laksh Raghavan, Marsh Ray, Julian Reschke, Eric Rescorla,
Tom Ritter, Peter Saint-Andre, Brian Smith, Robert Sparks, Maciej
Stachowiak, Sid Stamm, Andy Steingrubl, Brandon Sterne, Martin
Thomson, Daniel Veditz, and Jan Wrobel, as well as all the websec
working group participants and others for their various reviews and
Thanks to Julian Reschke for his elegant rewriting of the effective
request URI text, which he did when incorporating the ERU notion into
the updates to HTTP/1.1 [HTTP1_1-UPD]. Subsequently, the ERU text in
this spec was lifted from Julian's work in the updated HTTP/1.1
(part 1) specification and adapted to the [RFC2616] ABNF.
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