HTTP Strict Transport Security

HTTP Strict Transport Security (HSTS) is a web security policy mechanism which helps to protect websites against protocol downgrade attacks and cookie hijacking. It allows web servers to declare that web browsers (or other complying user agents) should only interact with it using secure HTTPS connections,^[1] and never via the insecure HTTP protocol. HSTS is an IETF standards track protocol and is specified in RFC 6797.

The HSTS Policy^[2] is communicated by the server to the user agent via an HTTP response header field named "Strict-Transport-Security". HSTS Policy specifies a period of time during which the user agent should only access the server in a secure fashion.

Specification history

The HSTS specification was published as RFC 6797 on 19 November 2012 after being approved on 2 October 2012 by the IESG for publication as a Proposed Standard RFC.^[3] The authors originally submitted it as an Internet-Draft on 17 June 2010. With the conversion to an Internet-Draft, the specification name was altered from "Strict Transport Security" (STS) to "HTTP Strict Transport Security", because the specification applies only to HTTP.^[4] The HTTP response header field defined in the HSTS specification however remains named "Strict-Transport-Security".

The last so-called "community version" of the then-named "STS" specification was published on 18 December 2009, with revisions based on community feedback.^[5]

The original draft specification by Jeff Hodges^[6] from PayPal, Collin Jackson^[7] and Adam Barth^[8] was published on 18 September 2009.^[9]

The HSTS specification is based on original work by Jackson and Barth as described in their paper “ForceHTTPS: Protecting High-Security Web Sites from Network Attacks”.^[10]

Additionally, HSTS is the realization of one facet of an overall vision for improving web security, put forward by Jeff Hodges and Andy Steingruebl in their 2010 paper The Need for Coherent Web Security Policy Framework(s).^[11]

HSTS mechanism overview

A server implements an HSTS policy by supplying a header over an HTTPS connection (HSTS headers over HTTP are ignored).^[12] For example, a server could send a header such that future requests to the domain for the next year (max-age is specified in seconds; 31,536,000 is equal to one non-leap year) use only HTTPS: Strict-Transport-Security: max-age=31536000.

When a web application^[13] issues HSTS Policy to user agents, conformant user agents behave as follows:^[14]

Automatically turn any insecure links referencing the web application into secure links. (For instance, http://example.com/some/page/ will be modified to https://example.com/some/page/ before accessing the server.)
If the security of the connection cannot be ensured (e.g. the server's TLS certificate is not trusted), show an error message and do not allow the user to access the web application.^[15]

The HSTS Policy helps protect web application users against some passive (eavesdropping) and active network attacks.^[16] A man-in-the-middle attacker has a greatly reduced ability to intercept requests and responses between a user and a web application server while the user's browser has HSTS Policy in effect for that web application.

Applicability

The most important security vulnerability that HSTS can fix is SSL-stripping man-in-the-middle attacks, first publicly introduced by Moxie Marlinspike in his 2009 BlackHat Federal talk "New Tricks For Defeating SSL In Practice".^[17]^[18] The SSL stripping attack works (on TLS as well) by transparently converting a secure HTTPS connection into a plain HTTP connection. The user can see that the connection is insecure, but crucially there is no way of knowing whether the connection should be secure. Many websites do not use TLS/SSL, therefore there is no way of knowing (without prior knowledge) whether the use of plain HTTP is due to an attack, or simply because the website hasn't implemented TLS/SSL. Additionally, no warnings are presented to the user during the downgrade process, making the attack fairly subtle to all but the most vigilant. Marlinspike's sslstrip tool fully automates the attack.

HSTS addresses this problem^[16] by informing the browser that connections to the site should always use TLS/SSL. The HSTS header can be stripped by the attacker if this is the user's first visit. Google Chrome, Mozilla Firefox, Internet Explorer and Microsoft Edge attempt to limit this problem by including a "pre-loaded" list of HSTS sites.^[19]^[20]^[21] Unfortunately this solution cannot scale to include all websites on the internet. See limitations, below.

HSTS can also help to prevent having one's cookie-based website login credentials stolen by widely available tools such as Firesheep.^[22]

Because HSTS is time limited, it is sensitive to attacks involving shifting the victim's computer time e.g. using false NTP packets.^[23]

Limitations

The initial request remains unprotected from active attacks if it uses an insecure protocol such as plain HTTP or if the URI for the initial request was obtained over an insecure channel.^[24] The same applies to the first request after the activity period specified in the advertised HSTS Policy max-age (sites should set a period of several days or months depending on user activity and behavior). Google Chrome, Mozilla Firefox and Internet Explorer/Microsoft Edge address this limitation by implementing a "STS preloaded list", which is a list that contains known sites supporting HSTS.^[19]^[20]^[21] This list is distributed with the browser so that it uses HTTPS for the initial request to the listed sites as well. As previously mentioned, these pre-loaded lists cannot scale to cover the entire Web. A potential solution might be achieved by using DNS records to declare HSTS Policy, and accessing them securely via DNSSEC, optionally with certificate fingerprints to ensure validity (which requires running a validating resolver to avoid last mile issues).^[25]

Even with a "STS preloaded list", HSTS can't prevent advanced attacks against TLS itself, such as the BEAST or CRIME attacks introduced by Juliano Rizzo and Thai Duong. Attacks against TLS itself are orthogonal to HSTS policy enforcement.

See RFC 6797 for a discussion of overall HSTS security considerations.^[26]

Privacy issues

HSTS can be used to near-indelibly tag visiting browsers with recoverable identifying data (Supercookies) which can persist in and out of browser "incognito" modes, by creating a web page that makes multiple HTTP requests to selected domains; if (for example) 20 different requests/domains are used, one million visitors can be distinguished (binary, e.g., 2^20) on account of which resulting requests arrive via HTTP .vs. HTTPS, the latter being the previously recorded binary "bits" established earlier via HSTS headers.^[27]

Browser support

Settings page for HTTPS Strict Transport Security within Chromium 45, showing the status of the security policy for the domain "en.wikipedia.org".

Chromium and Google Chrome since version 4.0.211.0^[28]^[29]
Firefox since version 4;^[30] with Firefox 17, Mozilla integrates a list of websites supporting HSTS.^[20]
Opera since version 12^[31]
Safari as of OS X Mavericks^[32]
Internet Explorer 11 on Windows 8.1 and Windows 7 when KB 3058515 is installed (Released on Windows Update in June 2015)^[33]
Microsoft Edge and Internet Explorer 11 on Windows 10 support HSTS.^[34]^[35]

Deployment best practices

Depending on the actual deployment there are certain threats (e.g. cookie injection attacks) that can be avoided by following best practices.

HSTS hosts should declare HSTS policy at their top-level domain name. For example, an HSTS host at https://sub.example.com should also answer with the HSTS header at https://example.com. The header should specify the includeSubDomains directive.^[36]
In addition to HSTS deployment, a host for https://www.example.com should include a request to a resource from https://example.com to make sure that HSTS for the parent domain is set and protects the user from potential cookie injection attacks performed by a MITM that would inject a reference to the parent domain (or even http://nonexistentpeer.example.com), which the attacker then would answer.