HTTP Strict Transport Security
HTTP Strict Transport Security (HSTS) is a web security policy mechanism whereby a web server declares that complying user agents (such as a web browser) are to interact with it using only secure HTTPS connections (i.e. HTTP layered over TLS/SSL). HSTS is an IETF standards track protocol and is specified in RFC 6797.
The HSTS Policy is communicated by the server to the user agent via a HTTP response header field named "Strict-Transport-Security". HSTS Policy specifies a period of time during which the user agent shall access the server in a secure-only fashion.
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. The authors originally submitted it as an Internet-Draft on 17 June 2010. It was with the conversion to an Internet-Draft that the specification name was altered from "Strict Transport Security" (STS) to "HTTP Strict Transport Security". The reason for this name change was given as being due to the specification being specific to HTTP. (Note: the HTTP response header field defined in the HSTS specification 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.
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”.
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).
HSTS mechanism overview
A server implements an HSTS policy by supplying a header over an HTTPS connection (HSTS headers over HTTP are ignored). For example, a server could send a header such that future requests to the domain for the next year use only HTTPS: Strict-Transport-Security: max-age=31536000.
- 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 self-signed), show an error message and do not allow the user to access the web application.
The HSTS Policy helps protect web application users against some passive (eavesdropping) and active network attacks. 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.
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". The SSL stripping attack works (on both SSL and TLS) 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 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 and Mozilla Firefox attempt to limit this problem by including a "pre-loaded" list of HSTS sites. Unfortunately this solution cannot scale to include all websites on the internet, see Limitations, below.
See also  for an analysis of HSTS deployment statistics, patterns, mistakes, and best practices.
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. 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 and Mozilla Firefox address this limitation by implementing a "STS preloaded list", which is a list that contains known sites supporting HSTS. This list is distributed with the browser so that it uses HTTPS for the initial request to the listed sites as well. Though, as previously mentioned above, 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 (although DNSSEC will have secure last mile issues for the foreseeable future).
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.
- Chromium and Google Chrome since version 220.127.116.11
- Firefox since version 4; with Firefox 17, Mozilla integrates a list of websites supporting HSTS.
- Opera since version 12
- Safari as of OS X Mavericks
- Internet Explorer does not support HSTS, but is expected to support it in the next major release after IE 11.
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
- 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.
- Such secure HTTP connections are denoted by the URI scheme name of "https", and this protocol stack is often colloquially referred to as the "HTTPS protocol". also known as HTTP Secure.
- Hodges, Jeff; Jackson, Collin; Barth, Adam (Nov 2012). "Section 5.2. HSTS Policy". RFC 6797. IETF. Retrieved 21 Nov 2012.
- "[websec] Protocol Action: 'HTTP Strict Transport Security (HSTS)' to Proposed Standard (draft-ietf-websec-strict-transport-sec-14.txt)". 2 Oct 2012. Retrieved 2 Oct 2012.
- Jeff Hodges (30 June 2010). "Re: [HASMAT] "STS" moniker (was: IETF BoF @IETF-78 Maastricht: HASMAT...)". Retrieved 22 July 2010.
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- Nations, Daniel. "Web Applications: What is a Web Application?". About.com. Retrieved 21 Nov 2012.
- Hodges, Jeff; Jackson, Collin; Barth, Adam (Nov 2012). "Section 5. HSTS Mechanism Overview". RFC 6797. IETF. Retrieved 21 Nov 2012.
- Hodges, Jeff; Jackson, Collin; Barth, Adam (Nov 2012). "Section 12.1. No User Recourse". RFC 6797. IETF. Retrieved 30 Jun 2014.
- Hodges, Jeff; Jackson, Collin; Barth, Adam (Nov 2012). "2.3. Threat Model". RFC 6797. IETF. Retrieved 21 Nov 2012.
- Adam Langley (8 July 2010). "Strict Transport Security". The Chromium Projects. Retrieved 22 July 2010.
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- Jeff Hodges (31 October 2010). "Firesheep and HSTS (HTTP Strict Transport Security)". Retrieved 8 Mar 2011.
- Garron, Lucas; Bortz, Andrew; Boneh, Dan (22 July 2014). "The State of HSTS Deployment: A Survey and Common Pitfalls". Retrieved 9 August 2014.
- Hodges, Jeff; Jackson, Collin; Barth, Adam (Nov 2012). "Section 14.6. Bootstrap MITM Vulnerability". RFC 6797. IETF. Retrieved 21 Nov 2012.
- Cricket Liu (11 February 2010). "Securing DNSSEC's "Last Mile"". Retrieved 18 Apr 2014.
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- Hodges, Jeff; Jackson, Collin; Barth, Adam (Nov 2012). "Section 14. Security Considerations". RFC 6797. IETF. Retrieved 21 Nov 2012.
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