Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-08

Abstract

This document describes how http URIs can be accessed using Transport Layer Security (TLS) to mitigate pervasive monitoring attacks.

Note to Readers

Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.

Working Group information can be found at http://httpwg.github.io/; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/opp-sec.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on May 4, 2017.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 1.1. Goals and Non-Goals
• 1.2. Notational Conventions
• 2. Using HTTP URIs over TLS
• 2.1. Alternative Server Opt-In
• 2.2. Interaction with “https” URIs
• 2.3. The “http-opportunistic” well-known URI
• 3. IANA Considerations
• 4. Security Considerations
• 4.1. Security Indicators
• 4.2. Downgrade Attacks
• 4.3. Privacy Considerations
• 4.4. Confusion Regarding Request Scheme
• 4.5. Server Controls
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Appendix A. Acknowledgements
• Authors' Addresses

1. Introduction

This document describes a use of HTTP Alternative Services [RFC7838] to decouple the URI scheme from the use and configuration of underlying encryption, allowing a http URI [RFC7230] to be accessed using Transport Layer Security (TLS) [RFC5246] opportunistically.

Serving https URIs requires avoiding Mixed Content [W3C.CR-mixed-content-20160802], which is problematic in many deployments. This document describes a usage model whereby sites can serve http URIs over TLS, thereby avoiding these issues, while still providing protection against passive attacks.

Opportunistic Security [RFC7435] does not provide the same guarantees as using TLS with https URIs; it is vulnerable to active attacks, and does not change the security context of the connection. Normally, users will not be able to tell that it is in use (i.e., there will be no “lock icon”).

1.1. Goals and Non-Goals

The immediate goal is to make the use of HTTP more robust in the face of pervasive passive monitoring [RFC7258].

A secondary (but significant) goal is to provide for ease of implementation, deployment and operation. This mechanism is expected to have a minimal impact upon performance, and require a trivial administrative effort to configure.

Preventing active attacks (such as a Man-in-the-Middle) is a non-goal for this specification. Furthermore, this specification is not intended to replace or offer an alternative to https, since it both prevents active attacks and invokes a more stringent security model in most clients.

1.2. Notational Conventions

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

2. Using HTTP URIs over TLS

An origin server that supports the resolution of http URIs can indicate support for this specification by providing an alternative service advertisement [RFC7838] for a protocol identifier that uses TLS, such as h2 [RFC7540], or http/1.1 [RFC7301]. Note that HTTP/1.1 requests MUST use the absolute form (see Section 5.3.2 of [RFC7230]).

A client that receives such an advertisement MAY make future requests intended for the associated origin ([RFC6454]) to the identified service (as specified by [RFC7838]), provided that the alternative service opts in as described in Section 2.1.

A client that places the importance of protection against passive attacks over performance might choose to withhold requests until an encrypted connection is available. However, if such a connection cannot be successfully established, the client can resume its use of the cleartext connection.

A client can also explicitly probe for an alternative service advertisement by sending a request that bears little or no sensitive information, such as one with the OPTIONS method. Likewise, clients with existing alternative services information could make such a request before they expire, in order minimize the delays that might be incurred.

Client certificates are not meaningful for URLs with the http scheme, and therefore clients creating new TLS connections to alternative services for the purposes of this specification MUST NOT present them. Connections that use client certificates for other reasons MAY be reused, though client certificates MUST NOT affect the responses to requests for http resources.

2.1. Alternative Server Opt-In

It is possible that the server might become confused about whether requests’ URLs have a http or https scheme, for various reasons; see Section 4.4. To ensure that the alternative service has opted into serving http URLs over TLS, clients are required to perform additional checks before directing http requests to it.

Clients MUST NOT send http requests over a secured connection, unless the chosen alternative service presents a certificate that is valid for the origin - as per [RFC2818] (this also establishes “reasonable assurances” for the purposes of {RFC7838}}) - and they have obtained a valid http-opportunistic response for an origin (as per Section 2.3).

For example, assuming the following request is made over a TLS connection that is successfully authenticated for those origins, the following request/response pair would allow requests for the origins “http://www.example.com” or “http://example.com” to be sent using a secured connection:

GET http://example.com/.well-known/http-opportunistic HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Content-Type: application/json
Connection: close

[ "http://www.example.com", "http://example.com" ]

2.2. Interaction with “https” URIs

When using alternative services, requests for resources identified by both http and https URIs might use the same connection, because HTTP/2 permits requests for multiple origins on the same connection.

Because of the potential for server confusion about the scheme of requests (see Section 4.4), clients MUST NOT send http requests on a connection prior to successfully retrieving a valid http-opportunistic resource that contains the origin (see Section 2.3). The primary purpose of this check is to provide a client with some assurance that a server understands this specification and has taken steps to avoid being confused about request scheme.

2.3. The “http-opportunistic” well-known URI

This specification defines the “http-opportunistic” well-known URI [RFC5785]. A client is said to have a valid http-opportunistic response for a given origin when:

• The client has obtained a 200 (OK) response for the well-known URI from the origin, and it is fresh [RFC7234] (potentially through revalidation [RFC7232]), and
• That response has the media type “application/json”, and
• That response’s payload, when parsed as JSON [RFC7159], contains an array as the root, and
• The array contains a string that is a case-insensitive character-for-character match for the origin in question, serialised into Unicode as per Section 6.1 of [RFC6454].

A client MAY treat an “http-opportunistic” resource as invalid if the contains values that are not strings.

3. IANA Considerations

This specification registers a Well-Known URI [RFC5785]:

• URI Suffix: http-opportunistic
• Change Controller: IETF
• Specification Document(s): Section 2.3 of [this specification]
• Related Information:

4. Security Considerations

4.1. Security Indicators

User Agents MUST NOT provide any special security indicia when an http resource is acquired using TLS. In particular, indicators that might suggest the same level of security as https MUST NOT be used (e.g., a “lock device”).

4.2. Downgrade Attacks

A downgrade attack against the negotiation for TLS is possible.

For example, because the Alt-Svc header field [RFC7838] likely appears in an unauthenticated and unencrypted channel, it is subject to downgrade by network attackers. In its simplest form, an attacker that wants the connection to remain in the clear need only strip the Alt-Svc header field from responses.

4.3. Privacy Considerations

Cached alternative services can be used to track clients over time; e.g., using a user-specific hostname. Clearing the cache reduces the ability of servers to track clients; therefore clients MUST clear cached alternative service information when clearing other origin-based state (i.e., cookies).

4.4. Confusion Regarding Request Scheme

HTTP implementations and applications sometimes use ambient signals to determine if a request is for an https resource; for example, they might look for TLS on the stack, or a server port number of 443.

This might be due to limitations in the protocol (the most common HTTP/1.1 request form does not carry an explicit indication of the URI scheme), or it may be because how the server and application are implemented (often, they are two separate entities, with a variety of possible interfaces between them).

Any security decisions based upon this information could be misled by the deployment of this specification, because it violates the assumption that the use of TLS (or port 443) means that the client is accessing a HTTPS URI, and operating in the security context implied by HTTPS.

Therefore, servers need to carefully examine the use of such signals before deploying this specification.

4.5. Server Controls

This specification requires that a server send both an Alternative Service advertisement and host content in a well-known location to send HTTP requests over TLS. Servers SHOULD take suitable measures to ensure that the content of the well-known resource remains under their control. Likewise, because the Alt-Svc header field is used to describe policies across an entire origin, servers SHOULD NOT permit user content to set or modify the value of this header.

5. References

5.1. Normative References

5.2. Informative References

Appendix A. Acknowledgements

Mike Bishop contributed significant text to this document.

Thanks to Patrick McManus, Stefan Eissing, Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin, Erik Nygren, Paul Hoffman, Adam Langley, Eric Rescorla, Julian Reschke, Kari Hurtta, and Richard Barnes for their feedback and suggestions.

Authors' Addresses