HTTP Working Group M. Thomson Internet-Draft Mozilla Intended status: Standards Track M. Nottingham Expires: November 4, 2018 Fastly W. Tarreau HAProxy Technologies May 3, 2018 Using Early Data in HTTP draft-ietf-httpbis-replay-03 Abstract Using TLS early data creates an exposure to the possibility of a replay attack. This document defines mechanisms that allow clients to communicate with servers about HTTP requests that are sent in early data. Techniques are described that use these mechanisms to mitigate the risk of replay. 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/ [1]. Working Group information can be found at http://httpwg.github.io/ [2]; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/replay [3]. 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 https://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 November 4, 2018. Thomson, et al. Expires November 4, 2018 [Page 1] Internet-Draft HTTP Early Data May 2018 Copyright Notice Copyright (c) 2018 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 (https://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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3 2. Early Data in HTTP . . . . . . . . . . . . . . . . . . . . . 3 3. Supporting Early Data in HTTP Servers . . . . . . . . . . . . 3 4. Using Early Data in HTTP Clients . . . . . . . . . . . . . . 5 5. Extensions for Early Data in HTTP . . . . . . . . . . . . . . 6 5.1. The Early-Data Header Field . . . . . . . . . . . . . . . 7 5.2. The 425 (Too Early) Status Code . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6.1. Gateways and Early Data . . . . . . . . . . . . . . . . . 8 6.2. Consistent Handling of Early Data . . . . . . . . . . . . 9 6.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 9 6.4. Out of Order Delivery . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . 11 8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction TLS 1.3 [TLS13] introduces the concept of early data (also known as zero round trip data or 0-RTT data). Early data allows a client to send data to a server in the first round trip of a connection, without waiting for the TLS handshake to complete if the client has spoken to the same server recently. When used with HTTP [HTTP], early data allows clients to send requests immediately, avoiding the one or two round trip delay needed Thomson, et al. Expires November 4, 2018 [Page 2] Internet-Draft HTTP Early Data May 2018 for the TLS handshake. This is a significant performance enhancement; however, it has significant limitations. The primary risk of using early data is that an attacker might capture and replay the request(s) it contains. TLS [TLS13] describes techniques that can be used to reduce the likelihood that an attacker can successfully replay a request, but these techniques can be difficult to deploy, and still leave some possibility of a successful attack. Note that this is different from automated or user-initiated retries; replays are initiated by an attacker without the awareness of the client. To help mitigate the risk of replays in HTTP, this document gives an overview of techniques for controlling these risks in servers, and defines requirements for clients when sending requests in early data. The advice in this document also applies to use of 0-RTT in HTTP over QUIC [HQ]. 1.1. Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. Early Data in HTTP Conceptually, early data is concatenated with other application data to form a single stream. This can mean that requests are entirely contained within early data, or only part of a request is early. In a multiplexed protocol, like HTTP/2 [RFC7540] or HTTP/QUIC [HQ], multiple requests might be partially delivered in early data. The model that this document assumes is that once the TLS handshake completes, the early data received on that TLS connection is known to not be a replayed copy of that data. However, it is important to note that this does not mean that early data will not be or has not been replayed on another connection. 3. Supporting Early Data in HTTP Servers A server decides whether or not to offer a client the ability to send early data on future connections when sending the TLS session ticket. Thomson, et al. Expires November 4, 2018 [Page 3] Internet-Draft HTTP Early Data May 2018 When a server enables early data, there are a number of techniques it can use to mitigate the risks of replay: 1. TLS [TLS13] mandates the use of replay detection strategies that reduce the ability of an attacker to successfully replay early data. These anti-replay techniques reduce but don't completely eliminate the chance of data being replayed and ensure a fixed upper limit to the number of replays. 2. The server can reject early data. A server cannot selectively reject early data, so this results in all requests sent in early data being discarded. 3. The server can choose to delay processing of early data until after the TLS handshake completes. By deferring processing, it can ensure that only a successfully completed connection is used for the request(s) therein. This provides the server with some assurance that the early data was not replayed. 4. If the server receives multiple requests in early data, it can determine whether to defer HTTP processing on a per-request basis. 5. The server can cause a client to retry a request and not use early data by responding with the 425 (Too Early) status code (Section 5.2), in cases where the risk of replay is judged too great. For a given request, the level of tolerance to replay risk is specific to the resource it operates upon (and therefore only known to the origin server). In general, if processing a request does not have state-changing side effects, the consequences of replay are not significant. The request method's safety ([RFC7231], Section 4.2.1) is one way to determine this. However, some resources do elect to associate side effects with safe methods, so this cannot be universally relied upon. It is RECOMMENDED that origin servers allow resources to explicitly configure whether early data is appropriate in requests. Absent such explicit information, origin servers MUST either reject early data or implement the techniques described in this document for ensuring that requests are not processed prior to TLS handshake completion. A request might be sent partially in early data with the remainder of the request being sent after the handshake completes. This does not necessarily affect handling of that request; what matters is when the server starts acting upon the contents of a request. Any time any Thomson, et al. Expires November 4, 2018 [Page 4] Internet-Draft HTTP Early Data May 2018 server instance might initiate processing prior to completion of the handshake, all server instances need to consider how a possible replay of early data could affect that processing (see also Section 6.2). A server can partially process requests that are incomplete. Parsing header fields - without acting on the values - and determining request routing is likely to be safe from side-effects, but other actions might not be. Intermediary servers do not have sufficient information to make this determination, so Section 5.2 describes a way for the origin to signal to them that a particular request isn't appropriate for early data. Intermediaries that accept early data MUST implement that mechanism. Note that a server cannot choose to selectively reject early data at the TLS layer. TLS only permits a server to accept all early data, or none of it. Once a server has decided to accept early data, it MUST process all requests in early data, even if the server rejects the request by sending a 425 (Too Early) response. A server can limit the amount of early data with the "max_early_data_size" field of the "early_data" TLS extension. This can be used to avoid committing an arbitrary amount of memory for deferred requests. A server SHOULD ensure that when it accepts early data, it can defer processing of requests until after the TLS handshake completes. 4. Using Early Data in HTTP Clients A client that wishes to use early data commences sending HTTP requests immediately after sending the TLS ClientHello. By their nature, clients have control over whether a given request is sent in early data - thereby giving the client control over risk of replay. Absent other information, clients MAY send requests with safe HTTP methods (see [RFC7231], Section 4.2.1) in early data when it is available, and SHOULD NOT send unsafe methods (or methods whose safety is not known) in early data. If the server rejects early data at the TLS layer, a client MUST start sending again as though the connection was new. This could entail using a different negotiated protocol [ALPN] than the one optimistically used for the early data. Any requests sent in early data MUST be sent again, unless the client decides to abandon those requests. Thomson, et al. Expires November 4, 2018 [Page 5] Internet-Draft HTTP Early Data May 2018 Automatic retry creates the potential for a replay attack. An attacker intercepts a connection that uses early data and copies the early data to another server instance. The second server instance accepts and processes the early data. The attacker then allows the original connection to complete. Even if the early data is detected as a duplicate and rejected, the first server instance might allow the connection to complete. If the client then retries requests that were sent in early data, the request will be processed twice. Replays are also possible if there are multiple server instances that will accept early data, or if the same server accepts early data multiple times (though this would be in violation of requirements in Section 8 of [TLS13]). Clients that use early data MUST retry requests upon receipt of a 425 (Too Early) status code; see Section 5.2. An intermediary MUST NOT use early data when forwarding a request unless early data was used on a previous hop, or it knows that the request can be retried safely without consequences (typically, using out-of-band configuration). Absent better information, that means that an intermediary can only use early data if the request either arrived in early data or arrived with the "Early-Data" header field set to "1" (see Section 5.1). 5. Extensions for Early Data in HTTP Because HTTP requests can span multiple "hops", it is necessary to explicitly communicate whether a request has been sent in early data on a previous connection. Likewise, some means of explicitly triggering a retry when early data is not desirable is necessary. Finally, it is necessary to know whether the client will actually perform such a retry. To meet these needs, two signalling mechanisms are defined: o The "Early-Data" header field is included in requests that might have been forwarded by an intermediary prior to the TLS handshake with its client completing. o The 425 (Too Early) status code is defined for a server to indicate that a request could not be processed due to the consequences of a possible replay attack. They are designed to enable better coordination of the use of early data between the user agent and origin server, and also when a gateway (also "reverse proxy", "Content Delivery Network", or "surrogate") is present. Thomson, et al. Expires November 4, 2018 [Page 6] Internet-Draft HTTP Early Data May 2018 Gateways typically don't have specific information about whether a given request can be processed safely when it is sent in early data. In many cases, only the origin server has the necessary information to decide whether the risk of replay is acceptable. These extensions allow coordination between a gateway and its origin server. 5.1. The Early-Data Header Field The "Early-Data" request header field indicates that the request has been conveyed in early data, and additionally indicates that a client understands the 425 (Too Early) status code. It has just one valid value: "1". Its syntax is defined by the following ABNF [ABNF]: Early-Data = "1" For example: GET /resource HTTP/1.0 Host: example.com Early-Data: 1 An intermediary that forwards a request prior to the completion of the TLS handshake with its client MUST send it with the "Early-Data" header field set to "1" (i.e., it adds it if not present in the request). An intermediary MUST use the "Early-Data" header field if it might have forwarded the request prior to handshake completion (see Section 6.2 for details). An intermediary MUST NOT remove this header field if it is present in a request. "Early-Data" MUST NOT appear in a "Connection" header field. The "Early-Data" header field is not intended for use by user agents (that is, the original initiator of a request). Sending a request in early data implies that the client understands this specification and is willing to retry a request in response to a 425 (Too Early) status code. A user agent that sends a request in early data does not need to include the "Early-Data" header field. A server cannot make a request that contains the Early-Data header field safe for processing by waiting for the handshake to complete. A request that is marked with Early-Data was sent in early data on a previous hop. Requests that contain the Early-Data field and cannot be safely processed MUST be rejected using the 425 (Too Early) status code. Thomson, et al. Expires November 4, 2018 [Page 7] Internet-Draft HTTP Early Data May 2018 The "Early-Data" header field carries a single bit of information and clients MUST include at most one instance. Multiple instances MUST be treated as equivalent to a single instance by a server. A "Early-Data" header field MUST NOT be included in responses or request trailers. 5.2. The 425 (Too Early) Status Code A 425 (Too Early) status code indicates that the server is unwilling to risk processing a request that might be replayed. User agents that send a request in early data MUST automatically retry the request when receiving a 425 (Too Early) response status code. Such retries MUST NOT be sent in early data. In all cases, an intermediary can forward a 425 (Too Early) status code. Intermediaries MUST forward a 425 (Too Early) status code if the request that it received and forwarded contained an "Early-Data" header field. Otherwise, an intermediary that receives a request in early data MAY automatically retry that request in response to a 425 (Too Early) status code, but it MUST wait for the TLS handshake to complete on the connection where it received the request. The server cannot assume that a client is able to retry a request unless the request is received in early data or the "Early-Data" header field is set to "1". A server SHOULD NOT emit the 425 status code unless one of these conditions is met. The 425 (Too Early) status code is not cacheable by default. Its payload is not the representation of any identified resource. 6. Security Considerations Using early data exposes a client to the risk that their request is replayed. A retried or replayed request can produce different side effects on the server. In addition to those side effects, replays and retries might be used for traffic analysis to recover information about requests or the resources those requests target. 6.1. Gateways and Early Data A gateway that forwards requests that were received in early data MUST only do so if it knows that the origin server that receives those requests understands the "Early-Data" header field and will correctly generate a 425 (Too Early) status code. A gateway that is uncertain about origin server support for a given request SHOULD Thomson, et al. Expires November 4, 2018 [Page 8] Internet-Draft HTTP Early Data May 2018 either delay forwarding the request until the TLS handshake with its client completes, or send a 425 (Too Early) status code in response. A gateway without at least one potential origin server that supports "Early-Data" header field expends significant effort for what can at best be a modest performance benefit from enabling early data. If no origin server supports early data, disabling early data entirely is more efficient. 6.2. Consistent Handling of Early Data Consistent treatment of a request that arrives in - or partially in - early data is critical to avoiding inappropriate processing of replayed requests. If a request is not safe to process before the TLS handshake completes, then all instances of the server (including gateways) need to agree and either reject the request or delay processing. Disabling early data, delaying requests, or rejecting requests with the 425 (Too Early) status code are all equally good measures for mitigating replay attacks on requests that might be vulnerable to replay. Server instances can implement any of these measures and be considered to be consistent, even if different instances use different methods. Critically, this means that it is possible to employ different mitigations in reaction to other conditions, such as server load. A server MUST NOT act on early data before the handshake completes if it and any other server instance could make a different decision about how to handle the same data. 6.3. Denial of Service Accepting early data exposes a server to potential denial of service through the replay of requests that are expensive to handle. A server that is under load SHOULD prefer rejecting TLS early data as a whole rather than accepting early data and selectively processing requests. Generating a 503 (Service Unavailable) or 425 (Too Early) status code often leads to clients retrying requests, which could result in increased load. 6.4. Out of Order Delivery In protocols that deliver data out of order (such as QUIC [HQ]) early data can arrive after the handshake completes. A server MAY process requests received in early data after handshake completion if it can rely on other instances correctly handling replays of the same requests. Thomson, et al. Expires November 4, 2018 [Page 9] Internet-Draft HTTP Early Data May 2018 7. IANA Considerations This document registers the "Early-Data" header field in the "Message Headers" registry located at https://www.iana.org/assignments/ message-headers [4]. Header field name: Early-Data Applicable protocol: http Status: standard Author/Change controller: IETF Specification document(s): This document Related information: (empty) This document registers the 425 (Too Early) status code in the "Hypertext Transfer Protocol (HTTP) Status Code" registry established in [RFC7231]. Value: 425 Description: Too Early Reference: This document 8. References 8.1. Normative References [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [HTTP] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Thomson, et al. Expires November 4, 2018 [Page 10] Internet-Draft HTTP Early Data May 2018 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", draft-ietf-tls-tls13-28 (work in progress), March 2018. 8.2. Informative References [ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, July 2014, . [HQ] Bishop, M., "Hypertext Transfer Protocol (HTTP) over QUIC", draft-ietf-quic-http-11 (work in progress), April 2018. [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015, . 8.3. URIs [1] https://lists.w3.org/Archives/Public/ietf-http-wg/ [2] http://httpwg.github.io/ [3] https://github.com/httpwg/http-extensions/labels/replay [4] https://www.iana.org/assignments/message-headers Acknowledgments This document was not easy to produce. The following people made substantial contributions to the quality and completeness of the document: David Benjamin, Subodh Iyengar, Benjamin Kaduk, Ilari Liusavaara, Kazuho Oku, Eric Rescorla, Kyle Rose, and Victor Vasiliev. Thomson, et al. Expires November 4, 2018 [Page 11] Internet-Draft HTTP Early Data May 2018 Authors' Addresses Martin Thomson Mozilla Email: martin.thomson@gmail.com Mark Nottingham Fastly Email: mnot@mnot.net Willy Tarreau HAProxy Technologies Email: willy@haproxy.org Thomson, et al. Expires November 4, 2018 [Page 12]