



Internet Engineering Task Force (IETF)                       B. Schwartz
Request for Comments: 9931                          Meta Platforms, Inc.
Updates: 9112, 9298                                           March 2026
Category: Standards Track                                               
ISSN: 2070-1721


Security Considerations for Optimistic Protocol Transitions in HTTP/1.1

Abstract

   In HTTP/1.1, the client can request a change to a new protocol on the
   existing connection.  This document discusses the security
   considerations that apply to data sent by the client before this
   request is confirmed and adds new requirements to RFCs 9112 and 9298
   to avoid related security issues.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9931.

Copyright Notice

   Copyright (c) 2026 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
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   in the Revised BSD License.

Table of Contents

   1.  Overview
   2.  Requirements Language
   3.  Background
   4.  Possible Security Issues
     4.1.  Request Smuggling
     4.2.  Parser Exploits
   5.  Operational Issues
   6.  Impact on HTTP Upgrade with Existing Upgrade Tokens
     6.1.  "TLS"
     6.2.  "WebSocket"/"websocket"
     6.3.  "connect-udp"
     6.4.  "connect-ip"
   7.  Guidance for Future Upgrade Tokens
     7.1.  Selection of Request Methods
   8.  Requirements for HTTP CONNECT
   9.  Security Considerations
   10. IANA Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgments
   Author's Address

1.  Overview

   This document discusses certain security considerations that arise
   when switching from HTTP/1.1 to a different protocol on the same
   connection.  It provides:

   *  a review of the relevant standards,

   *  a discussion of the security risks that may apply if a client
      sends data before the transition is confirmed,

   *  a security evaluation of existing upgrade tokens, and

   *  guidance for implementations and future standards documents.

   Updates to [HTTP/1.1] and [CONNECT-UDP], including new normative
   requirements, are provided in Sections 8 and 6.3, respectively.

2.  Requirements Language

   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.

3.  Background

   In HTTP/1.1 [HTTP/1.1] and later, a single connection can be used for
   many requests.  In HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3], these
   requests can be multiplexed, as each request is distinguished
   explicitly by its stream ID.  However, in HTTP/1.1, requests are
   strictly sequential, and each new request is distinguished implicitly
   by the closure of the preceding request.

   HTTP/1.1 is also the only version of HTTP that allows the client to
   change the protocol used for the remainder of the connection.  There
   are two mechanisms to request such a protocol transition.  One
   mechanism is the Upgrade header field ([HTTP], Section 7.8).  When
   included in a request, this field indicates that the client would
   like to use this connection for a protocol other than HTTP/1.1.  The
   server replies with a 101 (Switching Protocols) status code if it
   accepts the protocol change ([HTTP], Section 15.2.2).

   The other mechanism is the HTTP CONNECT method (Section 9.3.6 of
   [HTTP]).  This method indicates that the client wishes to establish a
   TCP connection to the specified host and port.  If accepted, the
   server replies with a 2xx (Successful) response to indicate that the
   request was accepted and a TCP connection was established.  After
   this point, the TCP connection is acting as a TCP tunnel, not an
   HTTP/1.1 connection.

   Both of these mechanisms also permit the server to reject the
   request.  For example, Section 7.8 of [HTTP] says:

   |  A server MAY ignore a received Upgrade header field if it wishes
   |  to continue using the current protocol on that connection.

   and Section 9.3.6 of [HTTP] says:

   |  A server MUST reject a CONNECT request that targets an empty or
   |  invalid port number, typically by responding with a 400 (Bad
   |  Request) status code.

   Rejected upgrades are common and can happen for a variety of reasons,
   such as:

   *  The server does not support any of the client's indicated upgrade
      tokens (i.e., the client's proposed new protocols), so it
      continues to use HTTP/1.1.

   *  The server knows that an upgrade to the offered protocol will not
      provide any improvement over HTTP/1.1 for this request to this
      resource, so it chooses to respond in HTTP/1.1.

   *  The server requires the client to authenticate before upgrading
      the protocol, so it replies with the status code 401
      (Authentication Required) and provides a challenge in an
      Authorization response header field ([HTTP], Section 11.6.2).

   *  The resource has moved, so the server replies with a 3xx
      (Redirection) status code ([HTTP], Section 3.4).

   Similarly, servers frequently reject HTTP CONNECT requests, such as
   when:

   *  The server does not support HTTP CONNECT.

   *  The specified destination is not allowed under server policy.

   *  The destination cannot be resolved, is unreachable, or does not
      accept the connection.

   *  The proxy requires the client to authenticate before proceeding.

   After rejecting a request, the server will continue to interpret
   bytes received on that connection in accordance with HTTP/1.1.

   Section 7.8 of [HTTP] also states:

   |  A client cannot begin using an upgraded protocol on the connection
   |  until it has completely sent the request message (i.e., the client
   |  can't change the protocol it is sending in the middle of a
   |  message).

   In other words, completion of the request message is a *necessary*
   condition for the client to begin using the new protocol.  However,
   it is important to clarify that this is not a *sufficient* condition
   because the server might reject the request.

   In some cases, the client might predict that the server is likely to
   accept a requested protocol transition.  For example, if a request
   using an upgrade token recently succeeded, the client might expect
   that subsequent requests with the same upgrade token will also
   succeed.  If this expectation is correct, the client can often reduce
   delay by immediately sending the first bytes of the new protocol
   "optimistically", without waiting for the server's response.  This
   document explores the security implications of this "optimistic"
   behavior.

4.  Possible Security Issues

   When there are only two distinct parties involved in an HTTP/1.1
   connection (i.e., the client and the server), protocol transitions
   introduce no new security issues: Each party must already be prepared
   for the other to send arbitrary data on the connection at any time.
   However, HTTP connections often involve more than two parties if the
   requests or responses include third-party data.  For example, a
   browser (party 1) might send an HTTP request to an origin (party 2)
   with the path, headers, or content controlled by a website from a
   different origin (party 3).  Post-transition protocols, such as
   WebSocket [WEBSOCKET], are also frequently used to convey data chosen
   by a third party.

   If the third-party data source is untrusted, then the data it
   provides is potentially "attacker-controlled".  The combination of
   attacker-controlled data and optimistic protocol transitions results
   in two significant security issues.

4.1.  Request Smuggling

   In a Request Smuggling attack ([HTTP/1.1], Section 11.2), the
   attacker-controlled data is chosen in such a way that it is
   interpreted by the server as an additional HTTP request.  These
   attacks allow the attacker to speak on behalf of the client while
   bypassing the client's own rules about what requests it will issue.
   Request Smuggling can occur if the client and server have distinct
   interpretations of the data that flows between them.

   If the server accepts a protocol transition request, it interprets
   the subsequent bytes in accordance with the new protocol.  If it
   rejects the request, it interprets those bytes as HTTP/1.1.  However,
   the client cannot know which interpretation the server will take
   until it receives the server's response status code.  If it uses the
   new protocol optimistically, this creates a risk that the server will
   interpret attacker-controlled data in the new protocol as an
   additional HTTP request issued by the client.

   As a trivial example, consider an HTTP CONNECT client providing
   connectivity to an untrusted application.  If the client is
   authenticated to the proxy server using a connection-level
   authentication method such as TLS Client Certificates ([TLS],
   Section 4.4.2), the attacker could send an HTTP/1.1 POST request
   ([HTTP], Section 9.3.3) for the proxy server at the beginning of its
   TCP connection.  If the client delivers this data optimistically, and
   the CONNECT request fails, the server would misinterpret the
   application's data as a subsequent authenticated request issued by
   the client.

 ## REQUESTS ##

 # The malicious application requests a TCP connection to a nonexistent
 # destination, which will fail.
 CONNECT no-such-destination.example:443 HTTP/1.1
 Host: no-such-destination.example:443

 # Before connection fails, the malicious application sends data on the
 # proxied TCP connection that forms a valid POST request to the proxy.
 # The vulnerable client optimistically forwards this data to the proxy.
 POST /upload HTTP/1.1
 Host: proxy.example
 Content-Length: 123456

 <POST body controlled by the malicious application>

 ## RESPONSES ##

 # When TCP connection establishment fails, the proxy responds by
 # rejecting the CONNECT request, but the client has already forwarded
 # the malicious TCP payload data to the proxy.
 HTTP/1.1 504 Gateway Timeout
 Content-Length: 0

 # The proxy interprets the smuggled POST request as coming from the
 # client.  If connection-based authentication is in use (e.g., using
 # TLS client certificate authentication), the proxy treats this
 # malicious request as authenticated.
 HTTP/1.1 200 OK
 Content-Length: 0

     Figure 1: Example Request Smuggling Attack Using HTTP CONNECT

4.2.  Parser Exploits

   A related category of attacks use protocol disagreement to exploit
   vulnerabilities in the server's request parsing logic.  These attacks
   apply when the HTTP client is trusted by the server, but the post-
   transition data source is not.  If the server software was developed
   under the assumption that some or all of the HTTP request data is not
   attacker-controlled, optimistic transmission can cause this
   assumption to be violated, exposing vulnerabilities in the server's
   HTTP request parser.

5.  Operational Issues

   If the server rejects the transition request, the connection can
   continue to be used for HTTP/1.1.  There is no general requirement to
   close the connection in response to a rejected transition, and
   keeping the connection open has performance advantages if additional
   HTTP requests to this server are likely.  Thus, it is normally
   inappropriate to close the connection in response to a rejected
   transition.

6.  Impact on HTTP Upgrade with Existing Upgrade Tokens

   This section describes the impact of this document's considerations
   on some registered upgrade tokens [IANA-UPGR] that are believed to be
   in use at the time of writing.

6.1.  "TLS"

   The "TLS" family of upgrade tokens was defined in [RFC2817], which
   correctly highlights the possibility of the server rejecting the
   upgrade.  If a client ignores this possibility and sends TLS data
   optimistically, the result cannot be valid HTTP/1.1: The first octet
   of a TLS connection must be 22 (ContentType.handshake), but this is
   not an allowed character in an HTTP/1.1 method (see [TLS],
   Section 5.1 and [HTTP/1.1], Section 3).  A compliant HTTP/1.1 server
   will treat this as a parsing error and close the connection without
   processing further requests.

6.2.  "WebSocket"/"websocket"

   Section 4.1 of [WEBSOCKET] says:

   |  Once the client's opening handshake has been sent, the client MUST
   |  wait for a response from the server before sending any further
   |  data.

   Thus, optimistic use of HTTP Upgrade is already forbidden in the
   WebSocket protocol.  Additionally, the WebSocket protocol requires
   high-entropy masking of client-to-server frames (Section 5.1 of
   [WEBSOCKET]).

6.3.  "connect-udp"

   Section 5 of [CONNECT-UDP] says:

   |  A client MAY optimistically start sending UDP packets in HTTP
   |  Datagrams before receiving the response to its UDP proxying
   |  request.

   However, in HTTP/1.1, this "proxying request" is an HTTP Upgrade
   request.  This upgrade is likely to be rejected in certain
   circumstances, such as when the UDP destination address (which is
   attacker-controlled) is invalid.  Additionally, the contents of the
   "connect-udp" protocol stream can include untrusted material (i.e.,
   the UDP packets, which might come from other applications on the
   client device).  This creates the possibility of Request Smuggling
   attacks.  To avoid these concerns, this document replaces that text
   to exclude HTTP/1.1 from any optimistic sending, as follows:

   |  A client MAY optimistically start sending UDP packets in HTTP
   |  Datagrams before receiving the response to its UDP proxying
   |  request but only if the HTTP version in use is HTTP/2 or later.
   |  Clients MUST NOT send UDP packets optimistically in HTTP/1.x due
   |  to the risk of Request Smuggling attacks.

6.4.  "connect-ip"

   The "connect-ip" upgrade token is defined in [CONNECT-IP].
   Section 11 of [CONNECT-IP] forbids clients from sending packets
   optimistically in HTTP/1.1, avoiding this issue.

7.  Guidance for Future Upgrade Tokens

   There are now several good examples of designs that reduce or
   eliminate the security concerns discussed in this document and may be
   applicable in future specifications:

   *  Forbid optimistic use of HTTP Upgrade (Section 4.1 of [WEBSOCKET]
      and Section 11 of [CONNECT-IP]).

   *  Embed a fixed preamble that deliberately terminates HTTP/1.1
      processing (Section 3.4 of [HTTP/2]).

   *  Apply high-entropy masking of client-to-server data (Section 5.1
      of [WEBSOCKET]).

   Future specifications for upgrade tokens should account for the
   security issues discussed here and provide clear guidance on how
   implementations can avoid them.

7.1.  Selection of Request Methods

   Some upgrade tokens, such as "TLS", are defined for use with any
   ordinary HTTP method.  The upgraded protocol continues to provide
   HTTP semantics and will convey the response to this HTTP request.

   The other upgrade tokens mentioned in Section 6 do not preserve HTTP
   semantics, so the method is not relevant.  All of these upgrade
   tokens are specified only for GET requests with no content.

   Future specifications for upgrade tokens should restrict their use to
   GET requests with no content if the HTTP method is otherwise
   irrelevant and the request does not need to carry any message
   content.  This improves consistency with other upgrade tokens and
   simplifies server implementation.

8.  Requirements for HTTP CONNECT

   This document updates [HTTP/1.1] to include the remaining text of
   this section.  The requirements in this section apply only to
   HTTP/1.1.

   Proxy clients that send CONNECT requests on behalf of untrusted TCP
   clients MUST do one or both of the following:

   1.  Wait for a 2xx (Successful) response before forwarding any TCP
       payload data.

   2.  Send a "Connection: close" request header.

   Proxy clients that don't implement at least one of these two
   behaviors are vulnerable to a trivial Request Smuggling attack
   ([HTTP/1.1], Section 11.2).

   At the time of writing, some proxy clients are believed to be
   vulnerable as described.  As a mitigation, proxy servers MUST close
   the underlying connection when rejecting a CONNECT request without
   processing any further requests on that connection.  This requirement
   applies whether or not the request includes a "close" connection
   option.

   Note that this mitigation will frequently cause slower connection
   establishment for correctly implemented clients, especially when
   returning a 407 (Proxy Authentication Required) response.  This
   performance loss can be avoided by using HTTP/2 or HTTP/3, which are
   not vulnerable to this attack.

   As a performance optimization, proxy servers MAY disable this
   mitigation if the client is known to wait for a 2xx (Successful)
   response before forwarding untrusted TCP payload data (i.e.,
   complying with item 1 above).  Proxy servers can identify compliant
   clients using the request's User-Agent header field and the user
   agent vendor's documentation regarding its compliance.

9.  Security Considerations

   This document describes security considerations related to optimistic
   use of protocol transitions in HTTP/1.1.

10.  IANA Considerations

   This document has no IANA actions.

11.  References

11.1.  Normative References

   [CONNECT-UDP]
              Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
              DOI 10.17487/RFC9298, August 2022,
              <https://www.rfc-editor.org/info/rfc9298>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <https://www.rfc-editor.org/info/rfc9112>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [CONNECT-IP]
              Pauly, T., Ed., Schinazi, D., Chernyakhovsky, A.,
              Kühlewind, M., and M. Westerlund, "Proxying IP in HTTP",
              RFC 9484, DOI 10.17487/RFC9484, October 2023,
              <https://www.rfc-editor.org/info/rfc9484>.

   [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/info/rfc9113>.

   [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/info/rfc9114>.

   [IANA-UPGR]
              IANA, "Hypertext Transfer Protocol (HTTP) Upgrade Token
              Registry",
              <https://www.iana.org/assignments/http-upgrade-tokens/>.

   [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
              HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000,
              <https://www.rfc-editor.org/info/rfc2817>.

   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [WEBSOCKET]
              Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,
              <https://www.rfc-editor.org/info/rfc6455>.

Acknowledgments

   This document benefited from valuable reviews and suggestions by:

   *  Mike Bishop

   *  Mohamed Boucadair

   *  Gorry Fairhurst

   *  Mark Nottingham

   *  Kazuho Oku

   *  Lucas Pardue

   *  David Schinazi

   *  Glenn Strauss

   *  Michael Sweet

   *  Willy Tarreau

   *  Martin Thomson

Author's Address

   Benjamin M. Schwartz
   Meta Platforms, Inc.
   Email: ietf@bemasc.net
