API Keys and Privacy
draft-ietf-httpapi-privacy-00

Building Blocks for HTTP APIs                                    R. Salz
Internet-Draft                                                 M. Bishop
Intended status: Best Current Practice               Akamai Technologies
Expires: 10 May 2025                                           M. Kleidl
                                                             Transloadit
                                                         6 November 2024

                          API Keys and Privacy
                     draft-ietf-httpapi-privacy-00

Abstract

   Redirecting HTTP requests to HTTPS, a common pattern for human-facing
   web resources, can be an anti-pattern for authenticated API traffic.
   This document discusses the pitfalls and makes deployment
   recommendations for authenticated HTTP APIs.  It does not specify a
   protocol.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   httpapi.github.io/draft-ietf-httpapi-privacy/draft-ietf-httpapi-
   privacy.html.  Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-httpapi-privacy/.

   Discussion of this document takes place on the Building Blocks for
   HTTP APIs Working Group mailing list (mailto:httpapi@ietf.org), which
   is archived at https://mailarchive.ietf.org/arch/browse/httpapi/.
   Subscribe at https://www.ietf.org/mailman/listinfo/httpapi/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-httpapi/httpapi-privacy.

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/.

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   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 10 May 2025.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Server Recommendations  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Pre-Connection Redirects  . . . . . . . . . . . . . . . .   3
     2.2.  Connection Blocking . . . . . . . . . . . . . . . . . . .   4
     2.3.  Credential Restriction  . . . . . . . . . . . . . . . . .   4
     2.4.  Credential Revocation . . . . . . . . . . . . . . . . . .   5
   3.  Client Recommendations  . . . . . . . . . . . . . . . . . . .   5
     3.1.  Implement Relevant Protocols  . . . . . . . . . . . . . .   5
     3.2.  Respect Credential Restrictions . . . . . . . . . . . . .   5
     3.3.  Disallow Insecure by Default  . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   It is a common pattern for HTTP servers to prefer serving resources
   over HTTPS.  Because HTTPS uses TLS, clients receive authentication
   of the server and confidentiality of the resource bodies supplied by
   the server.

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   In order to implement this preference, HTTP servers often listen for
   unencrypted requests and respond with a 3XX status code directing the
   client to the equivalent resource over an encrypted connection.  For
   unauthenticated web browsing, this is a reasonable user experience
   bridge.  Users often type bare hostnames (not URIs) into a user
   agent; if the user agent defaults to an unencrypted connection, the
   server can correct this default and require the use of encryption.
   This pattern is so well established that many HTTP server and
   intermediary implementations have a prominently displayed option to
   enable it automatically.

   When client authentication is used, more care must be taken.  The
   client's initial request may include a Bearer token or other
   credential; once the request has been sent on the network, any
   passive attacker who can see the traffic can acquire this credential
   and use it.

   If the server performs a redirect in this situation, it does not
   mitigate exposure of the credential.  Further, because the request
   will ultimately succeed if the client follows the redirect, an
   application developer or user who accidentally configures an
   unencrypted API endpoint will not necessarily notice the
   misconfiguration.

   This document describes actions API servers and clients should take
   in order to safeguard credentials.  These recommendations are not
   directed at resources where no authentication is used.

1.1.  Conventions and Definitions

   Although this document is not an IETF Standards Track publication, it
   adopts the conventions for normative language to provide clarity of
   instructions to the implementer.  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 BCPÂ 14 [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown
   here.

2.  Server Recommendations

2.1.  Pre-Connection Redirects

   Various mechanisms exist to inform clients that unencrypted requests
   to a server are never appropriate:

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   *  HTTP Strict Transport Security (HSTS) [RFC6797] informs clients
      who make a successful connection over HTTPS that secure
      connections are a requirement in the future.

   *  HTTPS DNS records [RFC9460] inform clients at connection time to
      use only secure connections to the indicated server.

   Neither mechanism is foolproof.  An attacker with control of the
   network or the DNS server could block resolution of HTTPS records on
   a client connecting to a new server, while HSTS requires a successful
   prior connection to the server and relies on the client to implement
   persistent storage of the HSTS directive.

   Used together, however, both approaches make clients less likely to
   send any requests over an insecure channel.  Servers with
   authenticated endpoints SHOULD employ both mechanisms.

2.2.  Connection Blocking

   If an API request succeeds despite having an unencrypted endpoint
   configured, the developer or user is less likely to notice the
   misconfiguration.  Where possible, it is advantageous for such a
   misconfiguration to fail immediately so that the error can be noticed
   and corrected.

   Servers MAY induce such an early failure by not accepting unencrypted
   connections, e.g. on port 80.  This makes it impossible for a client
   to send a credential over an insecure channel to the authentic
   server, as no such channel can be opened.

   However, this mitigation is limited against active network attackers,
   who can impersonate the server and accept the client's insecure
   connection attempt.

2.3.  Credential Restriction

   Whenever possible, credentials should include an indicator to clients
   that the credential is restricted to secure contexts.  For example,
   Cookie-based authentication SHOULD include the Secure attribute
   described in Section 4.1.2.5 of [RFC6265].  Bearer tokens MAY use the
   format described in [RFC8959] to indicate the expected usage to the
   client.

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2.4.  Credential Revocation

   Some deployments may not find it feasible to completely block
   unencrypted connections, whether because the hostname is shared with
   unauthenticated endpoints or for infrastructure reasons.  Therefore,
   servers need a response for when a valid credential has been received
   over an insecure channel.

   Because a difference in behavior would enable attackers to guess and
   check possible credentials, a server MUST NOT return a different
   client response between a valid or invalid credential presented over
   an insecure connection.  Differences in behavior MUST only be visible
   on subsequent use of the credential over a secure channel.

   When a request is received over an unencrypted channel, the presented
   credential is potentially compromised.  Servers SHOULD revoke such
   credentials immediately.  When the credential is next used over a
   secure channel, a server MAY return an error that indicates why the
   credential was revoked.

3.  Client Recommendations

   The following recommendations increase the success rate of the server
   recommendations above.

3.1.  Implement Relevant Protocols

   Clients SHOULD support and query for HTTPS records [RFC9460] when
   establishing a connection and SHOULD respect HSTS headers [RFC6797]
   received from a server.  This includes implementing persistent
   storage of HSTS indications received from the server.

3.2.  Respect Credential Restrictions

   Clients MUST NOT send a Cookie with the Secure attribute [RFC6265]
   over an insecure channel.  Clients MUST NOT send an Authorization
   header containing a token whose value begins with "secret-token:"
   over an insecure channel.

3.3.  Disallow Insecure by Default

   When authentication is used, clients SHOULD require an explicit
   indication from the user or caller that an insecure context is
   expected.  Without such an indication, attempts to send credentials
   should fail without producing any network traffic.

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4.  Security Considerations

   This entire document is about security of HTTP API interactions.

   The behavior recommended in Section 2.4 creates the potential for a
   denial of service attack where an attacker guesses many possible
   credentials over an unencrypted connection in hopes of discovering
   and revoking a valid one.  Servers implementing this mitigation MUST
   also guard against such attacks, such as by limiting the number of
   requests before closing the connection and rate-limiting the
   establishment of insecure connections.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [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/rfc/rfc2119>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6265>.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,
              <https://www.rfc-editor.org/rfc/rfc6797>.

   [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/rfc/rfc8174>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.

6.2.  Informative References

   [BLOG]     Viide, J., "Your API Shouldn't Redirect HTTP to HTTPS",
              May 2024, <https://jviide.iki.fi/http-redirects>.

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   [RFC8959]  Nottingham, M., "The "secret-token" URI Scheme", RFC 8959,
              DOI 10.17487/RFC8959, January 2021,
              <https://www.rfc-editor.org/rfc/rfc8959>.

Acknowledgments

   We are grateful to Joachim Viide for his [BLOG] blog posting that
   brought up the issue.

Authors' Addresses

   Rich Salz
   Akamai Technologies
   Email: rsalz@akamai.com

   Mike Bishop
   Akamai Technologies
   Email: mbishop@evequefou.be

   Marius Kleidl
   Transloadit
   Email: marius@transloadit.com

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