XEP-xxxx: SASL SCRAM Downgrade Protection

This specification provides a way to secure the SASL and SASL2 handshakes against method and channel-binding downgrades.
Thilo Molitor
© 1999 – 2021 XMPP Standards Foundation. SEE LEGAL NOTICES.


WARNING: This document has not yet been accepted for consideration or approved in any official manner by the XMPP Standards Foundation, and this document is not yet an XMPP Extension Protocol (XEP). If this document is accepted as a XEP by the XMPP Council, it will be published at <https://xmpp.org/extensions/> and announced on the <standards@xmpp.org> mailing list.
Standards Track
0.0.1 (2022-10-11)
Document Lifecycle
  1. Experimental
  2. Proposed
  3. Stable
  4. Final

1. Introduction

RFC 6120 [1] and Extensible SASL Profile (XEP-0388) [2] define a way to negotiate SASL mechanisms. When used together with SCRAM mechanisms (RFC 5802 [3]) and channel-binding (SASL Channel-Binding Type Capability (XEP-0440) [4]) the mechanism selection is protected against downgrade attacks by an active MITM tampering with the TLS channel and advertised SASL mechanisms, while the negotiation of the channel-binding types is still not protected against such downgrade attacks.

SASL Channel-Binding Type Capability (XEP-0440) [4] tries to mitigate this by making the "tls-server-end-point" (RFC 5929 [5]) channel-binding mandatory to implement for servers. But that leaves clients not able to implement this type, or any channel-binding at all, vulnerable to downgrades of channel-binding types and SASL mechanisms. Furthermore "tls-server-end-point" provides weaker security guarantees than other channel-bindings like for example "tls-exporter" (defined in RFC 5705 [6] and RFC 9266 [7]).

This specification aims to solve this issue by spcifying a downgrade protection for both SASL mechanisms and channel-binding types using an optional SCRAM attribute (see RFC 5802 [3]). This specification can be used for SASL1 (RFC 6120 [1]) and SASL2 (Extensible SASL Profile (XEP-0388) [2]) profiles as well as any other SASL profile.

Note: In the long term the author strives to publish this as an RFC rather than a XEP to also make this protection available to other protocols, after gaining implementation experience.

2. Requirements

This protocol was designed with the following requirements in mind:

Note that this specification intentionally leaves out support for SASL PLAIN. If server and client support PLAIN, no protection against SASL method or channel-binding downgrades is possible and the security relies solely on the underlying TLS channel. As explained in § 13.8.3 of RFC 6120 [1], servers and clients SHOULD NOT support SASL PLAIN unless it is required by the authentication backend.

3. Glossary

This specification uses some abbreviations:

4. Protocol

Sections 5.1 and 7 of RFC 5802 [3] allow for arbitrary optional attributes inside SCRAM messages. This specification uses those optional attribute to implement a downgrade protection.

4.1 Server Indicates Support

The server uses the optional attribute "d" with the value "ssdp" in its server-first-message to indicate support for this specification.

A client supporting this specification but not seeing this attribute advertised by the server MAY abort the authentication. It is RECOMMENDED to wait until the whole SCRAM flow hash been completed to distinguish the case of a server not supporting this specification from a MITM stripping out this optional SCRAM attribute.

4.2 Client Sends Downgrade Protection Hash

If the server indicated support for this spec in the server-first-message and the client supports it, the client calculates a hash for the server-advertised list of SASL mechanisms and channel-binding types as follows.

Note: All sorting operations MUST be performed using "i;octet" collation as specified in Section 9.3 of RFC 4790 [9].

  1. Initialize an empty ASCII string S
  2. Sort all server-advertised SASL mechanisms and append them to string S joined by delimiter "," (%x2C)
  3. If the server used SASL Channel-Binding Type Capability (XEP-0440) [4] to advertise channel-bindings, append "|" (%x7C) to S
  4. If the server used SASL Channel-Binding Type Capability (XEP-0440) [4] to advertise channel-bindings, sort all server-advertised channel-binding types and append them to string S joined by delimiter "," (%x2C)
  5. Hash S using the same hash mechanism as used for the SCRAM mechanism currently in use and encode the result using base64

The client then adds the optional attribute "d" with the base64 encoded hash obtained in step 5 to its client-final-message. The client MAY send this attribute even if the server did not advertise support.

Note: If the server simultaneously advertises SASL1 and SASL2, only the mechanism list of the SASL protocol the client uses for authentication MUST be considered for hashing.

4.3 Server Verifys The Downgrade Protection Hash

Upon receiving the client-final-message the server calculates its own base64 encoded hash using the list of SASL mechanisms and channel-binding types it advertised using SASL1 or SASL2 and SASL Channel-Binding Type Capability (XEP-0440) [4] by applying the same algorithm as defined in Client Sends Downgrade Protection Hash.

The server then extracts the base64 encoded hash presented by the client in the optional attribute "d" and compares it to its own hash. If the hashes match, the list of SASL mechanisms and channel-binding types has not been changed by an active MITM.

If the hashes do not match, the server MUST fail the authentication as specified in RFC 6120 [1] section 6.5 or Extensible SASL Profile (XEP-0388) [2] section 2.6.2 using the "aborted" error-condition. If Extensible SASL Profile (XEP-0388) [2] is used, the application-specific error-condition "downgrade-detected" in the namespace "urn:xmpp:ssdp:0" MUST be added, too. It MAY further include an optional descriptive text to further clarify this error as specified in Extensible SASL Profile (XEP-0388) [2] section 6.2.6 or RFC 6120 [1] section 6.5. If additional SCRAM data is provided, the used SCRAM "server-error-value" MUST be "downgrade-detected".

Non-XMPP implementations MAY use a SCRAM "server-error-value" of "downgrade-detected" alongside any protocol specific error-condition.

4.4 Full Example

This sections contains an example based on the ones provided in Extensible SASL Profile (XEP-0388) [2].

Example 1. Full SCRAM-SHA-1-PLUS authentication flow using the optional attribute defined in this spec
  Client sending stream header

  Server responding with stream header and features
  <authentication xmlns='urn:xmpp:sasl:2'>
    <inline xmlns='urn:xmpp:sasl:2'>
      <!-- Server indicates that XEP-0198 can be negotiated "inline" -->
      <enable xmlns='urn:xmpp:sm:3'/>
      <!-- Server indicates support for XEP-0386 Bind 2 -->
      <bind xmlns='urn:xmpp:bind2:1'/>
  <!-- Channel-binding information provided by XEP-0440 -->
  <sasl-channel-binding xmlns='urn:xmpp:sasl-cb:0'>
    <channel-binding type='tls-server-end-point'/>
    <channel-binding type='tls-exporter'/>

  Client initiates authentication using SCRAM-SHA-1-PLUS and channel-binding type "tls-exporter"
<authenticate xmlns='urn:xmpp:sasl:2' mechanism='SCRAM-SHA-1-PLUS'>
  <!-- Base64 of: 'p=tls-exporter,,n=user,r=12C4CD5C-E38E-4A98-8F6D-15C38F51CCC6' -->
  <user-agent id='d4565fa7-4d72-4749-b3d3-740edbf87770'>
    <device>Kiva's Phone</device>

  SCRAM-SHA-1-PLUS challenge issued by the server as defined in RFC 5802
  but including the optional attribute indicating support for this specification.
  Base64 of: 'r=12C4CD5C-E38E-4A98-8F6D-15C38F51CCC6a09117a6-ac50-4f2f-93f1-93799c2bddf6,s=QSXCR+Q6sek8bf92,i=4096,d=ssdp'
<challenge xmlns='urn:xmpp:sasl:2'>

  The client responds with the base64 encoded SCRAM-SHA-1-PLUS client-final-message (password: 'pencil')
  including the base64 encoded SHA-1 hash of the mechanism and channel-binding lists.
  Attribute "d" contains base64 encoded SHA-1 hash of 'SCRAM-SHA-1,SCRAM-SHA-1-PLUS|tls-exporter,tls-server-end-point'
  Base64 of: 'c=cD10bHMtZXhwb3J0ZXIsLMcoQvOdBDePd4OswlmAWV3dg1a1Wh1tYPTBwVid10VU,r=12C4CD5C-E38E-4A98-8F6D-15C38F51CCC6a09117a6-ac50-4f2f-93f1-93799c2bddf6,p=UApo7xo6Pa9J+Vaejfz/dG7BomU=,d=dRc3RenuSY9ypgPpERowoaySQZY='
  The c-attribute contains the GS2-header and channel-binding data blob (32 bytes) as defined in RFC 5802.
<response xmlns='urn:xmpp:sasl:2'>

  The server accepted this authentication, no tampering with the advertised SASL mechanisms or channel-bindings was detected.
<success xmlns='urn:xmpp:sasl:2'>
  <!-- Base64 of: 'v=sQq8A1dePL5DxWX22Sz4TJMD7t4=' -->

5. Security Considerations

Using SCRAM attributes makes them part of the HMAC signatures used in the SCRAM protocol flow efficiently protecting them against any MITM attacker not knowing the password used.

6. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [10].

7. XMPP Registrar Considerations

This specification does not need any interaction with the XMPP Registrar [11].

8. XML Schema

This specification does not specify any new XML elements.


Appendix A: Document Information

XMPP Standards Foundation
Standards Track
Last Updated
Approving Body
XMPP Council
XMPP Core, RFC 5802, XEP-0388
Superseded By
Short Name

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Appendix B: Author Information

Thilo Molitor


This XMPP Extension Protocol is copyright © 1999 – 2020 by the XMPP Standards Foundation (XSF).


Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

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Limitation of Liability

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IPR Conformance

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Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/about/discuss.shtml> for a complete list.

Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <xsf-ietf@xmpp.org> list might also be appropriate.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

1. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.

2. XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>.

3. RFC 5802: Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms <http://tools.ietf.org/html/rfc5802>.

4. XEP-0440: SASL Channel-Binding Type Capability <https://xmpp.org/extensions/xep-0440.html>.

5. RFC 5929: Channel Bindings for TLS <http://tools.ietf.org/html/rfc5929>.

6. RFC 5705: Keying Material Exporters for Transport Layer Security (TLS) <http://tools.ietf.org/html/rfc5705>.

7. RFC 9266: Channel Bindings for TLS 1.3 <http://tools.ietf.org/html/rfc9266>.

8. RFC 7677: SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms <http://tools.ietf.org/html/rfc7677>.

9. RFC 4790: Internet Application Protocol Collation Registry <http://tools.ietf.org/html/rfc4790>.

10. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

11. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.

Appendix H: Revision History

Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/

  1. Version 0.0.1 (2022-10-11)
    Initial version.

Appendix I: Bib(La)TeX Entry

  title = {SASL SCRAM Downgrade Protection},
  author = {Molitor, Thilo},
  type = {XEP},
  number = {xxxx},
  version = {0.0.1},
  institution = {XMPP Standards Foundation},
  url = {https://xmpp.org/extensions/xep-xxxx.html},
  date = {2022-10-11/2022-10-11},