A Pre-Authentication Mechanism for SSH
draft-gutmann-ssh-preauth-03

Network Working Group                                         P. Gutmann
Internet-Draft                                    University of Auckland
Intended status: Informational                          17 February 2025
Expires: 21 August 2025

                 A Pre-Authentication Mechanism for SSH
                      draft-gutmann-ssh-preauth-03

Abstract

   Devices running SSH are frequently exposed on the Internet, either
   because of operational considerations or through misconfiguration,
   making them vulnerable to the constant 3-degree background radiation
   of scanning and probing attacks that pervade the Internet.  This
   document describes a simple pre-authentication mechanism that limits
   these attacks with minimal changes to SSH implementations and no
   changes to the SSH protocol itself.

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   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Threat Model  . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Usage Scenarios . . . . . . . . . . . . . . . . . . . . .   4
   3.  Description . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Test Vectors  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Contributors/Acknowledgements . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Devices running SSH are frequently exposed on the Internet, either
   because of operational considerations or through misconfiguration,
   making them vulnerable to the constant 3-degree background radiation
   of scanning and probing attacks that pervade the Internet.  This
   document describes a simple pre-authentication mechanism that limits
   these attacks with minimal changes to SSH implementations and no
   changes to the SSH protocol itself.

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1.1.  Conventions Used in This Document

   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
   [RFC2119] and [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Background

   This section covers the background and threat model for the SSH pre-
   authentication process and mechanism.

2.1.  Requirements

   The mechanism to limit scanning and probing attacks needs to meet the
   following requirements:

   *  It should stop attackers at the gate, preventing probing past the
      first message exchanged.  This both limits information leakage and
      mitigates against exploitation of pre-auth vulnerabilities in
      implementations.

   *  It should require no changes to the SSH protocol, for example the
      addition of new handshake messages or changes to existing
      handshake messages.

   In addition to these requirements there are also additional desirable
   properties:

   *  Ideally it would require no user-visible changes to the operation
      of an SSH client or server, in other words no need to supply
      additional or auxiliary keying material or perform other
      configuration changes.  Unfortunately this goal can't easily be
      met, see the comments in Section 7, with one configuration change
      on the client and server being required to enable pre-
      authentication.

   *  In order to encourage adoption by implementers of embedded SSH, it
      should require minimal effort to retrofit to existing SSH
      implementations, both because embedded systems using SSH are
      frequent targets and because these systems often have minimal
      effort applied to keep current with new mechanisms.

   Note that although this mechanism can be applied to any SSH
   implementation, its primary intended target is embedded SSH where few
   if any mitigations such as privilege separation or frequent patches
   to address vulnerabilities are possible.

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2.2.  Threat Model

   This document considers two different attacker types:

   1.  The generic three-degree background radiation of non-targeted
       Internet scanning and probing from off-path attackers.  Any pre-
       authentication measure, for example including a static non-public
       value at the start of the handshake, will stop this type of
       attack.

   2.  More targeted attacks from on-path attackers, which require
       something like a challenge/response mechanism to stop.

   The pre-authentication mechanism described here targets both off-path
   and on-path attackers.

2.3.  Usage Scenarios

   This document considers three different SSH usage scenarios:

   1.  A conventional server, possibly behind a firewall.  Firewall
       rules and security/access-control proxies, if available, would
       typically be used to handle any required SSH access control.

   2.  An embedded device that, for operational reasons or possibly just
       through misconfiguration, is exposed to the Internet.

   3.  As above, but on a private network that's been penetrated by
       attackers who are probing it for targets.  In other words the
       call is coming from inside the building.

   The pre-authentication mechanism described in this document is
   primarily targeted at the latter two scenarios.

3.  Description

   The pre-authentication mechanism for SSH takes the existing exchange
   of client and server ID strings and adds a simple challenge/response
   to them, preventing the exchange of any SSH handshake messages, in
   other words any actual SSH protocol messages, unless the pre-
   authentication succeeds.  It does this by adding a random challenge
   in the Comment field of the server's SSH ID, with the client
   responding with the response in the comment field of its SSH ID.  The
   server challenge in the comment field is denoted with 'C=<challenge>'
   and the client response with 'R=<response>'.  When the pre-
   authentication mechanism used in this document is used, these MUST be
   the first values in the Comment field, with any further entries that
   follow separated by a comma.

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   The challenge is a 64-bit server-generated nonce which is then
   base64-encoded to create a text string suitable for use in the
   Comment field.  This encoded form, and the base64-encoded response
   from the client, are sent without any base64 padding characters '='
   at the end, so that the encoded values consist of 11 alphanumeric
   characters.

   The response to the challenge is an HMAC-SHA256 of the challenge,
   with the MAC value truncated to 64 bits and base64-encoded to an
   11-character form in the same manner as the server's challenge.  The
   server challenge is MAC'd in 11-character base64 form as sent,
   without decoding back to binary form.

   Computing the response to the challenge requires a shared secret
   'preAuthSecret' between the client and server.  This SHOULD NOT be
   the same as any user password or other authentication value that
   might be used for authentication but should be created or generated
   independently and only used for pre-authentication.  In the situation
   where more than a single user or account exists on the device, the
   preAuthSecret functions in a manner similar to a WiFi password, with
   the preAuthSecret granting access to the SSH server and subsequent
   user authentication granting access to whatever sits behind the
   server.

   The HMAC key is calculated as:

       key = SHA256( string    challenge
                     string    preAuthSecret )

   The response is then computed as a truncated HMAC:

       rawResponse = HMAC-SHA256( key, challenge )
       response = base64( rawRespone[ 0...7 ]

   In other words the response is the base64 encoding (without adding
   base64 padding) of the first 64 bits of the HMAC value to give the
   required 11-byte response value.

4.  Test Vectors

   Test vectors to be added when the format is finalised.

5.  Contributors/Acknowledgements

6.  IANA Considerations

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7.  Security Considerations

   As the introduction points out, using this pre-authentication
   mechanism for SSH is not intended to be all things to all people but
   to address a specific problem, stopping scanning and probing attacks
   of SSH-enabled devices at the gates.  Conceptually it works like a
   WiFi password, granting initial access to the SSH server while
   subsequent user authentication grants access to whatever sits behind
   the server.  Its primary usage scenario is embedded devices with few
   users, most commonly only one.  Use with public SSH systems with
   large numbers of users is optionally possible, although unlike
   embedded devices these are expected to have up-to-date software and
   proper mitigations in place.

   However even in the latter case it can provide some protection
   against protocol and implementation vulnerabilities by reducing the
   possibility of compromise of the vulnerable implementation or system
   until the software is updated.  Legitimate users could continue to
   access the system while attackers who couldn't pass the pre-
   authentication check would be prevented from attacking the vulnerable
   system behind it.

   (When fuzzing an implementation that provides pre-authentication
   protection, remember to disable the authentication check for the
   fuzzing process otherwise the fuzzer will be prevented from
   progressing through to fuzzing the SSH protocol implementation
   itself).

   The pre-authentication can also be used to provide risk-based
   security in the same way that CAPTCHAs are used on some web sites in
   which suspicious accesses requiring a CAPTCHA in addition to the
   normal logon process.  For example accesses from the local subnet or
   only one or two hops away could be permitted without pre-
   authentication while ones that don't fall into these requirements
   could only be permitted with pre-authentication, complicating the
   task for remote attackers while leaving local users unaffected.

   The use of a separate preAuthSecret is the lesser of two evils, the
   other option being to reuse existing authentication information like
   a password, after due cryptographic processing, for the pre-
   authentication.  This makes the pre-authentication process
   transparent without requiring the management of additional keying
   material, since more than two decades of SSH compromise history have
   taught us that many users are not good at managing such keying
   material.  However with a simple gatekeeper pre-authentication
   mechanism of the kind described here it appears to be impossible to
   implement it in a manner that doesn't open it up to an offline
   dictionary attack by an on-path attacker who, even in the presence of

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   countermeasures like a severely truncated MAC that leads to false
   positives, can intercept many challenge/response pairs over time and
   use those to get to true positives.

   An additional benefit of using a distinct preAuthSecret is that it
   makes enabling and use of pre-authentication explicit.  A downside of
   using a distinct preAuthSecret is that it requires explicit
   configuration actions to enable and use pre-authentication.

   It is recommended that implementations perform rate-limiting on pre-
   authentication attempts, throttling back responses if too many pre-
   authentication failures occur in a given time interval.  To further
   confound attackers, servers may in addition opt to continue with an
   emulated handshake if the pre-authentication fails, eventually
   failing anyway or dropping the attacker into a tarpit.

   Since the authentication is unidirectional, a pass-the-hash attack is
   possible in the presence of an active attacker who convinces a victim
   to connect to a fake server and then MITMs the challenge and response
   from/to the genuine server.  However this is merely a pre-
   authentication mechanism whose main design goal is to protect the SSH
   implementation behind it from scanning and probing attacks rather
   than a full-blown (and complex) client/server authentication
   protocol.  The SSH protocol behind the protective pre-authentication
   step provides the required full client/server authentication,
   assuming the client verifies the server fingerprint [fingerprints].

   The server nonce generation process is described explicitly rather
   than in general terms like "a random string" to avoid implementations
   using things like the server name as a nonce value, or more generally
   just hardcoding something into the otherwise static server ID.

   Following Grigg's Law, "There is only one mode and that is secure",
   the pre-authentication mechanism hardcodes use of SHA256, the de
   facto universal standard hash in SSH implementations.  Since the
   security property required of the hash function is preimage
   resistance rather than collision resistance, and even beyond that the
   ability to find one specific preimage rather than any valid preimage,
   almost any hash function would suffice; SHA256 is chosen because of
   its universal acceptance and use.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.ietf.org/rfc/rfc2119.txt>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", RFC 8174, May 2017,
              <http://www.ietf.org/rfc/rfc8174.txt>.

8.2.  Informative References

   [fingerprints]
              Gutmann, P., "Do Users Verify SSH Keys?", ;login Volume
              36, Number 4, August 2011,
              <https://github.com/jscep/jscep>.

Author's Address

   Peter Gutmann
   University of Auckland
   Department of Computer Science
   Auckland
   New Zealand
   Email: pgut001@cs.auckland.ac.nz

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