NetBSD Security Advisory 2023-006: KDC-spoofing in pam_krb5

To: current-users%NetBSD.org@localhost
Subject: NetBSD Security Advisory 2023-006: KDC-spoofing in pam_krb5
From: NetBSD Security-Officer <security-officer%netbsd.org@localhost>
Date: Wed, 28 Jun 2023 13:59:04 -0400
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		 NetBSD Security Advisory 2023-006
		 =================================

Topic:		KDC-spoofing in pam_krb5

Version:	NetBSD-current:		affected prior to 2023-06-20
		NetBSD 10.0_BETA:	affected prior to 2023-06-21
		NetBSD 9.3:		affected
		NetBSD 9.2:		affected
		NetBSD 9.1:		affected
		NetBSD 9.0:		affected
		NetBSD 8.2:		affected
		NetBSD 8.1:		affected
		NetBSD 8.0:		affected

Severity:	Remote attacker may be able to log in as any user

Fixed:		NetBSD-current:		2023-06-20
		NetBSD-10 branch:	2023-06-21
		NetBSD-9 branch:	2023-06-21
		NetBSD-8 branch:	2023-06-21

Please note that NetBSD releases prior to 8.2 are no longer supported.
It is recommended that all users upgrade to a supported release.


Abstract
========

On a NetBSD system with:

- - ftpd, sshd, or some other network services enabled with default
  configuration, and

- - /etc/krb5.conf created (possibly an empty file),

an adversary on the network can log in as any user via pam_krb5 by
spoofing a Kerberos KDC (Key Distribution Center) on the network,
bypassing Kerberos's normal cryptographic verification methods.

NetBSD installations without /etc/krb5.conf (the default) are not
affected.

This vulnerability has been assigned CVE-2023-3326.


Technical Details
=================

Normally, Kerberos works like this:

1. An organization is named by a realm like EXAMPLE.COM and has a
   server called a KDC storing secrets shared with all users and
   services in the organization.

2. Each user and service is identified by a `principal name' like
   user%EXAMPLE.COM@localhost or imap/mail.example.com%EXAMPLE.COM@localhost.

3. Each principal (user or service) has a secret shared with the KDC:
   . passwords, for users;
   . random keys, for services, stored in a file called a `keytab' with
     the service.

4. To log in as a user, you run `kinit user%EXAMPLE.COM@localhost' and enter your
   password.  kinit(1) talks to the KDC to get a `ticket-granting
   ticket' (tgt) which is typically good for 10h or 24h, so you only
   have to do this once per day to get access to any services as a
   single-sign-on (SSO) system.

   In this process, the KDC and kinit(1) use your password in a
   cryptographic protocol to authenticate one another -- if you enter
   the wrong password, or if the KDC is actually spoofed and doesn't
   know your password, kinit(1) fails to obtain a tgt.

5. To use a service like IMAP at mail.example.com:
   (a) your mail reader requests a service ticket for
       imap/mail.example.com%EXAMPLE.COM@localhost from the KDC using the tgt,
   (b) your mail reader sends the service ticket to mail.example.com,
   (c) mail.example.com uses its keytab to verify the service ticket
       came from the legitimate KDC before letting you read your mail.


pam_krb5 is an authentication module that verifies a password on behalf
of login programs like sshd and xdm.  It works by effectively running
kinit(1) with the password, and getting a tgt from the KDC, as a way to
verify the password.

- - For local logins, like xdm, pam_krb5 incorporates the single-sign-on
  aspect of Kerberos into the local login prompt, so users can proceed
  to use the tickets obtained by pam_krb5 for applications in their
  login session.

- - For logins from remote users, like sshd, pam_krb5 serves as a
  fallback for users who are familiar with passwords, but are
  unfamiliar with Kerberos or the normal kinit(1) path.

  (Note: pam_krb5 is not for normal Kerberos authentication in sshd --
  for that, you must set `GSSAPIAuthentication yes' in
  /etc/ssh/sshd_config.)

If there is a keytab, pam_krb5 also uses the tgt to get a service
ticket and verifies the service ticket with the keytab.

But without a keytab, the only way pam_krb5 can verify the KDC's
response and get a tgt is with the password, and yet pam_krb5's job is
to verify the password with a tgt.

If both the password _and_ the KDC are controlled by an adversary, the
adversary can provide a password that the adversary's spoofed KDC will
return a valid tgt for.  Currently, without a keytab, pam_krb5 is
vulnerable to this attack.


pam_krb5 is not necessary for you to use Kerberos just for client-side
SSO, but to use that, you need to create an /etc/krb5.conf (which may
be empty) before kinit(1) and any kerberized client software like
Thunderbird or Firefox will use Kerberos.  Creating /etc/krb5.conf --
even an empty file -- has the side effect of enabling pam_krb5, which
is on by default in /etc/pam.d for system, ftpd, display_manager, and
sshd.

Even if /etc/krb5.conf is empty and doesn't specify a default realm or
KDC, pam_krb5 will attempt to discover the realm and KDC by DNS TXT and
SRV records based on the local hostname.  All of these can be forged by
an adversary on the network.

Alternatively, for client-side SSO, you use ~/.krb5/config, or create
a different file and set the environment variable KRB5_CONFIG to point
to it in any processes like kinit(1) and client programs that use
Kerberos.  This approach does not enable pam_krb5.


Solutions and Workarounds
=========================

Any one of the following steps will prevent the attack:

- - If you are not using Kerberos at all:

        Ensure you have no /etc/krb5.conf.

- - If you are using Kerberos for client-side SSO, as in kinit/klist for
  login to remote hosts, but not to authenticate users logging in with
  sshd, xdm, etc.:

        Either:
        . Comment out the `pam_krb5.so' lines in /etc/pam.d.
        . Delete /etc/krb5.conf and use ~/.krb5/config instead.

- - If you want to use Kerberos for authenticating users logging in:

        Obtain a keytab for your host from your Kerberos administrator.

        (For sshd, you should generally use `GSSAPIAuthentication yes'
        in /etc/ssh/sshd_config to get native Kerberos authentication,
        not pam_krb5, which is a form of password authentication that
        just uses Kerberos to check a password.)

If you want to use Kerberos for authenticating users logging in with
passwords, and you do not have a keytab from your Kerberos
administrator: This use-case is inherently vulnerable to the attack,
and you must rely on physical security or a closed network to prevent
it.  Once you update NetBSD, you must enable the new `allow_kdc_spoof'
option on any pam_krb5.so lines in /etc/pam.d for this to continue to
function.


To apply a fixed version from a releng build, fetch a fitting
base.tar.xz (or base.tgz on older architectures) from nycdn.NetBSD.org
and extract the fixed binaries:

cd /var/tmp
ftp https://nycdn.NetBSD.org/pub/NetBSD-daily/REL/BUILD/ARCH/binary/sets/base.tar.xz
cd /
tar xzpf /var/tmp/base.tar.xz ./usr/lib/security/pam_krb5.so.4

with the following replacements:
REL   = the release version you are using
BUILD = the source date of the build. 20230621* and later will fit
ARCH  = your system's architecture


The following instructions describe how to upgrade your pam_krb5
binaries by updating your source tree and rebuilding and installing
a new version of pam_krb5:

* NetBSD-current:

	Systems running NetBSD-current dated from before 2023-06-20
	should be upgraded to NetBSD-current dated 2023-06-21 or later.

	The following files/directories need to be updated from the
	netbsd-current CVS branch (aka HEAD):
		lib/libpam/modules/pam_krb5

	To update from CVS, re-build, and re-install pam_krb5:
		# cd src
		# cvs update -d -P lib/libpam/modules/pam_krb5
		# cd lib/libpam/modules/pam_krb5
		# make USETOOLS=no cleandir dependall
		# make USETOOLS=no install

* NetBSD 10.*:

	Systems running NetBSD 10.* sources dated from before
	2023-06-21 should be upgraded from NetBSD 10.* sources dated
	2023-06-22 or later.

	The following files/directories need to be updated from the
	netbsd-10 branch:
		lib/libpam/modules/pam_krb5

	To update from CVS, re-build, and re-install pam_krb5:
		# cd src
		# cvs update -r netbsd-10 -d -P lib/libpam/modules/pam_krb5
		# cd lib/libpam/modules/pam_krb5
		# make USETOOLS=no cleandir dependall
		# make USETOOLS=no install


* NetBSD 9.*:

	Systems running NetBSD 9.* sources dated from before 2023-06-21
	should be upgraded from NetBSD 9.* sources dated 2023-06-22 or
	later.

	The following files/directories need to be updated from the
	netbsd-9 branch:
		lib/libpam/modules/pam_krb5

	To update from CVS, re-build, and re-install pam_krb5:
		# cd src
		# cvs update -r netbsd-9 -d -P lib/libpam/modules/pam_krb5
		# cd lib/libpam/modules/pam_krb5
		# make USETOOLS=no cleandir dependall
		# make USETOOLS=no install


* NetBSD 8.*:

	Systems running NetBSD 8.* sources dated from before 2023-06-21
	should be upgraded from NetBSD 8.* sources dated 2023-06-22 or
	later.

	The following files/directories need to be updated from the
	netbsd-8 branch:
		lib/libpam/modules/pam_krb5

	To update from CVS, re-build, and re-install pam_krb5:

		# cd src
		# cvs update -r netbsd-8 -d -P lib/libpam/modules/pam_krb5
		# cd lib/libpam/modules/pam_krb5
		# make USETOOLS=no cleandir dependall
		# make USETOOLS=no install

Thanks To
=========

Taylor R. Campbell for finding and fixing the issue.


Revision History
================

	2023-06-28	Initial release


More Information
================

Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at

	https://cdn.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2023-NNN.txt.asc

Information about NetBSD and NetBSD security can be found at

	https://www.NetBSD.org/
	https://www.NetBSD.org/Security/

Copyright 2023, The NetBSD Foundation, Inc.  All Rights Reserved.
Redistribution permitted only in full, unmodified form.

$NetBSD: NetBSD-SA2023-006.txt,v 1.1 2023/06/28 15:33:55 christos Exp $
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