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Date: Wed, 21 Jun 2023 19:45:26 -0700
From: Russ Allbery <>
To: Taylor R Campbell <>
Subject: Re: Re: PAM/Kerberos issue on NetBSD

Taylor R Campbell <> writes:

> Linux pam_krb5[1] and sssd-krb5[2] are both affected by the same attack,
> but they have always been _documented_ to be affected; unlike BSD
> pam_krb5, it's just not news that they are affected.

Yes, this is a long-standing and well-known issue with Kerberos PAM
modules.  It's hard to solve properly because PAM modules are often
invoked as non-root users for other good security reasons and thus do not
have access to the system keytab, which means that doing proper KDC
verification requires providing them with a separate keytab for some other
principal that is safe to expose to whatever unprivileged user the PAM
module is running as.  Sometimes there is no safe option.

The correct fix without requiring protocol changes is probably a system
service running in a different privilege domain that exposes an API that
allows the PAM module to verify tickets with a keytab but protects the
keytab from being read by the PAM module.  I am aware of private
implementations of this approach, but I don't know if anyone has put one
in a public module.  I was going to do that at one point and never found
the time.

There are other ways to fix this problem by using more sophisticated
Keberos protocols.  For example, I think (although am not 100% sure) that
using FAST for the authentication protects against this attack because I
believe the FAST armor will authenticate the Kerberos KDC.

> (Side note: pam_krb5 (and sssd-krb5) is not and never has been the
> normal way to do Kerberos authentication in network services.  (E.g., in
> sshd, you set `GSSAPIAuthentication yes' for that.)  pam_krb5 has always
> been an abuse of Kerberos as a method to check a password, which
> Kerberos was designed to avoid, through SSO.)

This is only mostly true.  The primary exception is using a Kerberos PAM
module with console / desktop environment login on a Linux desktop or
laptop, where often people do prefer to use their Kerberos password as
their local system password so that they acquire Kerberos credentials at
the same time that they gain access to their local system.  In that case,
pam_krb5 is a replacement for kinit, and is using Kerberos as intended.
This used to be a fairly popular configuration for people using Linux at
institutions that used Kerberos for the site-wide authentication system.
It's been a while since I've worked for one of those institutions, so I'm
not sure if it still is.

But yes, a lot of the uses are places that should use GSSAPI but don't for
various reasons (usually because the client is not capable of GSSAPI, such
as lots of mail clients).

> The verify_ap_req_nofail option rates pretty high among the worst-named
> knobs I have ever seen, and has been confusing people for
> decades[9][10].  I filed an issue to make it default-secure in
> Heimdal[11]; this could pose compatibility issues, but sites that
> continue rely on the insecure option can always set it in their
> krb5.conf.

The primary thing that I would expect this to break is screen lock
programs for people who use Kerberos for local authentication.  (Also IMAP
and POP servers not configured with their own keytab and not running as
root, but there the solution is generally to get them a keytab.)

Russ Allbery (             <>

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