Samba impact of "ZeroLogin" CVE-2020-1472
kseeger at samba.org
Wed Sep 16 12:57:06 UTC 2020
The following applies to Samba used as domain controller only.
(Both as classic/NT4-style and active direcory DC.)
Samba users have reported that the exploit for "ZeroLogin" passes
Samba has some protection for this issue because since Samba 4.8 we have
set a default of 'server schannel = yes'.
Users who have changed this default are hereby warned that Samba
implements the AES netlogon protocol faithfully and so falls to the same
fault in the cryptosystem design.
Vendors supporting Samba 4.7 and below should patch their installations
and packages to change this default, as values of:
- server schannel = no
- server schannel = auto
are NOT secure and we expect can result in full domain compromise,
particularly for AD domains.
Some public exploit tests, such as
only confirm that a ServerAuthenticate3 call operates, but not that the
ServerPasswordSet2 call required to exploit the domain also operates.
We are well aware of administrator concern and are looking to provide
patches that provide mitigation here, to make the ServerAuthenticate3
call also fail.
We, like Microsoft, suggest that 'server schannel = yes' must be set for
secure operation. This is our equivalent to Microsoft's
FullSecureChannelProtection=1 registry key, with the difference
that it's already enabled by default in all Samba major versions
released in the last three years.
Finally, we would note that Samba's audit logging will record
ServerAuthenticate3 and ServerPasswordSet calls including the source IP,
details will be provided later on the options to enable.
There seem to be some legacy software, which still requires
"server schannel = auto". See the following bugs:
We'll add additional hardening that will allow
administrators to use "server schannel = yes" globally
and define exceptions only for specified computer accounts.
Our progress can be monitored via this bug:
Karolin Seeger https://samba.org/~kseeger/
Release Manager Samba Team https://samba.org
Team Lead Samba SerNet https://sernet.de
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