Hardening Samba as an AD DC
The Samba Team values the effort put by others into documenting Samba, and so rather than duplicate that work, please do first see:
Samba smb.conf options to set/not set
While this is not a comprehensive list, the below options some of which are defaults often disabled, and others are stronger options off by default should be carefully checked to confirm that Samba's behaviour is as strict as is possible in your organisation.
Samba's AD DC Audit logging is configured via the
log level parameter, and should be configured and processed by a log analysis stem on a hardened installation.
ldap server require strong auth
This smb.conf option was introduced due to the attacks on LDAP known as NTLM forwarding or 'pass the hash' attacks. If
ldap server require storong auth it set to
allow_sasl_over_tls then the authentication to an SMB server by a privileged user or computer can be redirected to our LDAP server, allowing impersonation.
A hardened installation will retain the default value of
See also notes on this change.
It is more secure to use Kerberos with Kerberos encryption directly.
Future possible hardening
Microsoft has chosen a different path to addressing this issue, and instead would like AD clients to include a session-specific value in the NTLMv2 response, known a channel binding. Samba doesn't set this as a client nor does it check this as a server, at this time. A future Samba may add a new option here allowing NTLM and Kerberos connections over TLS, when this is sent and checked.
NTLM authentication is 56-bit encryption at best and compromised badly for well-known word lists.
For a passive attacker, the options
allow-mschapv2 are at their core equally insecure, as they boil down to the same cryptography, but the latter option is permitted because MSCHAPv2 is harder to eliminate, is not presented on administrative interfaces and may be protected by a TLS tunnel.
A fully hardened installation should set this to
disabled, to require Kerberos or simple-bind authentication only, as even NTLMv2 is quite poor security in 2023.
Samba 4.17 removed LanMan authentication, if you are running Samba 4.16 and earlier, ensure that
lanman auth has the default setting of
allow store nt hash
Related to the above, Samba 4.17 has an the
allow store nt hash option, allowing Samba to avoid storing the NT hash. Well known for being unsalted the NT hash is required for
MSCHAPv2 authentication, as well as for
arcfour-hmac-md5 Kerberos authentication. However in 2023 all of these authentication mechanisms are considered as weak, and should not be used on hardened networks.
The purpose of this option is to allow Samba to not store the NT hash that will not be used, as it presents a vulnerability if the DB is exposed.
A hardened installation will set the value
never, which avoids this storage.
server min protocol, client min protocol, client ipc min protocol
Modern Samba versions default to SMB2 and above. A hardened Samba installation should not set
server min protocol
client min protocol or
client ipc min protocol to values such as
NT1, which enable the old SMB1 support.
Avoiding SMB1 avoids support for NTLMv2 without NTLMSSP, which is not as secure as NTLMv2 as used within NTLMSSP - eg as used in SMB2, as well as a significant body of older C code in Samba. The SMB message signing (integrity protection) protocols are also much better in SMB2.
smb1 unix extensions, unix extensions
This is only relevant if SMB1 is enabled.
The SMB1 unix extensions allow the creation of a server-side symlink to a user-chosen location, and significant work had to be done to avoid Samba following such a link. While this work is now complete, it is still better to never allow the creation of such symbolic links.
A hardened installation will retain the default of
NETLOGON Secure channel options
The default values of these options should not be changed:
allow nt4 crypto = no,
reject md5 clients = yes,
server schannel require seal = yes,
server schannel = yes
If specific client hosts need to use outdated NETLOGON cryptography, then see the smb.conf documentation for the per-host overrides that are permitted.
GnuTLS TLS option configuration
Samba can feed the GnuTLS cryptography provider an option string to describe the TLS protocols that should be supported and not supported. See GnuTLS manual for valid strings. The
tls priority option is is useful to disable a newly-vulnerable TLS protocol. The default in Samba (and in modern GnuTLS) is set to ensure that SSLv3 is disabled.
Enforce stronger password quality (not upper/lower/special chars)
A hardened installation of Samba will set
check password script to a secure script or binary that checks the most common cracked passwords as well as dictionary words. Samba provides an example
crackcheck binary, but cracklib which it uses has been superseded by tools checking large breach databases.
Insecure DNS updates
Samba can be configured to allow un-authenticated clients to update Dynamic DNS records. The default value for
allow dns updates is
secure only and this should be kept.
Samba Host hardening
These are things to do, on the Samba host, to improve security of Samba and your AD Domain.
The Samba Team does not maintain these hardening methods, but does recommend using (eg) packages with them prepared and tested.
Samba as packaged for some distributions (SuSE has been observed) has an extensive AppArmor rule set to restrict Samba only to the parts of the filesystem that it requires access to. This should not be disabled.
Samba as packaged on Fedora and RHEL has SELinux rules set to likewise restrict Samba only to the parts of the filesystem that it requires access to. This should not be disabled.
TLS Configuration (LDAP over TLS, ldaps://, StartTLS)
Samba will autogenerate a TLS certificate for the LDAP service. This should be replaced with a properly signed certificate trusted and verified by the clients in your organisation.
tls dh params file,
tls keyfile control the location of these files.
Currently Samba will need to be restarted when these files are updated.
Encryption at rest for the Samba AD DC
Samba's AD DC supports encryption at rest of passwords and other secret values (for a fixed list of attributes).
To use this feature to the full advantage, two hardening steps and then key management are required.
samba-tool domain provision or
samba-tool domain join
By default encryption at rest is enabled. Even without key management (below) this avoids a number of in-memory attacks on our database by separating the key from the encrypted data.
Ensure Samba's AD DC sam.ldb was created after Samba 4.8
A Samba AD DC database that was continuously updated in-place from an earlier Samba version will not gain the encrypted secret feature, it will continue to read and write plaintext secrets into the sam.ldb file.
A new AD DC running a modern Samba version should be joined to the domain, and the DC that was historically pre-Samba 4.8 DC should be demoted.
Arrange offline storage of the
Samba does not attempt key management of the
A hardened AD DC could arrange to have this file be supplied to Samba - say on a ramdisk after obtaining the key from a key vault - at startup.
Note that this would still be a partial solution, there are other secret files in
private/ that could be managed, but no specific engineering has been done for
secrets.keytab and host passwords in
Samba compilation hardening
Samba's build system and that of many Samba packages harden Samba against some attacks on the C codebase.
See Samba compiler hardening for details.
AD DC configuration options to consider
LDAP Max Query Duration
Samba honours the lDAPAdminLimits MaxQueryDuration however the default is 120 seconds. This is a very high value and the worker processing this query will not process other requests in this time. Consider lowering the value. Queries over 25% the maximum time will be logged at log level
Protected users, implemented in Samba 4.17 and later, are not permitted to use NTLM, are restricted from Kerberos delegations (were one server can act as the user to another) and have shorter ticket lifetimes. Windows clients also detect protected users and implement local protections for these accounts.
Samba supports domain-wide and Fine-Grained Password Policies / Password Setting Objects (PSOs).
The domain-wide policies can be set via GPO if
apply group policies = yes is set, but the direct method is to update the values in LDAP or via
samba-tool domain passwordsettings.
To configure fine-grained password policies, use
samba-tool domain passwordsettings pso per Tranquil IT's Applying a password strategy with Samba-AD guide
The default values in Samba are
Password complexity: on Store plaintext passwords: off Password history length: 24 Minimum password length: 7 Minimum password age (days): 1 Maximum password age (days): 42 Account lockout duration (mins): 30 Account lockout threshold (attempts): 0 Reset account lockout after (mins): 30
These values match Windows, but should be strengthened in a hardened domain. A hardened domain should have a password lockout set (by changing the threashold from 0) and should have a much longer minimum password length.