SMB1/2/3 connections may not require signing where they should

Description

There are several code paths where the code doesn’t enforce SMB signing:

• The fixes for CVE-2015-5296 didn’t apply the implied signing protection
  when enforcing encryption for commands like ‘smb2mount -e’, ‘smbcacls -e’ and
  ‘smbcquotas -e’.

• The python binding exported as ‘samba.samba3.libsmb_samba_internal’
  doesn’t make use of the “client signing” smb.conf option.

• libgpo as well as ‘net ads gpo’ doesn’t require SMB signing when fetching
  group policies.

• Commandline tools like ‘smbclient’, ‘smbcacls’ and ‘smbcquotas’ allow
  a fallback to an anonymous connection when using the ‘–use-ccache’
  option and this happens even if SMB signing is required.

Patch Availability

A patch addressing this defect has been posted to

https://www.samba.org/samba/security/

Additionally 4.6.8, 4.5.14 and 4.4.16 have been issued as
security releases to correct the defect. Samba vendors and administrators
running affected versions are advised to upgrade or apply the patch as
soon as possible.

Workaround

The missing implied signing for ‘smb2mount -e’, ‘smbcacls -e’ and
‘smbcquotas -e’ can be enforced by explicitly using ‘–signing=required’
on the commandline or “client signing = required” in smb.conf.

Credits

This vulnerability was discovered and researched by Stefan Metzmacher of
SerNet (https://samba.plus) and the Samba Team (https://www.samba.org),
who also provides the fixes.