Bug 1553553 (CVE-2018-1057)

Upstream describes the following workaround:

Rewoke the change passwords right for everyone from all user objects (including
computers) in the directory. Note that this will prevent users from being able
to change their own expired passwords, so the maximum password age should be set
to a value that prevents user passwords from expiring while the workaround is in
place.

The change password right in AD is an extended object access right with the GUID
ab721a53-1e2f-11d0-9819-00aa0040529b. By default every user and computer object
in the directory grants the change password right to self and everyone.

The corresponding ACEs are

self:  (OA;;CR;ab721a53-1e2f-11d0-9819-00aa0040529b;;PS)
world: (OA;;CR;ab721a53-1e2f-11d0-9819-00aa0040529b;;WD)

in SDDL. The components of these ACEs are

OA: object access allowed
CR: extended rights
PS,WD: trustee: self, everyone

Of these, the ACE granting the right to everyone (world, WD) must be removed.

The ACL of any object in the Samba directory can be shown as SDDL with

# ldbsearch -H /var/lib/samba/private/sam.ldb cn=USER ntSecurityDescriptor

Alternatively, the ACL can be shown in NDR dump format by appending
"--show-binary":

# ldbsearch -H /var/lib/samba/private/sam.ldb \
  cn=USER ntSecurityDescriptor --show-binary
...
aces: struct security_ace
    type                     : SEC_ACE_TYPE_ACCESS_ALLOWED_OBJECT (5)
    flags                    : 0x00 (0)
           0: SEC_ACE_FLAG_OBJECT_INHERIT
           0: SEC_ACE_FLAG_CONTAINER_INHERIT
           0: SEC_ACE_FLAG_NO_PROPAGATE_INHERIT
           0: SEC_ACE_FLAG_INHERIT_ONLY
           0: SEC_ACE_FLAG_INHERITED_ACE
        0x00: SEC_ACE_FLAG_VALID_INHERIT (0)
           0: SEC_ACE_FLAG_SUCCESSFUL_ACCESS
           0: SEC_ACE_FLAG_FAILED_ACCESS
    size                     : 0x0028 (40)
    access_mask              : 0x00000100 (256)
    object                   : union security_ace_object_ctr(case 5)
    object: struct security_ace_object
        flags                    : 0x00000001 (1)
               1: SEC_ACE_OBJECT_TYPE_PRESENT
               0: SEC_ACE_INHERITED_OBJECT_TYPE_PRESENT
        type                     : union security_ace_object_type(case 1)
        type                     : ab721a53-1e2f-11d0-9819-00aa0040529b
        inherited_type           : union security_ace_object_inherited_type(case 0)
    trustee                  : S-1-1-0
...

To temporarily remove this ACE you can use ldbedit:

# ldbedit -H /var/lib/samba/private/sam.ldb cn=USER ntSecurityDescriptor

This invokes the configured editor. In the editor search for the string

  (OA;;CR;ab721a53-1e2f-11d0-9819-00aa0040529b;;WD)

and remove it (be careful about line wraps). Saving and exiting the editor will
apply the change to the directory.

Mitigation:

Revoke the change passwords right for everyone from all user objects (including computers) in the directory. Note that this will prevent users from being able to change their own expired passwords, so the maximum password age should be set to a value that prevents user passwords from expiring while the workaround is in place. For more information please refer to: https://bugzilla.redhat.com/show_bug.cgi?id=1553553#c3

Acknowledgments:

Name: the Samba project
Upstream: Björn Baumbach (Sernet)

Statement:

The versions of samba shipped with Red Hat Enterprise Linux 6 and 7 do not support Active Directory Domain Controller (AD-DC) mode. Therefore this flaw does not affect Red Hat Enterprise Linux 6 and 7.

External References:

https://www.samba.org/samba/security/CVE-2018-1057.html

Created samba tracking bugs for this issue:

Affects: fedora-all [bug 1554756]

Upstream patches available at:

https://www.samba.org/samba/ftp/patches/security/samba-4.4.16-CVE-2018-1057.patch
https://www.samba.org/samba/ftp/patches/security/samba-4.3.13-CVE-2018-1057.patch