A flaw was found in the way Samba AD DC validated user permissions. An authenticated attacker could use this flaw to change any other users passwords, including administrative users.
As per samba upstream advisory:
On a Samba 4 AD DC the LDAP server in all versions of Samba from 4.0.0 onwards incorrectly validates permissions to modify passwords over LDAP allowing authenticated users to change any other users passwords, including administrative users.
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
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
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
# 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)
0x00: SEC_ACE_FLAG_VALID_INHERIT (0)
size : 0x0028 (40)
access_mask : 0x00000100 (256)
object : union security_ace_object_ctr(case 5)
object: struct security_ace_object
flags : 0x00000001 (1)
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
and remove it (be careful about line wraps). Saving and exiting the editor will
apply the change to the directory.
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
Name: the Samba project
Upstream: Björn Baumbach (Sernet)
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.
Created samba tracking bugs for this issue:
Affects: fedora-all [bug 1554756]
Upstream patches available at: