Description of problem:

Since a recent Samba upgrade (4.14 / 4.15 ?), unix group membership rules are erroneously applied to /etc/samba/smbuser unix user accounts (i.e. entries without the '@' prefix).


Version-Release number of selected component (if applicable):

Tested with latest samba-4.15.5-3.el8.x86_64


How reproducible:

Always


Steps to Reproduce:
1. /etc/samba/smbusers entry : "root = administrator admin"
2. start a Samba session with a Samba user account belonging to the Unix group "admin"


Actual results:

User access is refused


Expected results:

User should be granted access


Additional info:

1. user "didier" is member of the unix group "admin". The user is mapped to the 'root'-account, which does not have a valid Samba account ; hence access is denied.

[2022/02/28 09:53:40.769579,  3, pid=4179984, effective(0, 0), real(0, 0), class=auth] ../../source3/auth/auth.c:202(auth_check_ntlm_password)
  check_ntlm_password:  Checking password for unmapped user [SAMBA]\[didier]@[CP0043] with the new password interface
[2022/02/28 09:53:40.769593,  3, pid=4179984, effective(0, 0), real(0, 0), class=auth] ../../source3/auth/auth.c:205(auth_check_ntlm_password)
  check_ntlm_password:  mapped user is: [SAMBA]\[root]@[CP0043]
[2022/02/28 09:53:40.770828,  3, pid=4179984, effective(0, 0), real(0, 0), class=auth] ../../source3/auth/check_samsec.c:399(check_sam_security)
  check_sam_security: Couldn't find user 'root' in passdb.
[2022/02/28 09:53:40.770854,  2, pid=4179984, effective(0, 0), real(0, 0), class=auth] ../../source3/auth/auth.c:348(auth_check_ntlm_password)
  check_ntlm_password:  Authentication for user [didier] -> [root] FAILED with error NT_STATUS_NO_SUCH_USER, authoritative=1

2. https://www.samba.org/samba/docs/current/man-html/smb.conf.5.html#USERNAMEMAP states : "The list of usernames on the right may contain names of the form @group in which case they will match any UNIX username in that group."

The @-prefix is not present, nevertheless group membership rules are applied.


3. Mitigation : 
3a. accounts which are not a member of unix group "admin" are granted access ;
3b. commenting out the "root = administrator admin" smbusers entry grants access to members of unix group "admin".



4. Summary :
It seems (unwanted) group membership rules are applied *without* the "@" prefix.