[Samba] Account is sensitive and cannot be delegated (userAccountControl NOT_DELEGATED flag 0x00100000)

Andrew Bartlett abartlet at samba.org
Sat Jan 6 21:55:09 UTC 2018

On Sat, 2018-01-06 at 11:11 +0000, Antonios Kalkakos via samba wrote:
> I have an AD with two Debian Stretch Samba 4.5.12 DCs. The Samba and Heimdal Kerberos 7.1.0 packages are installed from Debian repositories. Management is done from MS-RSAT installed on a Windows 7 Pro client.
> When I select the option "Account is sensitive and cannot be delegated" (in Active Directory Users and Computers under the Account tab) for a user account regardless of its privileges, the user cannot logon on any client PC. Windows 7 responds "Logon failure: user account restriction. Possible reasons are blank passwords are not allowed, logon hour restrictions, or a policy restriction has been enforced" and a Debian Stretch client responds "You are not allowed to logon from this workstation". The Samba DC will provide a non-forwardable TGT, if you ask for it with kinit -F command from the Linux client. Issuing the command kinit -f will again fail with "krb5_get_init_creds: Ticket may not be forwardable".
> Investigation with Wireshark showed that after receiving an AS-REQ for a TGT with the forwardable flag set, the Samba 4.5.12 DC responds a KRB5KDC_ERR_POLICY with e-text "Ticket may not be forwardabale" (same as kinit -f). This behavior is correct according to CVE-2016-2125 (https://www.samba.org/samba/security/CVE-2016-2125.html) which states:
> 0x00100000: UF_NOT_DELEGATED:
> The UF_NOT_DELEGATED can be used to disable the ability to get forwardable TGT
> for the account. It means the KDC will respond with an error if the client asks
> for the forwardable ticket.  The client typically gives up and removes the
> GSS_C_DELEG_FLAG flag and continues without passing delegated credentials.
> Administrators can use this to disable possible delegation for the most
> privileged accounts (e.g. administrator accounts).
> Upon the initial logon procedure however, both Samba 4.5.12 and Windows 7 clients will actually give up and not continue asking for a non-forwardable TGT, which means that the user will be locked out. 
> Testing with Wireshark on another AD with one Windows 2008 R2 DC showed that the DC ignored the forwardable flag on AS-REQ and the user logged in normally having a non-forwardable TGT. All subsequent TGS requests on the same logon session from a Windows 7 client didn't have the forwardable flag set.
> Should I fill a bug for that, request to be added on Samba wiki or am I doing something wrong?

Yes, please file a bug.  Clearly we need a test for this. 

(Regarding Rowland's point, the Heimdal package on Debian won't
actually be used by the Samba 4.5 package). 

Andrew Bartlett
Andrew Bartlett                       http://samba.org/~abartlet/
Authentication Developer, Samba Team  http://samba.org
Samba Developer, Catalyst IT          http://catalyst.net.nz/services/samba

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