CVE-2016-2125

It was found that Samba always requested forwardable tickets when using Kerberos authentication. A service to which Samba authenticated using Kerberos could subsequently use the ticket to impersonate Samba to other services or domain users.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

The following mitigation is suggested by upstream.

The samba-tool command and the AD DC mode honours the undocumented "gensec_gssapi:delegation=no" option in the [global] section of the smb.conf file.

Controlling Kerberos forwarding

In the Active Directory world it's possible for administrators to limit the delegation. User and computer objects can both act as Kerberos users and also as Kerberos services. Both types of objects have an attribute called 'userAccountControl' which is a bitmask that controls the behavior of the account. The following three values have impact on possible delegation:

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).

0x00080000: UF_TRUSTED_FOR_DELEGATION

If the UF_TRUSTED_FOR_DELEGATION is set on an account a KDC will include the OK_AS_DELEGATE flag in a granted service ticket. If the client application uses just GSS_C_DELEG_POLICY_FLAG (instead of GSS_C_DELEG_FLAG) gssapi/Kerberos libraries typically only include delegated credentials when the service ticket includes the OK_AS_DELEGATE flag. Administrators can use this to control which services will get delegated credentials, for example if the service runs in a trusted environment and actually requires the presence of delegated credentials.

0x01000000: UF_TRUSTED_TO_AUTHENTICATE_FOR_DELEGATION

The UF_TRUSTED_TO_AUTHENTICATE_FOR_DELEGATION is not really relevant for this CVE and just listed here for completeness. This flag is relevant for the S4U2Proxy feature, where a service can ask the KDC for a proxied service ticket which can impersonate users to other services.