Re: Help regarding the security descriptor creation algorithms

Hi Nadezhda:

We have identified the flaw in the algorithm and as soon as I have the final version of the algorithm, I’ll be in touch. Your finding is correct and algorithm needs modification.

 

Regards,

Obaid Farooqi

Sr. Support Escalation Engineer | Microsoft

 

 

 

 

Hi Obaid,

I was wandering if there is any progress on this issue?

 

Regards,

Nadezhda Ivanova

 

 

Hi Nadezhda:

Just an update. I am still working on your issue. I’ll update you as soon as I have something concrete.

 

Regards,

Obaid Farooqi

Sr. Support Escalation Engineer | Microsoft

 

 

Hi Nadezhda:

My name is Obaid Farooqi and I am a member of protocol documentation team. I’ll be helping you with your question regarding security descriptor creation algorithms.

I’ll keep you updated as appropriate with my investigation.

Feel free to contact me if you have any further question or clarification about this issue.

 

Regards,

Obaid Farooqi

Sr. SEE | Microsoft

 

 

 

Hi,

I have been working on implementing correct nTSecurityDeascriptor creation in the directory service of Samba 4, and have come upon a problem in the ComputeInheritedACLfromParent  subroutine described in MS-DTYP 2.5.2.6. The way the algorithm is described, the purpose of this algorithm is to determine which ACE’s from an object’s parent are to be inherited by the new object actively, and which are to be inherited only. The ComputeInheritedACLfromParent as described, walks the parent ACL twice. The first time it determines the active inherited ACE’s, the second time the ones that are inherited but inactive.

I have been testing our implementation with the CN=Schema partition, as the attributes and objects by default are not given a security descriptor during creation, and the defaultSecurityDescriptor of attribute-Schema is empty DACL and SACL.

So, they inherit all their DACL ACE’s from their parent, CN=Schema.

 

In a Win2008R2, CN=Schema has three inheritable DACL ACE’s:

 

(A;CI;RPLCLORC;;;AU)

(A;CI;RPWPCRCCLCLORCWOWDSW;;;SA)

(A;CI;RPWPCRCCDCLCLORCWOWDSDDTSW;;;SY)

 

ComputeInheritedACLfromParent has the following arguments:

                         ACL: ACL that contains the parent's ACEs from which to compute the inherited ACL.

                         IsContainerObject: TRUE if the object is a container, FALSE otherwise.

                         ObjectTypes: Array of GUIDs for the object type being created.

 

So if we invoke the ComputeInheritedACLfromParent with the above DACL,and  isConatinerObject = true (According to MS-ADTS 7.1.3, true is always the value), the first walk of the input

 

Initialize ExplicitACL to Empty ACL

FOR each ACE in ACL DO

IF ACE.Flags contains INHERIT_ONLY

THEN

        CONTINUE

ENDIF

           

IF(((ACE.Flags contains CONTAINER_INHERIT) AND

        (IsContainerObject = TRUE))OR

                                     ((ACE.Flags contains OBJECT_INHERIT) AND (IsContainerObject = FALSE)))

THEN

CASE ACE.Type OF

ALLOW:

DENY:

Set NewACE to ACE

Set NewACE.Flags to INHERITED

Append NewACE to ExplicitACL

OBJECT_ALLOW:

OBJECT_DENY:

IF (ObjectTypes contains ACE.ObjectGUID) THEN

Set NewACE to ACE

Set NewACE.Flags to INHERITED

Append NewACE to ExplicitACL

ENDIF

ENDCASE

ENDIF

END FOR

 

Will give:

 

D:AI(A;CIID;RPLCLORC;;;AU)(A;CIID;RPWPCRCCLCLORCWOWDSW;;;SA)(A;CIID;RPWPCRCCDCLCLORCWOWDSDDTSW;;;SY)

 

Which is as expected, as this is the DACL of all attributes and classes in Win 2008.

However, the algorithm then walks the input a second time:

 

Initialize InheritableACL to Empty ACL

IF (IsContainerObject = TRUE) THEN  //In our case this is always true

FOR each ACE in ACL DO

IF ACE.Flags contains NO_PROPAGATE THEN  //This flag is not set

CONTINUE

ENDIF

           

IF((ACE.Flags contains CONTAINER_INHERIT) OR

(ACE.Flags contains OBJECT_INHERIT))

THEN

Set NewACE to ACE

Add INHERITED to NewACE.Flags

Add INHERIT_ONLY to NewACE.Flags

Append NewACE to InheritableACL

ENDIF

                END FOR

ENDIF

 

This second loop yields:

 

(A;CIIOID;RPLCLORC;;;AU)(A;CIIOID;RPWPCRCCLCLORCWOWDSW;;;SA)(A;CIIOID;RPWPCRCCDCLCLORCWOWDSDDTSW;;;SY)

 

Which after:

RETURN concatenation of ExplicitACL and InheritableACL

 

Makes the final DACL look like:

 

D:AI(A;CIID;RPLCLORC;;;AU)(A;CIID;RPWPCRCCLCLORCWOWDSW;;;SA)(A;CIID;RPWPCRCCDCLCLORCWOWDSDDTSW;;;SY)(A;CIIOID;RPLCLORC;;;AU)(A;CIIOID;RPWPCRCCLCLORCWOWDSW;;;SA)(A;CIIOID;RPWPCRCCDCLCLORCWOWDSDDTSW;;;SY)

 

So ACE’s are duplicated.

 

However, an attribute’s DACL in Win2008 does not have these last three ACE’s, so I am obviously missing something. How should the flow actually go with this same example in order to avoid this duplication? Or am I providing the wrong argument?

 

Best Regards,

Nadezhda Ivanova

Nadezhda Ivanova Software Engineer Software Development nadezhda.ivanova@...	CISCO SYSTEMS BULGARIA EOOD 18 Macedonia Blvd. Sofia 1606 Bulgaria Cisco home page
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