2.5.3.2 Access Check Algorithm Pseudocode

Artigo
04/04/2023

In overview, the Access Check algorithm takes an access request and a security descriptor. It iterates through the DACL of the security descriptor, processing each ACE. If the ACE contains a SID that is also in the Token authorization context, then the ACE is processed, otherwise it is skipped. If an ACE grants access to that SID, then those access rights from the Access Request Mask are considered satisfied, and removed from the mask. If the ACE denies access to that SID, and the access rights in the ACE are present in the request mask, the whole request is denied. At the end of the algorithm, if there are any access rights still pending in the Access Request Mask, then the request is considered denied.

There are two noteworthy configurations of the security descriptor in light of the access check algorithm: an empty DACL, and a NULL (or absent) DACL. No DACL in the security descriptor implies that there is no policy in place to govern access to the object; any access check will succeed. An empty DACL, where the DACL is marked as being present but contains no ACEs, means that no principal can gain access to the object, except through the implied access of the owner.

If the access request is MAXIMUM_ALLOWED, the algorithm operates in a different mode. It iterates through every ACE in the DACL of the security descriptor, remembering which access rights were granted or denied for each ACE. After all ACEs have been examined, the complete set of grantable access rights is computed and returned via the GrantedAccess parameter (described later in this section).

Note that the use of MAXIMUM_ALLOWED is not recommended; instead, callers can request the specific minimum level of access required to accomplish their requirements.

The detailed processing of the list is as follows.

On entrance:

SecurityDescriptor: SECURITY_DESCRIPTOR structure that is assigned to the object.
Token: Authorization context as described above.
Access Request mask: Set of permissions requested on the object.
Object Tree: An array of OBJECT_TYPE_LIST structures representing a hierarchy of objects for which to check access. Each node represents an object with three values: A GUID that represents the object itself; a value called Remaining, which can be zero, and which specifies the user rights requests for that node that have not yet been satisfied; and a value called Level, which indicates the level of the object type in the hierarchy.
PrincipalSelfSubst SID: A SID that logically replaces the SID in any ACE that contains the well-known PRINCIPAL_SELF SID. It can be null.

GrantedAccess: An optional ACCESS_MASK output parameter used when the Access Request Mask parameter equals MAXIMUM_ALLOWED. Upon return this parameter contains the set of permissions granted to Token by the SecurityDescriptor.

 STATUS_CODE
 EvaluateTokenAgainstDescriptor(
     TOKEN Token,
     SECURITY_DESCRIPTOR SecurityDescriptor,
     ACCESS_MASK Access_Request_mask,
     OBJECT_TYPE_LIST Object Tree,
     Sid PrincipalSelfSubstitute,
     [out] ACCESS_MASK GrantedAccess)
  
     Dim OBJECT_TYPE_LIST LocalTree
     Dim ULONG Result
  
     Set DACL to SecurityDescriptor Dacl field
     Set SACL to SecurityDescriptor Sacl field
     Set RemainingAccess to Access Request mask
     Set AllowedAccesses to 0
     Set DeniedAccesses to 0
     Set MaxAllowedMode to FALSE
  
     IF RemainingAccess contains ACCESS_SYSTEM_SECURITY access bit THEN
         IF Token.Privileges contains SeSecurityPrivilege THEN
             Remove ACCESS_SYSTEM_SECURITY access bit from RemainingAccess
             Set GrantedAccess to GrantedAccess or ACCESS_SYSTEM_SECURITY
  
             IF RemainingAccess to 0 THEN
                 Return success
             Else
  
         ELSE
             Set GrantedAccess to 0
             Return access_denied
  
         END IF
     END IF
  
     IF RemainingAccess contains WRITE_OWNER access bit and Token.Privileges is not NULL THEN
         IF  Token.Privileges contains SeTakeOwnershipPrivilege THEN
           Remove WRITE_OWNER access bit from RemainingAccess
           Set GrantedAccess to GrantedAccess or WRITE_OWNER
        END IF
     END IF
  
     -- the owner of an object is always granted READ_CONTROL and WRITE_DAC.
     CALL SidInToken(Token, SecurityDescriptor.Owner, PrincipalSelfSubst)
     IF SidInToken returns True THEN
         IF DACL does not contain ACEs from object owner THEN
             Remove READ_CONTROL and WRITE_DAC from RemainingAccess
             Set GrantedAccess to GrantedAccess or READ_CONTROL or WRITE_DAC
         END IF
     END IF
  
     -- Support for MAXIMUM_ALLOWED 
     IF RemainingAccess contains MAXIMUM_ALLOWED access bit THEN
         Set MaxAllowedMode to TRUE
     END IF
  
     IF Object Tree is not NULL THEN
         Set LocalTree to Object Tree
  
     -- node is of type OBJECT_TYPE_LIST
         FOR each node in LocalTree DO
             Set node.Remaining to RemainingAccess
         END FOR
  
         END IF
  
     FOR each ACE in DACL DO
         IF ACE.AceFlags does not contain INHERIT_ONLY_ACE THEN
        
             CASE ACE.Type OF
  
                 CASE Allow Access:
  
                     CALL SidInToken( Token, ACE.Sid, and PrincipalSelfSubst )
                     IF SidInToken returns True THEN
                         IF MaxAllowedMode equals TRUE THEN
                           Set AllowedAccesses to AllowedAccesses or ACE.AccessMask
                           Set GrantedAccess to GrantedAccess or ACE.AccessMask
                         ELSE
                             Remove ACE.AccessMask from RemainingAccess
                             Set GrantedAccess to GrantedAccess or(RemainingAccess and 
                                 ACE.AccessMask)
                             FOR each node in LocalTree DO
                                 Remove ACE.AccessMask from node.Remaining                                
                             END FOR
                         END IF
                     END IF
        
                 CASE Deny Access:
                     IF ACE.AccessMask equals 0 returns True THEN Break             
  
                     CALL SidInToken( Token, ACE.Sid, PrincipalSelfSubst )
                     IF SidInToken returns True THEN
                         IF MaxAllowedMode equals TRUE THEN
                             Set DeniedAccesses to DeniedAccesses or ACE.AccessMask
                         ELSE
                             IF any bit of RemainingAccess is in ACE.AccessMask THEN
                                 Set GrantedAccess to 0
                                 Return access_denied
                             END IF
                         END IF
                     END IF
        
                 CASE Object Allow Access:
  
                     CALL SidInToken( Token, ACE.Sid, PrincipalSelfSubst )
                     IF SidInToken returns True THEN
                         IF ACE.Object is contained in LocalTree THEN
                             Locate node n in LocalTree such that 
                               n.GUID is the same as ACE.Object
                             Remove ACE.AccessMask from n.Remaining
                             FOR each node ns such that ns is a descendent of n DO
                                 Remove ACE.AccessMask from ns.Remaining
                             END FOR
                             FOR each node np such that np is an ancestor of n DO
                               Set np.Remaining to np.Remaining or np-1.Remaining
  
                           --  the 'or' above is a logical bitwise OR operator.  For
                           --  Some uses (like Active Directory), a hierarchical list
                           --  of types can be passed in; if the requestor is granted 
                           --  access to a specific node, this will grant access to 
                           --  all its children. The preceding lines implement this by 
                           --  removing, from each child, the permissions just found for
                           --  the parent. The change is propagated upwards in
                           --  the tree: once a permission request has been satisfied
                           --  we can tell the next-higher node that we do not need
                           --  to inherit it from the higher node (we already have it 
                           --  in the current node). And since we must not blindly 
                           --  replace the parent's RemainingAccess, we BIT_OR the  
                           --  parent's RemainingAccess with the current node's. This 
                           --  way, if the parent needs, say, READ_CONTROL, and the
                           --  current node was just granted that, the parent's 
                           --  RemainingAccess still contains this bit since satisfying 
                           --  the request at a lower level does nothing to affect 
                           --  the higher level node. Active Directory has its own  
                           --  checking rules--see [MS-ADTS] section 3.1.1.4.3. 
                           
                             END FOR
                         END IF
                     END IF
        
                 CASE Object Deny Access:
  
                     CALL SidInToken( Token, ACE.Sid, PrincipalSelfSubst )
                     IF SidInToken returns True THEN
                         Locate node n in LocalTree such that 
                           n.GUID is the same as ACE.Object
                         IF n exists THEN
                             If any bit of n.Remaining is in ACE.AccessMask THEN
                                 Set GrantedAccess to 0
                                 Return access_denied
                             END IF
                         END IF
                     END IF
  
                 CASE Allow Access Callback Ace:
  
                     EvaluateAceCondition(Token,
                                          Sacl,
                                          ApplicationData,
                                          ApplicationDataSize) returning Result 
  
                     IF Result is 1 THEN
                         IF (SidInToken(Token, ACE.Sid, PrincipalSelfSubst)) THEN
                             IF MaxAllowedMode equals TRUE THEN
                                 Set GrantedAccess to GrantedAccess or ACE.AccessMask
                                 Set AllowedAccesses to AllowedAccesses or ACE.AccessMask
  
                             ELSE
                                 Remove  ACE.AccessMask from RemainingAccess
  
                                 SET n = root node of object tree
                                 FOR each node np such that np is an ancestor of n DO
                                   Set np.Remaining to np.Remaining or np-1.Remaining
  
                           --  the 'or' above is a logical bitwise OR operator.  For
                           --  Some uses (like Active Directory), a hierarchical list
                           --  of types can be passed in; if the requestor is granted 
                           --  access to a specific node, this will grant access to 
                           --  all children. The preceding lines implement this by 
                           --  removing, from each child, the permissions just found for 
                           --  the parent. The change is propagated upwards in
                           --  the tree: once a permission request has been satisfied
                           --  we can tell the next-higher node that we do not need
                           --  to inherit it from the higher node (we already have it 
                           --  in the current node). And since we must not blindly 
                           --  replace the parent's RemainingAccess, we BIT_OR the  
                           --  parent's RemainingAccess with the current node's. This 
                           --  way, if the parent needs, say, READ_CONTROL, and the
                           --  current node was just granted that, the parent's 
                           --  RemainingAccess still contains this bit since satisfying 
                           --  the request at a lower level does nothing to affect 
                           --  the higher level node.
  
                                 END FOR
                             END IF
                         END IF
                     END IF
             END CASE            
         END IF
     END FOR
  
     IF MaxAllowedMode equals TRUE THEN
         -- The not operator below is a bit-wise operator
         Set GrantedAccess to AllowedAccesses and (not DeniedAccesses)
         IF GrantedAccess not equals 0 THEN
             Return success
         ElSE
             Return access_denied
  
 END IF
  
 SET GrantedAccess to 0
  
     IF RemainingAccess to 0 THEN
         Return success
     Else
         Return access_denied
     END IF
  
 END-SUBROUTINE
  
 STATUS_CODE
 AccessCheck(
     TOKEN Token,
     SECURITY_DESCRIPTOR SecurityDescriptor,
     ACCESS_MASK Access Request mask,
 OBJECT_TYPE_LIST Object Tree,
 Sid PrincipalSelfSubstitute,
     [out] ACCESS_MASK GrantedAccess)
  
     Dim CentralAccessPolicy CentralizedAccessPolicy
     Dim SECURITY_DESCRIPTOR CaprSecurityDescriptor
     Dim SECURITY_DESCRIPTOR StagedCaprSecurityDescriptor
     Dim ACCESS_MASK DesiredAccess
     Dim ACCESS_MASK CentralAccessPolicyEffectiveAccess
     Dim ACCESS_MASK CentralAccessPolicyEntryEffectiveAccess
     Dim ACCESS_MASK CentralAccessPolicyStagedAccess
     Dim ACCESS_MASK CentralAccessPolicyEntryStagedAccess
     Dim ULONG Result
     Dim STATUS_CODE Status
     
     Set DACL to SecurityDescriptor Dacl field
     Set SACL to SecurityDescriptor Sacl field
     Set RemainingAccess to Access Request mask
     Set AllowedAccesses to 0
     Set DeniedAccesses to 0
     Set DesiredAccess to Access Request mask
  
     CALL EvaluateTokenAgainstDescriptor(Token,
                                         SecurityDescriptor,
                                         DesiredAccess,
                                         Object Tree,
                                         PrincipalSelfSubstitute,
                                         GrantedAccess) returning Status
  
     IF Status is access_denied THEN
         return Status
     END IF
  
     CALL GetCentralizedAccessPolicy(SACL) returning CentralizedAccessPolicy
  
     IF CentralizedAccessPolicy is not NULL THEN
  
         Set CentralAccessPolicyEffectiveAccess to GrantedAccess
         Set CentralAccessPolicyStagedAccess to GrantedAccess
  
         FOR each CentralAccessPolicyRule in CentralAccessPolicy.RulesList
  
            EvaluateAceCondition(Token,
                                 SACL,
                                 AppliesTo,
                                 AppliesToSize) returning Result
  
             IF Result is not 1 THEN
                 GOTO NextRule
             END IF
  
             Copy SecurityDescriptor to CaprSecurityDescriptor
             Set CaprSecurityDescriptor.DACL to
     CentralAccessPolicyRule.EffectiveCentralAccessPolicy.AccessCondition.DACL
  
             EvaluateTokenAgainstDescriptor
                                    (Token,
                                     CaprSecurityDescriptor,
                                     DesiredAccess,
                                     NULL,
                                     PrincipalSelfSubstitute,
                                     CentralAccessPolicyEntryEffectiveAccess)
  
             -- The and operator below is a bit-wise operator
             Set CentralAccessPolicyEffectiveAccess to 
          CentralAccessPolicyEffectiveAccess and CentralAccessPolicyEntryEffectiveAccess
  
            -- StagingLocalPolicyEnabled = True if MS-GPAC ADM variable
            -- "System Advanced Audit Policy" (MS-GPAC section 3.2.1.1) contains the GUID
            -- for "Central Access Policy Staging" as specified in MS-GPAC section 2.2.1.2
  
             IF IfStagingLocalPolicyEnabled THEN 
  
                 Copy SecurityDescriptor to StagedCaprSecurityDescriptor
                 Set StagedCaprSecurityDescriptor.DACL to 
          CentralAccessPolicyRule.StagedCentralAccessPolicy.AccessControl.DACL
  
                 EvaluateTokenAgainstDescriptor
                                         (Token,
                                         StagedCaprSecurityDescriptor,
                                         DesiredAccess,
                                         NULL,
                                         PrincipalSelfSubstitute,
                                         CentralAccessPolicyEntryStagedAccess)
  
                 -- The and operator below is a bit-wise operator
  
                 Set CentralAccessPolicyStagedAccess to CentralAccessPolicyStagedAccess 
                                             and CentralAccessPolicyEntryStagedAccess
  
             ELSE IF CentralAccessPolicyEffectiveAccess is 0 THEN
                 Set GrantedAccess to 0
                 return access_denied
             END IF
  
  
 NextRule:
         END FOR
  
         IF CentralAccessPolicyEffectiveAccess is not equal to 
                                         CentralAccessPolicyStagedAccess THEN
             -- Log the difference between the Effective and Staged Access
         END IF
  
         -- The "not" and "and" operator below is a bit-wise operator
  
         Set AllowedAccess to AllowedAccess and CentralAccessPolicyEffectiveAccess
         Set RemainingAccess to DesiredAccess and not CentralAccessPolicyEffectiveAccess
  
         FOR each node in Object Tree DO
             Set node.Remaining to RemainingAccess
         END FOR
  
     ELSE
        Return success
     END IF
  
     IF MaxAllowedMode equals TRUE THEN
         -- The not operator below is a bit-wise operator
         Set GrantedAccess to AllowedAccesses and (not DeniedAccesses)
  
         IF GrantedAccess is 0 THEN
             Return access_denied
         Else
             Return success
         END IF
  
     END IF
  
     SET GrantedAccess to 0
    
     IF RemainingAccess is 0 THEN
         Return success
     Else
         Return access_denied
     END IF
  
 END-SUBROUTINE

2.5.3.2 Access Check Algorithm Pseudocode

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