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Access Control Lists for COM

Windows Server XP Service Pack 2 (SP 2) and Windows Server 2003 Service Pack 1 (SP 1) introduce security enhancements for the Distributed Component Object Model (DCOM). One of these enhancements is more specific access rights for use in access control lists (ACLs). The access rights are:

COM_RIGHTS_EXECUTE 1
COM_RIGHTS_EXECUTE_LOCAL 2
COM_RIGHTS_EXECUTE_REMOTE 4
COM_RIGHTS_ACTIVATE_LOCAL 8
COM_RIGHTS_ACTIVATE_REMOTE 16

To provide backward compatibility, an ACL may exist in the format used before Windows XP SP 2 and Windows Server 2003 SP 1, which uses only the access right COM_RIGHTS_EXECUTE, or it may exist in the new format used in Windows XP SP 2 and Windows Server 2003 SP 1, which uses COM_RIGHTS_EXECUTE together with a combination of COM_RIGHTS_EXECUTE_LOCAL, COM_RIGHTS_EXECUTE_REMOTE, COM_RIGHTS_ACTIVATE_LOCAL, and COM_RIGHTS_ACTIVATE_REMOTE.

Note   COM_RIGHTS_EXECUTE must always be present; the absence of this right generates an invalid security descriptor.

You must not mix the old format and the new format within a single ACL; either all access control entries (ACEs) must grant only the COM_RIGHTS_EXECUTE access right, or they all must grant COM_RIGHTS_EXECUTE together with a combination of COM_RIGHTS_EXECUTE_LOCAL, COM_RIGHTS_EXECUTE_REMOTE, COM_RIGHTS_ACTIVATE_LOCAL, and COM_RIGHTS_ACTIVATE_REMOTE.

The following is an example of an incorrectly formatted ACL:

Revision 1
Sbz1 0
Control 0x8004
    SE_DACL_PRESENT
    SE_SELF_RELATIVE
Owner: S-1-5-21-1597522630-148096252-1166023319-500 (no name mapped)
Group: S-1-5-21-1597522630-148096252-1166023319-500 (no name mapped)
DACL:
    AclRevision 2
    Sbz1 0
    AclSize 128
    AceCount 4
    Sbz2 0
    Ace[0]
        AceType 0: ACCESS_ALLOWED_ACE_TYPE
        AceFlags 0
        AceSize 36
        AccessMask 0x1
        S-1-5-21-1597522630-148096252-1166023319-500 (no name mapped)
    Ace[1]
        AceType 0: ACCESS_ALLOWED_ACE_TYPE
        AceFlags 0
        AceSize 20
        AccessMask 0xb
        S-1-5-18 (Well Known Group: NT AUTHORITY\SYSTEM)
    Ace[2]
        AceType 0: ACCESS_ALLOWED_ACE_TYPE
        AceFlags 0
        AceSize 20
        AccessMask 0x9
        S-1-5-11 (Well Known Group: NT AUTHORITY\Authenticated Users)
SACL:
    (null)

Note that the first access control entry (ACE) grants COM_RIGHTS_EXECUTE (0x1) only, while the second ACE grants COM_RIGHTS_EXECUTE, COM_RIGHTS_EXECUTE_LOCAL, and COM_RIGHTS_ACTIVATE_LOCAL (0xb), and the third grants COM_RIGHTS_EXECUTE and COM_RIGHTS_ACTIVATE_LOCAL (0x9).

To correct this, the first ACE should be changed to grant COM_RIGHTS_EXECUTE in combination with one of the other four access rights, or else the second and third ACEs should be changed to grant only COM_RIGHTS_EXECUTE.

Related topics

DCOM Security Enhancements in Windows XP Service Pack 2 and Windows Server 2003 Service Pack 1
Security in COM

 

 

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