Access Control Lists in the Security Descriptor

An ACL is an ordered list of ACEs (Access Control Entries) that define the access attributes that apply to an object and its properties. Each ACE identifies a security principal (user or group account) and specifies a set of access rights that are allowed, denied, or audited for that security principal.

The security descriptor for an object can contain two ACLs:

  • A DACL that identifies the users and groups that are allowed or denied access
  • An SACL that controls how access is audited

The following table shows the data structure for an ACL:

ACL data structure

ACE Field

Description

ACL Size

Number of bytes, allocated for the ACL. This includes the ACL header, ACEs, and remaining free space in the buffer. The size of an ACL varies with the number and size of its ACEs. The maximum size of an ACL is 64K, or approximately 1,820 ACEs, depending on the size of the ACEs. However, for performance reasons, the maximum size is not practical.

ACL revision

Revision number for the ACL's data structure. The structure of an ACL is the same for all revisions, but the structure of ACEs in the ACL can vary

ACE count

Number of ACEs in the ACL. A value of zero means the ACL has no ACEs — it is empty; therefore, access-checking can stop. Note that an empty DACL differs from a NULL DACL. An empty DACL gives no access to anyone, but a NULL DACL gives unconditional access to everyone, and should therefore be avoided.

ACE[1]

An ordered list containing zero or more ACEs.During an access check, ACEs are processed in the order in which they are listed.

ACE[...]

An ordered list containing zero or more ACEs.During an access check, ACEs are processed in the order in which they are listed.

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