Access Control Entries (ACE)

In the Windows operating system, Access Control Entries (ACE) are the individual entries within an Access Control List (ACL) that define the permissions granted or denied to a specific user, group, or security principal on a securable object, such as files, folders, registry keys, or Active Directory objects.

Each ACE contains the following elements:

1. Security Identifier (SID): This uniquely identifies the user, group, or security principal to which the ACE applies. SIDs are used by Windows to represent security principals.

2. Permissions: These define the specific actions or operations that are allowed or denied for the security principal identified by the SID. Examples of permissions include read, write, execute, delete, modify, full control, etc.

3. Inheritance Flags: These flags indicate whether the permissions defined by the ACE should be inherited by child objects (containers or subfolders) within the object to which the ACL is applied. Inheritance flags control how permissions propagate throughout the object hierarchy.

4. Propagation Flags: These flags specify how inherited permissions should be applied to child objects. Propagation flags determine whether inherited permissions should be automatically applied to child objects, whether they should be propagated only to child objects created after the ACE is added, and whether they should replace or accumulate with existing permissions on child objects.

There are two main types of ACEs:

1. Allow ACE: This type of ACE grants specific permissions to a security principal. For example, an Allow ACE might grant read and write permissions to a particular user on a file.

2. Deny ACE: This type of ACE explicitly denies specific permissions to a security principal, overriding any permissions granted elsewhere. For example, a Deny ACE might deny delete permission to a particular group on a folder.

ACEs are stored within the security descriptor of a securable object along with other security information, such as the owner and group of the object. The ACL containing the ACEs determines who can access the object and what actions they can perform on it.

In summary, Access Control Entries (ACEs) are the building blocks of Access Control Lists (ACLs) in Windows, providing fine-grained control over access to resources by specifying permissions for individual users or groups.