The Pane ACL Permissions

Introduction to Permissions

Every file and every folder accessed by your computer is associated with a specific set of rights that define which users are allowed to perform what operations with these objects, e.g. reading the contents of a file, or removing a file from a folder. This set of rights associated with a file system object is called permissions. macOS uses the classic permissions found on every UNIX system, the so-called POSIX Permissions, an extended set of permission-like markers, called Special Permissions, and an advanced set of right definitions used by Microsoft® Windows, most modern UNIX systems, and many other operating systems, the Access Control Lists, abbreviated ACLs. ACLs are also called POSIX.1e permissions, because they behave very similar to a draft document called POSIX.1e which was planned to become an industry-wide standard for permissions one day. However, the 1e documents have been officially withdrawn for various reasons, so actually no standard exists by that name. The 1e draft contained very good ideas, however, so permissions very similar to the intentions of 1e exist in most operating systems today. But you should keep in mind that the exact meaning of ACL permissions may differ slightly between different OS vendors.

POSIX Permissions

The minimum set of permission definitions used in all UNIX systems and many other operating systems which are compliant to the POSIX standard (IEEE 1003) is based on three predefined “parties” for which rights can be granted:

Apple identifies the third category by the term everyone. Unfortunately, this term is incorrect, because this category does explicitly not include the owner or any member of the primary group. If you grant or deny a right for “everyone” via the Finder, those users won’t be included, which is not really what the word everyone suggests. For this reason, TinkerTool System uses the correct term other only.

For each of the three categories, the following permissions can be granted:

If one of these rights is not explicitly granted for a user, this will mean that the user doesn’t have permission to access. The right is denied, although there is no possibility to explicitly define denials in this model.

By default, most applications create files with the following permission settings:

Applications can grant less rights for specific files if they have been programmed to do so. For example, an e-mail application is designed to “know” that a new mail folder should be kept confidential, so it won’t grant any group and other permissions when creating it. Only the owner should have read/write permission in this case.

Additional Permission Markers

macOS supports some other special permission settings. They can be found on most other UNIX systems as well.

Access Control Lists

Introduction to Access Control Lists

Access Control Lists, or, in short, ACLs, are a supplement to the existing POSIX permissions, so you don’t necessarily need to use ACLs. The conventional rules for access rights outlined above still apply, but some optional new rules can be added.

Technically seen, an ACL is a list of individual rights which can be attached to a file system object. The ACL can either be empty — in this case, the conventional POSIX permissions apply only —, or it can contain one or more objects called Access Control Entries (ACEs). An Access Control Entry includes the following information:

ACL Rights

ACLs allow the definition of 13 different rights to access a file-system object:

These rights can be joined in any possible combination.

ACL Inheritance Settings

Each Access Control Entry is allowed to contain additional information that specifies how this entry is inherited to objects located at deeper levels in the file system hierarchy, for example, a file in a folder which is enclosed in another folder. The top folder may have an ACL which is automatically inherited to objects inside this folder.

Inheritance operations take only place in the moment when new objects are created. For example, when a file B is created in a folder A, the file B will inherit ACEs from A only at that time. When somebody changes the permissions of B later, the system will not automatically reinforce a new inheritance operation from A to B. Also, a change in the ACEs of folder A won’t be “re-inherited” to the already existing object B.

There are four different settings which control how ACE permissions should be inherited from a certain folder onto the objects that will later be created in that folder. The settings basically control how “deep” the inheritance should take effect.

There are 16 possible combinations of these four settings, but only 12 of them really make sense in practice.

Inherited and Explicit Entries

Because ACE settings can be inherited from folders to the objects they contain, the system has to keep track which ACEs in an ACL are inherited and which are not. Only ACEs which are not inherited can be changed. Non-inherited entries are called explicit. To change an inherited entry, it is either necessary to change the entry at the parent level (where this inherited entry came from), or to delete the ACL for this object (hereby breaking the inheritance), replacing the inherited entries by explicit entries.

The Evaluation Rules for Access Control Entries

As mentioned before, an Access Control List consists of several Access Control Entries. Certain rules define how macOS evaluates the entries when a specific user wants to access an object in the file system. Note that ACEs could contradict each other. For example, if user A is allowed to access the file B, but user A is also member of a user group which is denied access to file B, we have a contradiction which must be resolved. The following rules apply:

Important Recommendations

Access Control Lists are a powerful tool to define specific rights at a low level of granularity. However, you should keep in mind that ACLs are also very complex.

There are 13 different permissions which can be granted or denied, and 12 possible ways to define inheritance. This results in a total of 2^13 * 12 = 98,304 different concepts of access rights you can define.

Each of these nearly 100,000 different access rights can be applied to a user or a user group to form an ACE, and a nearly unlimited number of ACEs can be combined into an ACL. Each file or folder in your system can be attached to a different ACL, so maintaining all these entries can easily become a nightmare. For this reason you should define ACL permissions with greatest care only.

File Systems Supporting ACLs

Access Control Lists can only be used on file systems which are capable of storing them. macOS allows using ACLs when working with the following types of file systems, under the prerequisite that the computers hosting these file systems are using an operating system version generally capable of handling ACLs:

Other file systems, e.g. disk volumes formatted using UFS, FAT, VFAT, FAT32, ExFAT, NTFS, or ZFS, and network volumes accessed via NFSv2, NFSv3, FTP, or WebDAV cannot support Access Control Lists. Not supporting ACLs over a file server connection means that the client computer cannot “see” or modify ACLs stored on the server. However, if the file server is capable of using ACLs, it will still respect them, no matter if the accessing computer may notice this or not.

Show or Set Permissions

Displaying Permissions

TinkerTool System can display the full set of POSIX and ACL permissions which are currently set for a specific file or folder. The settings are displayed in a clear table, sorted by the same order in which evaluation of effective rights takes place. The table can also be used to change permission settings.

The Finder of macOS is not capable of displaying the “true” permission settings of a file system object. Due to several design flaws, the section Sharing & Permissions in the Get Info panel of the Finder may show a very simplified or even wrong summary of the permission settings. TinkerTool System, however, will display the true settings, as they are defined and stored by the core operating system. For this reason, some permission details shown can differ between the two applications. In such a case you should not trust the display of the Finder.

To display or change the current permission settings of a file system object, perform the following steps:

  1. Open the tab item Show or Set Permissions on the pane ACL Permissions.
  2. Drag the file or folder from the Finder into the field File or folder. You can also click the button […] to navigate to the object, or click on the white area to enter the UNIX path of the object.
  3. The current settings will be shown in the table.
Show or set permissions
Show or set permissions

Header lines in the table show which rights are ACEs of an ACL, and which are based on the conventional POSIX settings. The columns specify the following information:

If a permission is being displayed as Custom, it will indicate that the rights cannot be described by simple terms, like read only. Remember that there are 98,304 different concepts of permissions which can be defined by combining ACL rights. To see the 13 detail rights and 4 inheritance settings (for folders) exactly, double-click a line of the table. Alternatively, you can click on the button with the pencil icon directly below the table.

In cases where ACLs or even permissions in general are not supported for the selected object, a red warning will appear below the File or folder box, together with a question mark help button. You can click the button to get detailed information why there could be issues when reviewing or editing permissions on the affected volume.

There can never be any file system object without permission settings, so macOS will automatically convert rights of other systems to “plausible” permissions for the local system if this should be necessary, or it will completely synthesize artificial settings which aren’t actually stored on the volume. TinkerTool System will show you the effective settings as macOS is simulating them for your current user account in such a case, but you should keep in mind that processes running for other users may receive different settings. It is also possible to edit and save such artificial permissions, but macOS will “re-translate” them back to the affected volume when applying them, so the results may not always be what you expect. We don’t recommend to apply new permission settings in cases where TinkerTool System shows a support warning.

Changing permissions

After you have chosen an item and TinkerTool System is displaying its permission settings in the table, every aspect of the settings can be changed. After you have made all desired changes, you can click the button Apply in the lower right corner to save the current settings. The button Revert will discard all changes you have made and TinkerTool System will go back to the original settings currently stored for the object in question.

If you like to modify the Type of an entry, or want to change one of the Permission concepts to one of the simple standard terms, you can do so by using the pop-up buttons in the table.

Set permission details
Set permission details

To change user or group of an entry, perform the following steps:

  1. Double click the respective line of the table, or select the line and click the pencil button.
  2. In the detail sheet, click the button Set… at the top of the panel.
  3. In the new sheet, select either Users or Groups (if applicable).
  4. Select a user or group in the table and click the OK button.
  5. In the detail sheet, click the Close button.

The entry type and the detail rights can be changed in the same fashion. Note that the detail sheet is grouping the rights and inheritance settings into four categories. You can enable or disable all rights in a category by setting or removing the check mark in the respective group header. Enabling all rights of an ACE is also possible by selecting the item Full Control in the Permission pop-up. The inheritance settings will be set to appropriate defaults in this case.

To add an ACE, click the button [+] below the table. To remove one or more ACEs, use the [—] button. To reorder an ACL, drag a line in the ACE section of the table and drop it at its intended new position. Note that objects always have well-defined POSIX permissions and that POSIX permissions are always evaluated in the predefined user-group-others order, so it won’t be possible to remove or reorder one of the lines below the POSIX headline.

Selecting users and groups

You may have to select a user or group account when making changes to access rights. TinkerTool System uses several dialog panels and sheets in this context which allow you to easily choose an entry from the list of all accounts available on your computer.

In large organizations, the list of available accounts can be very long. In some cases, it might not be stored on your local computer alone, but also on other computers (directory servers) in your network which need to be contacted. For efficiency, TinkerTool System may not show the complete list of accounts when you open a user or group dialog for the first time. The list can be restricted to accounts which have already been used somewhere in the operating system since the computer was started. In order to retrieve the full account list, click the button Fetch all entries in the lower left corner of the window. This makes sure the list of users or groups is complete.

Fetching all entries may take a considerable time, especially in environments with directory servers.

Additional Operations

Additional operations can be performed by selecting one of the items in the pull-down menu Operations at the bottom of the window. The operations vary depending on whether you have selected a file or a folder.

If you have selected a folder, you can:

There is an additional option when propagating Access Control Lists: It is either possible to copy the existing Access Control List from the top folder to the entire hierarchy as it is, or to let TinkerTool System simulate inheritance in retrospect. In the latter case, the inheritance attributes of the top folder may cause the results to be different. For example, if the top folder does not have the option apply to all subfolder levels enabled for a particular ACE, inheritance of that ACE will stop at the first level.

Another option controls how TinkerTool System should handle objects which have the attribute “locked” set. The default behavior of macOS is to stop the propagation, cancelling the running operation with an error when such an object is found, because macOS does not permit that a permission setting of a locked object is changed. The policy to stop the operation makes sure there won’t be any undetected security problems that could arise when the access rights for a locked object remain unchanged unexpectedly. However, you may like to ignore such cases, simply continuing the operation silently, which was the behavior of old versions of macOS Server.

When propagating permissions in folders containing symbolic links, the program will operate on the links themselves. The objects referred by the links will remain unchanged. Folders referred by a link won’t be traversed. Access Control Lists won’t be propagated to symbolic links because macOS does not support this.

If you like to ensure that no object is omitted during propagation of permissions, it is recommended to remove all protection attributes prior to the propagation. This can be done with the feature Protection on the Files pane.

A propagation is automatically limited to the volume where the top folder is located.

If you have selected a file, you can:

With exception of the propagation feature, the operations will modify the permissions table first, not the actual settings on disk. The changes will take effect after clicking the Apply button.

Saving and reusing rights

In practice, it can happen that you want to apply a very specific assignment of rights to different objects multiple times, e.g. when specifying access rights for several network shares that are located on different volumes. In order to avoid having to enter all settings for users, groups, access control entries and their options again and again, you can save a set of rights once you have defined them in the program. Later, you can select them on the same computer again, reusing the permissions for a different entry in the file system. TinkerTool System refers to this as Predefined Rights. The associated features can be accessed via the pull-down menu Predefined Rights.

After you have set and applied all access rights for an object, you can save the associated set of permissions independent from that object on the same computer as follows:

  1. Make sure TinkerTool System displays the desired set of permissions settings in the table of the tab item Show or Set Permissions, and that the settings have already been applied (the Apply button is gray). You can load the rights of an existing object into the program any time by selecting the object via File or Folder.
  2. Open the Predefined rights button to select the menu item Save as predefined rights….
  3. In the dialog box that appears, specify a new name for this set of permissions that should be used to retrieve the entry later. Click Save.

TinkerTool System saves these predefined rights as part of your personal preferences for this computer.

If you would like to apply a set of rights saved in this way to another object later, proceed as follows:

  1. Open the tab item Show or Set Permissions on the pane ACL Permissions.
  2. Drag the file or folder from the Finder into the field File or folder. You can also click the button […] to navigate to the object, or click on the white area to enter the UNIX path of the object.
  3. Activate the desired predefined rights by selecting the corresponding menu item Load xxx via the Predefined rights button, where xxx is the name you assigned previously. The current permission settings are now overwritten with the predefined rights.
  4. Save the rights to the selected object with the Apply button.

Rights for folders and rights for files (or other non-folder objects) have slightly different meanings. They also provide different options for Access Control Entries. Because of this, you cannot apply permission settings for a file to a folder and vice versa.

Settings for rights usually refer to accounts. Because accounts are unique, you cannot transfer predefined rights from one computer to another.

You can rename or delete a set of access permissions any time using the Predefined Rights button.

Effective Permissions

The combination of several Access Control Entries and the POSIX permissions can make it difficult to estimate how the final rights for a certain user will be. TinkerTool System can compute and display the effective permissions of a user. This feature is helpful if you don’t have much experience with permission settings yet. To display effective permissions, perform the following steps:

  1. Open the tab item Effective Permissions on the pane ACL Permissions.
  2. Drag a file or folder from the Finder into the field File or folder. You can also click the button […] to navigate to the object, or click on the white area to enter the UNIX path of the object.
  3. Click the button Select… to choose one of the known user accounts of the current computer.
  4. TinkerTool System will display the results in the table at the bottom. Rights currently granted to this user will be displayed by a green marker, rights currently denied by a red marker.
Effective permissions
Effective permissions

Special Permissions

The set of POSIX permissions contains three special settings, named SUID, GUID, and sticky. For their individual meanings, please see the introductory sections earlier in this chapter. TinkerTool System can display and change any of the three settings. Perform the following steps:

Special permissions
Special permissions
  1. Open the tab item Special Permissions on the pane ACL Permissions.
  2. Drag a file or folder from the Finder into the field File or folder. You can also click the button […] to navigate to the object, or click on the white area to enter the UNIX path of the object.
  3. The current settings will be displayed. You can modify the fields Owner, Group owner, SUID, GUID, and Sticky as desired.
  4. Click the button Apply to save the new settings.

Attention Warning: As mentioned in the introduction, setting the SUID or GUID markers may cause very serious security problems affecting the whole operating system. It should never be necessary to set the SUID/GUID markers for programs when their installers have not set the flags already. Removing flags can cause the affected programs to malfunction. You should not use this feature if you don’t know exactly what you are doing.

Set permissions in a user folder to defaults

If you have used the Finder or other means to change permissions for files in your home folder in such a way that programs no longer work correctly, cannot save settings any longer, or you lost access to your own data, TinkerTool System can reset the permission settings to suggested default values. This applies to all files and folders in the home folder of any local user.

For older versions of macOS, Apple had temporarily provided a Unix command-line program where a similar procedure could be executed in the recovery system of macOS. In addition, this was tied to resetting the affected user’s password. This option is no longer available in current versions of macOS.

This procedure is sometimes incorrectly called “repairing permissions”. This term is misleading, because permission settings can only ever be changed by a program or user, but they cannot be damaged.

Attention Warning: You should never use this feature when it isn’t necessary, and never regularly. The default rights are a clean suggestion that guarantees affected users can work with their own files without any problems. However, these default values could also make files readable by other users, even though the application that created the data may have originally saved them with a “readable only by this user” setting. Neither macOS nor TinkerTool System can “know” the meaning of each individual file and folder and whether it should be classified as confidential or public from the owner’s point-of-view. This is unmanaged user data where any setting could make sense. In other words: The standard settings could be too insecure for confidential data in some cases. The affected user has to make sure that no unauthorized persons can read or delete the data, by subsequently checking and possibly tightening the rights after the defaults have been set. Some (not all) applications may automatically correct insecure permission settings the next time they automatically save data for this user.

Incorrect permission settings in a home folder can be set to default values
Incorrect permission settings in a home folder can be set to default values

The following basic rules apply:

If you like, a test run can be performed to check all files and folders in a user folder without actually modifying any permission setting. You get a report how many settings would change or would remain unchanged, and which files would be affected.

Attention Due to the particular nature of ACL inheritance, there are cases where the results of a test run won’t exactly match the results of an actual run. Changing an Access Control List can indirectly trigger future changes to other objects, which is not always predicted in detail.

Attention For technical reasons, the entries date of last modification will change for each folder in the user folder during the reset process. This also applies to a test run. Files are not affected.

To reset rights in a local user’s home folder to working default settings, perform the following steps:

  1. Open the tab item Reset Home on the pane ACL Permissions.
  2. At User account, select the user whose folder should be processed.
  3. Use the Test run option to choose whether permissions should actually be modified, or you only like to get a preliminary preview.
  4. Click the button Reset.

The selected procedure will be executed, ending with a detailed report which can also be saved to a text file. While the operation is running, a preview of the report is shown as scrolling text.

Depending on individual case, the report may contain several millions of lines. In order not to overload macOS with this amount of data, the text won’t be shown in a scrollable text editor box exceptionally.

Finding internal identifications of user and group accounts

Each user and group account is a record maintained by macOS to hold the data of individuals who have permission to access certain information. Depending on circumstances, the operating system can use different representations to identify each account:

ID Finder
ID Finder

If you specify one of these four identifications, TinkerTool System can help you to find the remaining three items for you. This can be helpful, for example, when the operating system refers to “an issue with user 502” in an internal log message and you need to find out which user account is actually affected.

A matching identification can sometimes be used for both a user and a group, even if they are completely different objects, so you also need to specify which type of account you are searching for. This is not necessary for UUIDs however, because they are always unique. Accounts may not only be stored on your Mac, but your network may keep one or more databases for accounts which are valid for all computers of the network. A server which hosts such a central account database provides a so-called directory service.

  1. Open the tab item ID Finder on the pane ACL Permissions.
  2. Enter data into the field Specify either account name, full name, POSIX identifier or UUID.
  3. Select either User or Group if you did not specify a UUID.
  4. If your Mac is configured to access a network directory service, searching for items may affect thousands of accounts and may cause a high amount of network traffic. You can choose whether you like to permit an Exhaustive search through all records on all directory servers, or whether you like to limit the search on accounts which have actively been in use on your local Mac. (A non-exhaustive search may still consider accounts from remote directory services if the OS referred to these accounts recently.)
  5. Press the return key.

The result of the search will be shown in the box Result.