This week brought a timely revisit to remind myself just how common the three security extended attributes (xattr) have become, and to see whether we can make use of any of them for our own purposes.

How common?

Checking through one of my ~/Documents folders containing a modest 57,884 files, nearly 60% of them have at least one xattr. By far the most common is com.apple.quarantine on 48%, followed by com.apple.provenance at 14%. Some way behind those, but still one of the most frequent, is com.apple.macl on 2.8%.

Having explained what I think macOS does with all those xattrs, the next step is to ask whether we can use them for our direct benefit. Of the three, quarantine seems least useful to anything beyond Gatekeeper. MACL is like a boil on the bum, and the only time you’ll notice it is when it gets in your way. I can’t make sense from its contents either, but as it’s protected by SIP, there’s little a utility could do to alleviate our suffering, so we just have to learn to live with it. I’m surprised how uncommon it is in comparison with the nuisance it can cause.

Promising provenance

It’s the provenance xattr that looks most promising, and Koh M. Nakagawa has followed up his recent research into its function with an open-source command tool ShowProvenanceInfo that can look up provenance IDs found on files, in the ExecPolicy database’s Provenance Tracking table, although that requires root privileges for access.

Apps and executable code signed by third-parties rather than Apple are added to that table when they’ve successfully completed their first run. Each is given a unique provenance ID number that is attached to them in a com.apple.provenance xattr. When they then perform any of 11 types of file operation, such as creating a file or opening one in write mode, that app’s provenance ID is attached to the file in its own provenance xattr.

As apps and other executables that have been entered into that table have their own provenance xattrs, it shouldn’t be too much of a burden to build an independent database from those, together with other information about the executable with that provenance ID. That can then be used to examine provenance xattrs on arbitrary files, to identify which app last worked with that file, the primary task of a new GUI utility I’ve already dubbed Providable.

In addition to telling you which app with a provenance ID last changed a document, there are other functions that Providable could perform. Those property lists forming the basis for Background Items listed in Login Items & Extensions settings are normally created and changed by their owning app. When a third-party app with a provenance ID does that, the property list gains a provenance xattr that can be used to identify the app responsible. That can in turn provide information that’s often sadly lacking from the list of extensions, including the location of the app rather than that of the property list.

One obvious hole in this plan is the fact that apps that are signed by Apple, including those bundled in macOS and everything installed from the App Store, don’t get assigned provenance IDs. They therefore can’t be traced and identified from the files they create or change, as they operate outside the provenance tracking system.

Providable

My outline design for Providable is therefore to inspect provenance IDs saved as xattrs to the apps in the Data volume, and to display their details in a list you can refer to. Alongside that, a window lets you drop files on it for checking. Each will be examined for a provenance xattr, and those that have one will then be associated with the app in the list, providing its path and other details.

Provenance IDs can also be assigned to command tools and other executables, and a later version will allow you to check those in popular locations, and add them to the database so files can be matched to them as well. I don’t currently know how useful that might be, but we should get a better idea once the first version of Providable is in use.

I invite your ideas and comments, please, before I start coding.

Over the last few years, files and apps on our Macs have started to bristle with unfamiliar extended attributes (xattr). The oldest is the quarantine xattr, containing the quarantine flag, dating back to the introduction of Gatekeeper in 2012. Although its primary purpose is to determine which apps should undergo first run checks, it’s also to be found on many files. Then in macOS Catalina, the MACL xattr appeared and now seems to get attached to pretty well everything, no matter where it has come from. It was joined by the provenance xattr in macOS Ventura, and that too is spreading like wildfire on both apps and files. This article reviews why they’re there, and what you can do about them.

Quarantine

Since its introduction, Gatekeeper has drawn a distinction between apps that originated outside the Mac, and those that can be fully trusted, when performing security assessments on the first occasion. To enable that, apps that download items from the internet, or transfer them from another system on the same network, attach a quarantine xattr to every file that arrives on your Mac. When archives are decompressed, for example, the quarantine xattr is propagated to every file they contain. Gatekeeper then performs full first run checks on those apps, and in the right circumstances they may be run in translocation.

This com.apple.quarantine xattr is also attached to non-executable files, where its role isn’t clear, as they aren’t checked by Gatekeeper, and their quarantine flag isn’t cleared after they have been opened for the first time. However, you’ll find them on all items that have been downloaded by an app or tool that attaches them. As this xattr isn’t protected in any way, it’s straightforward to remove, although you should avoid doing so for apps whose origins could be suspicious, as that would prevent Gatekeeper from running its additional checks.

MACL

This is thought to be an abbreviation for Mandatory Access Control List, and might be intended to preserve privacy while allowing the user to open files. The com.apple.macl xattr is now probably the most common of all, as these get attached to any and every file, including apps, even if they were created on that Mac and never left its local storage.

This xattr contains 72 bytes of what could be two UUIDs, or just binary data. However, it’s protected by SIP, preventing any user from stripping it. This can be responsible for problems, for example files that can’t be opened in their default editor app, and some that can’t be saved. In the past one way of triggering this blocking behaviour was to set a document to be opened by default using an app other than its normal app, then saving it from that app before trying to open it again.

Perhaps the simplest way to remove this xattr is to copy the file to another volume, where the xattr is no longer protected by SIP, stripping it using my free editor xattred or the xattr command tool, then copying that back to its original location. Although it’s likely to be given another MACL xattr shortly, that should be less prone to cause problems.

Provenance

Most recent versions of macOS have what’s known as a Provenance Sandbox that enables the security system to track the origins of files, and trace which app has altered them. This has recently been detailed in full in Koh M. Nakagawa’s account of XProtect Remediator. It operates quite differently from the regular app sandbox, and doesn’t appear to impose any restrictions.

Apps that aren’t signed by Apple are assigned an 11-byte integer when they first clear Gatekeeper’s checks, and those are entered into the Provenance Tracking table in the ExecPolicy database, and attached to the app in the com.apple.provenance xattr. When that app performs operations like opening a file in write mode, or creating a new one, the same xattr with that app’s provenance ID is attached to the file. Thus, by checking the provenance ID on any file with the xattr, the app that last wrote to the file can be identified.

Provenance IDs and xattrs aren’t assigned to Apple’s own apps, or those installed from the App Store, but they are to apps that are signed using certificates other than Apple’s, and those that are notarised. When a file is created or changed by an app without a provenance ID, no xattr is attached to that file, and any existing xattr is left unchanged.

This is a powerful tool in gathering security intelligence. For example, suppose a Mac has just installed previously unknown malware that started to write files in one of the locations watched by behavioural XProtect under one of its Bastion rules. Those could be inspected, perhaps by one of the scanning modules in XProtect Remediator, the provenance ID checked against details in the Provenance Tracking table, and information forwarded to Apple for further investigation.

Evidence so far suggests that you don’t want to try to tamper with the provenance xattr, as it doesn’t appear to have any role in blocking access to files, and is working on our side. Like the MACL xattr, it’s now normally protected by SIP, so can’t be removed directly.

Summary

• com.apple.quarantine is likely to be found on any app or file downloaded or transferred from another system, but appears harmless.
• com.apple.macl is likely to be attached to most apps and files, even those that have remained local at all times. It can sometimes cause problems including blocking the file from being opened or saved, but is hard to remove as it’s protected by SIP.
• com.apple.provenance is used to track which app has created or modified files. This can be important in security intelligence, so shouldn’t be removed, although it appears harmless and is working for our benefit.