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Sinter: New user-mode security enforcement for macOS

by Mike Myers on August 12, 2020

TL;DR: Sinter is the first available open-source endpoint protection agent written entirely in Swift, with support for Apple’s new EndpointSecurity API from first principles. Sinter demonstrates how to build a successful event-authorization security agent, and incorporates solutions to many of the challenges that all endpoint protection agents will face as they migrate from kernel-mode to user-mode agents before the release of macOS 11 Big Sur.

Simple, open-source, and Swift

Sinter is our new open-source endpoint security enforcement agent for macOS 10.15 and above, written in Swift. We built it from scratch as a 100% user-mode agent leveraging the new EndpointSecurity API to receive authorization callbacks from the macOS kernel for a set of security-relevant event types. Sinter is controlled with simple rules to allow or deny events—and uses none of the expensive full-system scans or signature-based detection of traditional anti-virus solutions.

Grab an installer for the beta version today and try it out!

Currently, Sinter lets you write a set of rules to block or allow process execution events, and to provide the rules to the agent with a Santa-compatible sync server or with a local configuration file (here is an example rule that demonstrates an explicit-allow). However, we’re planning to develop a more sophisticated rule syntax and add blocking capability for the many other kinds of events supported by the API, which would also mean an end to the Santa rule compatibility.

The quest for the 100% user-mode security agent

Implementing an endpoint security solution (e.g., anti-virus, anti-malware) requires interception and authorization of OS-level events in real time. Historically, that has meant the use of kernel-mode callback APIs or hooking kernel-mode operating system code when a proper API was not provided. Operating system developers have long known that third-party kernel-mode code like this was the leading source of system instability and insecurity, because any small error in kernel code tends to have large consequences.

Enter the macOS EndpointSecurity API. In late 2019, Apple announced that support for all third-party kernel extensions would be deprecated in macOS, and that they would introduce user-mode APIs and frameworks to replace the functionality needed for third-party products. All security vendors were put on notice: Deprecate your existing kernel-mode solutions within the next year and migrate to the EndpointSecurity API before the next release of macOS (macOS 11 Big Sur). It’s clearly not a fun prospect for many teams, and soon after the announcement a client tapped us to develop a user-mode solution that would make migration less painful.

What is the EndpointSecurity API?

EndpointSecurity is an API that implements a callback from the macOS kernel, in real time, as a particular event is about to happen. EndpointSecurity clients subscribe to one or more event types that are either a NOTIFY type or an AUTH (Authorization) type. Notify is just what it sounds like, and is useful for capturing a simple activity log on the host. An Authorization callback is much more powerful; it lets a client process make a decision to allow or deny the event from happening.

EndpointSecurity replaces the kernel-mode equivalents for real-time event authorizing on macOS (Kauth KPI and other unsupported kernel methods) and the read-only event monitoring OpenBSM audit trail. Any real-time monitoring or protection product for macOS must be rewritten to use EndpointSecurity for macOS 11 Big Sur.

Note that there are no network-related events in the EndpointSecurity API (except UNIX domain sockets). All of these are in the Network Extension framework. You can combine the use of both APIs from one System Extension, but here we focus on the EndpointSecurity API specifically.

Using this API, Stephen Davis at FireEye and Patrick Wardle at Objective-See quickly released event monitoring applications that could display, for example, process-related and file-related events in real time. But read-only monitoring tools following in the footsteps of Process Monitor (“ProcMon”), while useful, are only using the Notify half of the functionality of the EndpointSecurity API (the ability to monitor). Google’s Santa, an open-source macOS process allow/deny solution in Objective-C, demonstrates the ability to authorize events using EndpointSecurity: Its agent now receives and makes allow/deny decisions for process events from EndpointSecurity.

We saw that it would be critically important to master the EndpointSecurity API, as many teams would need to migrate to it in their existing macOS security applications. With the development of Sinter, we’ve delved into EndpointSecurity, learned some lessons from the experience, and incorporated solutions for various challenges we encountered—so you don’t have to. Sinter also demonstrates an implementation of an EndpointSecurity client in the Swift programming language, which promises better memory safety and performance than Objective-C, while maintaining compatibility with all of the other new macOS APIs.

Developing Sinter: Not for the faint of heart

Implementing an event-authorizing agent is an order of magnitude more difficult than implementing a read-only event-subscriber. We also learned—the hard way—certain shortcomings of the EndpointSecurity API. Here’s some of the more significant heavy lifting we did in the course of Sinter’s development.

1. Making decisions in real-time without impacting the system

Possibly the most difficult part of implementing a security event authorization agent is that authorization decisions must be made in real time. You cannot block forever to make a decision, and EndpointSecurity enforces a deadline for each authorization message: If your client blows the deadline, EndpointSecurity will terminate your client process to preserve a functioning system.

Decisions shouldn’t be made synchronously; Sinter uses the es_copy_message to dequeue the message from EndpointSecurity and allow its dispatcher to immediately send you the next message. Decisions should be made in separate threads, responding asynchronously as soon as possible from each one. Some decisions will take longer than others to process, but often the APIs needed to perform signature checks can’t be interrupted.

With Sinter, we ran into this problem head-on when a quick burst of execution events involving large programs caused Sinter to lock up the machine. We solved this by implementing an efficient queuing system, with one queue for small programs and another one for big programs, so events would never get stuck waiting in the queue. The big-programs queue works out-of-process so a long-running verification can be aborted whenever necessary. This new approach performs reliably on all of our tests.

2. Mitigating the TOCTOU risks in real-time security decisions

The TOCTOU (time of check, time of use) race condition vulnerability pattern commonly occurs when making security decisions. Any security agent performing a check must not allow the checked resource to be modified in the time between the check and the action’s approval.

When authorizing macOS execution events, the resource being checked is the executable file, which is mapped into memory before executing. Here’s one TOCTOU attack scenario:

A malicious actor executes Bad.app. The bad executables are mapped into memory and an execution authorization event is emitted by EndpointSecurity. But then the attacker immediately replaces or modifies the executable file to make it Good.app. The EndpointSecurity client gets the event, verifies that the bundle and its files all look good, and allows the execution.

This problem is not unique to EndpointSecurity, and was always a risk with the KAuth framework that preceded it (e.g., an issue was raised about this TOCTOU in Santa not long ago). It’s still a challenge that must be solved by any agent that wants to authorize events. As mentioned, Sinter attempts to monitor file events to catch TOCTOU attacks. It would have been much easier if Apple handled this responsibility within the EndpointSecurity API itself (submitted to Apple as a developer feedback suggestion FB8352031; see OpenRadar).

3. macOS executable files live in application bundles

Execution events occur in the context of a single executable file, but most macOS executables exist within application bundles, the directory-like structure that appears as a single “.app” file in macOS Finder. A bundle itself is code-signed, and code-signing verification must be done at the bundle level. This means that a security agent that catches an execution event must discover if the executable has a containing app bundle, then verify the code signature on the entire bundle—these are tasks not performed by EndpointSecurity itself. Some bundles like Apple’s Xcode.app are upwards of a gigabyte in size, and processing a verification in real time isn’t possible. Execution events have to be denied at first, until the verification completes.

EndpointSecurity does provide a built-in caching mechanism, a single cache shared by all EndpointSecurity clients. However, as a client you cannot invalidate a single entry in this cache; you can only clear the entire cache at once. EndpointSecurity will automatically invalidate a cache item if the related file is changed/deleted/etc., but it does this on a per-file basis, not per-application-bundle. Currently, Sinter works with two caches: the one managed by EndpointSecurity, and another custom cache containing the application bundle code-signing verification results.

In theory, malware could be added into an application bundle and EndpointSecurity would not react by clearing a cached approval decision, if the previously approved executable file in the bundle had not changed. EndpointSecurity clients would have to monitor for this themselves, and invalidate the entire cache in response. This is less than ideal, and we hope Apple will make improvements to this caching mechanism. In the near term, EndpointSecurity clients may have to implement their own integrity monitoring on application bundles to avoid being circumvented this way. Sinter attempts its own bundle file integrity monitoring capability to detect when this custom cache should be cleared.

4. Advantages of installing your agent as a System Extension

“System Extensions” is Apple’s name for “user-mode components that extend the system” and is the umbrella term for what replaces the now-deprecated third-party Kernel Extensions. EndpointSecurity is one API under this umbrella; DriverKit and Network Extensions are a couple of others. A System Extension is also a new kind of managed plug-in package for macOS through which you can install your executable.

Installing an EndpointSecurity client as a System Extension is not required—you can implement all of the EndpointSecurity functionality from any kind of executable, even a basic command-line application—but is highly encouraged. There are additional benefits and system-enforced protections for your agent when it is installed as a System Extension. System Extensions can opt to be loaded before all other third-party applications at startup. Apple also announced that macOS extends SIP (System Integrity Protection) to cover System Extensions, meaning it prevents even root users from unloading your security agent. Historically this was only possible if you developed your own kernel-mode anti-tamper logic, but installing your agent as a System Extension frees you from reinventing this wheel. Sinter is currently a background daemon, but now that Apple has documented the anti-tamper protection benefit of installing your agent as a System Extension, we will be converting Sinter to this format.

5. Mastering the Entitlements, Signing, and Notarization workflow

The EndpointSecurity API is only usable within code-signed and notarized applications by Apple-approved developers like Trail of Bits. In other words, the API is gated by a special entitlement. Unlike most entitlements, this one requires a manual application and approval by Apple, after which you are granted a code-signing certificate with the EndpointSecurity entitlement. In our case, the time to be approved was six calendar weeks, but your mileage may vary. Apple is apparently being careful with this entitlement, because a misbehaving or malicious EndpointSecurity client could put a halt to everything on a host system.

Apple’s code-signing and notarization steps are difficult to troubleshoot when they fail, so it’s essential to set up and automate the process early, so you will immediately notice when they break and easily narrow down the breaking changes. For Sinter, we created our own CMake-driven approach that automates the workflow for Apple’s notarization, packaging, package signing, and package notarization steps. All of that now integrates perfectly into our CI with minimal fuss.

One last entitlement that EndpointSecurity agents need is related to user privacy. Because most agents will be inspecting files (whether in the context of file events or the executables of process events), they need the user’s permission to access the filesystem. On or before the first run of your application, the user must manually go to Privacy settings in System Preferences, and enable “Full Disk Access.” There are MDM payloads that can automatically enable the permission and sidestep this manual user approval step.

Those were the thornier challenges we addressed when writing Sinter, and of course there were more miscellaneous gotchas and lessons learned (e.g., determining whether files are binaries, signature verification, and multiple EndpointSecurity clients). We’ll update the most compelling details as development continues—stay tuned.

The upshot

With the deprecation of kernel extensions, Apple is leveling the playing field for endpoint protection agents: Everyone must use the same user-mode APIs. This will benefit everyone with improved system stability and reduced attack surface, but existing security product developers first have to replace their kernel extensions with a user-mode approach. In user mode, they can now work in any language, not just C/C++.

So instead of starting from scratch with just the example code in C, we hope organizations will help us build and rely upon an open-source platform in Swift, a forward-looking choice for long-term investment as Apple’s successor to Objective-C.

Get involved with Sinter

The beta version of Sinter is available today. It’s a significant first step, and here’s a peek at some of the larger items we’re working on now:

We invite you to partner with us to sponsor the continued development of Sinter, or to discuss the integration of EndpointSecurity-based capability into your existing agent—just contact us to get started.

Contributors are welcome, too! Give us your feedback on GitHub, or join us in the #sinter channel on the Empire Hacking Slack.


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*** This is a Security Bloggers Network syndicated blog from Trail of Bits Blog authored by Mike Myers. Read the original post at: https://blog.trailofbits.com/2020/08/12/sinter-new-user-mode-security-enforcement-for-macos/

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