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xnumon - monitor macOS for malicious activity

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Project Status

To fully support macOS 10.15 Catalina, including reliable acquisition of executable images, xnumon will need major refactoring in order to replace the kernel extension and audit(4) code with two new System Extensions. That work has not been started yet and there is currently no roadmap. When or if there are any specific plans, this notice will be updated. Feel free to contact me if you are interested in working on this.


xnumon is a monitoring agent that produces system activity logs intended to be suitable for monitoring potentially large fleets of macOS systems for malware and intrusions. It aims at providing similar capabilities on macOS that sysmon provides on Windows.

Currently implemented are the following log events:

  • xnumon-ops[0] and xnumon-stats[1]: for in-band monitoring of agent status and activity metrics. *
  • image-exec[2]: a process has replaced its executable image as a result of calling execve(2) or posix_spawn(2). *
  • process-access[3]: a process has accessed and possibly manipulated another process using either task_for_pid or ptrace(2). *
  • launchd-add[4]: a process has added or modified a launch daemon or launch agent plist. 
  • socket-listen[5]: a process has started listening on a socket. *
  • socket-accept[6]: a process has accepted an incoming connection. *
  • socket-connect[7]: a process has initiated an outgoing connection. 

* stable
experimental and under active development
stable, but limited to blocking sockets due to an unresolved bug in audit(4)

xnumon provides context information such as executable image hashes, code signature meta-data, script shebang handling, and the history of previous executable images that led to the current process state. It does so by tracking fork and other syscalls instead of relying only on the ppid, which can change over the lifetime of a process. For the reliable acquisition of image hashes even from short-living or self-modifying executables, xnumon comes with an optional kernel extension.

xnumon is configurable. It supports different log formats and hash algorithms. In order to reduce log volume close to the source, xnumon implements a number of suppression mechanisms and allows tuning the level of information per event. The log subsystem was designed to be easy to extend with custom log drivers.


A supported version of macOS, currently:

  • macOS 10.14 Mojave
  • macOS 10.13 High Sierra
  • macOS 10.12 Sierra

Releases of xnumon are considered safe to deploy in production environments.


See the xnumon wiki for further documentation. While the wiki is still work in progress, some useful starting points:

  • cat /var/log/xnumon.log | jq 'select(.eventcode==0)'
  • xnumonctl
  • xnumon -h
  • dmesg | grep xnumon


The installer package published on the xnumon website will install the daemon, the control utility and a default configuration which by default will log to /var/log/xnumon.log in JSON Lines format. It will also install a matching newsyslog configuration and the kernel extension.

As of macOS 10.13 High Sierra, the kext needs to be explicitly approved by the user before it can be loaded. For enterprise deployments, you will want to allow the Team ID C9BFEG985N to bypass user approval using spctl kext-consent from Recovery OS or NetBoot/NetInstall/NetRestore images, or using Mobile Device Management (MDM). For details, refer to TN2459.

The extensively commented default configuration is installed to /Library/Application Support/ch.roe.xnumon/configuration.plist-default. While the defaults are as sensible as possible, you will most likely want to maintain a custom configuration at /Library/Application Support/ch.roe.xnumon/configuration.plist to be used in favour of the default configuration, especially for enterprise deployments.

In addition to installing xnumon, you will want to make sure that auditd does not clobber the global kernel audit policy. Make sure the argv policy flag is enabled in /etc/security/audit_control, which is the default. If you are using envlevel dyld or full, then arge is needed in addition to argv.

In order to make the logs useful and to get them out of reach of malware and attackers, it is recommended to continuously forward logs to central log collection infrastructure. A minimal sample Splunk configuration for ingesting xnumon logs can be found in extra/splunk.


xnumonctl uninstall

This will remove all traces of this package from your system, including logs at the default location /var/log/xnumon.log*, but not including the config at /Library/Application Support/ch.roe.xnumon/configuration.plist unless it is the same as the default config.

Build Dependencies

Building an unsigned userland binary and kernel extension requires Xcode command line tools. The userland binary requires only the CoreFoundation and Security frameworks and libbsm; there are no third-party dependencies.

Building a signed userland binary requires an Application Developer ID certificate from Apple.

Building a signed kernel extension requires a Kext Developer ID certificate from Apple.

Building signed binary packages requires pandoc and an Installer Developer ID certificate from Apple.


Use make test while xnumon is logging to /var/log/xnumon.log to execute a set of automated test cases, exercising different APIs and automatically checking the log file for the expected events. Submitting a pull request with a failing testcase is the best way to report bugs.

Use the metrics in eventcode 1 events to monitor xnumon internals, possibly reducing the interval it gets generated in the configuration.

Enable debug in the configuration and run xnumonctl logstderr to change the launchd plist for xnumon to send stderr to /var/log/xnumon.stderr. This will allow you to get context information for fatal events that would otherwise only be visible in one of the eventcode 1 metrics.

For short-term debugging during development you can also just unload xnumon using xnumonctl unload and run xnumon with -o debug=true on the command line.

Pass DEBUG=1 to make in order to build a debug version of xnumon that includes symbols, assertions and additional debugging code. See make file for details.

To load an unsigned, modified kext for testing and development, you need to disable System Integrity Protection (SIP) for kexts. Reboot to Recovery OS by pressing cmd⌘+r during boot and from within the repair console, run csrutil enable --without kext. This will also turn off the kext user consent requirement of High Sierra and later.

Copyright and License

Copyright (c) 2017-2019, Daniel Roethlisberger.
All rights reserved.
Licensed under the Open Software License version 3.0.
Contains components licensed under BSD and MIT licenses as well as components released under the Unlicense.

See LICENSE, LICENSE.contrib and LICENSE.third as well as the respective source file headers for details.