Little Snitch cannot be installed if the “/mach_kernel” file is missing. This file is an important component of OS X and its absence can not only cause problems for Little Snitch’s installation, but also for other software and OS X itself.
The /mach_kernel file is located at the top level of the startup disk and is normally hidden and therefore not visible in the Finder, so most users will never see it.
In October 2013, Apple released the OS X Mountain Lion 10.8.5 Supplemental Update. Probably due to an error, installing this update makes the /mach_kernel file visible in Finder and therefore more likely to be mistaken for a superfluous file that can be deleted without consequences. However, deleting this file may lead to severe consequences, in some cases even preventing the system from starting.
Whenever a new kernel extension is installed (which the Little Snitch Installer does) the system attempts to update the so–called kernel cache to speed up the startup of OS X. However, if the /mach_kernel file is missing, it’s not possible to update the kernel cache and Little Snitch won’t work.
To solve this problem, the /mach_kernel file must be recovered by restoring it from a backup or by re–installing the latest OS X Combo Update.
Restoring the file from a backup may be problematic if the backup’s version of the /mach_kernel file does not match the current system’s version. Therefore, re–installing the latest OS X update is recommended.
At the time of writing (October 15th, 2013), the latest OS X update is 10.8.5. Unfortunately, trying to install this update will not work if /mach_kernel is missing on the startup disk. But the OS X update installer can be tricked into allowing the installation by following these steps while logged in as an administrator:
The second step creates an empty /mach_kernel file, which is enough for the 10.8.5 update to work. As part of the update, the empty /mach_kernel file is replaced with the correct one. After that, installing Little Snitch should work as expected.
Please note that simply creating the empty /mach_kernel file is not sufficient for Little Snitch’s installation to succeed.