These instructions are outdated because only macOS 10.15.0 is affected. Install macOS 10.15.1 or later to fix the issue descibed here.
When you install Little Snitch on the initial version of macOS Catalina (version 10.15.0), you may get an error message “Little Snitch version mismatch detected” after restarting your Mac. This is due to a bug in the operating system.
Catalina sometimes refuses to perform the necessary boot cache update for the newly installed kernel extension. As a consequence, macOS still uses the previous version of the kernel extension from the outdated cache, which then results in a version conflict with all other updated components of Little Snitch.
Unfortunately it’s not possible to fix this problem in the Little Snitch Installer. macOS provides no means to repair such an invalid cache (except for some more elaborate actions in macOS Recovery). This problem can only be fixed by Apple with an update of macOS, so we are hoping that Apple will be able to address this issue soon.
In the meantime you can work around the problem by downgrading to the previously installed version of Little Snitch. It’s important to install the version that matches the currently installed kernel extension. To figure out which version has to be installed, please do the following:
1. Open Terminal
Open Terminal.app from your Applications > Utilities folder, type the following command and then press Return:
kextstat | grep obdev
2. Find the version number
You will see a result like this:
[...] at.obdev.nke.LittleSnitch (5422) [...]
The four-digit number in parentheses indicates the version of the currently installed kernel extension.
3. Downgrade Little Snitch
Download and install the corresponding Little Snitch version:
If your version is not listed here, please follow the instructions in this article instead.