Some Linux distributions, for example Debian, do not provide tools that VirtualBox Guest Additions uses to verify kernel module signatures. If VirtualBox Guest Additions can't verify a module's signature, they will attempt to rebuild the kernel module on system boot. This can cause boot delays and the need to re-sign modules every time the VM is rebooted.
On the VM, create a file
This tells VirtualBox Guest Additions not to check signatures on boot, but to load the modules and leave it to the kernel to check module signatures.
If you set the variable to any value other than 1, it has no effect.