How can I fix Broadcom driver Wifi with 4.15.0-xx kernel on Ubuntu 16.04

Canonical updated HWE stack for Ubuntu 16.04 to the 4.15 kernel. But they forgot to update the Broadcom wireless driver, so it can be built. It is a critical and a very stupid bug, that will cause trouble for many people using the LTS.

Now the issue is being fixed and a compatible driver version has been built

This issue can be solved by installing a compatible version of bcmwl-kernel-source that should build on the 4.15 kernels.

Download http://archive.ubuntu.com/ubuntu/pool/restricted/b/bcmwl/bcmwl-kernel-source_6.30.223.271+bdcom-0ubuntu1~1.3_amd64.deb

and install it using dkpg.

This can be done by

wget http://archive.ubuntu.com/ubuntu/pool/restricted/b/bcmwl/bcmwl-kernel-source_6.30.223.271+bdcom-0ubuntu1~1.3_amd64.deb
sudo dpkg -i bcmwl-kernel-source_6.30.223.271+bdcom-0ubuntu1~1.3_amd64.deb

Windows is not related at all.

The problem is that you have a kernel that is not compatible with the driver.

Important: If you don't have any other network connection but the Broadcom Wi-Fi, you can boot with a previous kernel using grub menu and install the driver. It should build for all kernels.

Apropos to Pilot6's nice answer to this question, it looks like an official fix for xenial (i.e. not requiring one to install packages from bionic) has been released (if you use package broadcom-sta), or else committed (if you use package bcmwl which provides bcmwl-kernel-source).

For further info, see:

broadcom-sta: https://bugs.launchpad.net/ubuntu/+source/broadcom-sta/+bug/1777444

bcmwl: https://bugs.launchpad.net/ubuntu/+source/bcmwl/+bug/1777646