LuLu by Objective-See

LuLu is the free, shared-source firewall for macOS. Its goal is simple; block any unknown outgoing connections, until approved by the user. While it was designed to generically detect malware by flagging unauthorized networking connections, LuLu can also be used to block OS components or 3rd-party applications from transmitting information to remote servers.

What's to like about LuLu? Lots!

100% free As in no ads, no time trials, no missing features. Because why not!? And no, it doesn't track, monitor, or spy on you - as that'd just be pure evil!

Shared source The full source code for LuLu is available on GitHub. Such transparency allows anybody to audit its code, or understand exactly what is going on.

Protects LuLu aims to alert you whenever an unauthorized network connection is attempted. As such, it can generically detect malware, or be used to block legitimate applications that may be transmitting private data to remote servers.

Simple "Do one thing, do it well!" LuLu is designed as simply as possible. Sure this means complex features may not be available, but it also means it's easier to use and has a smaller attack surface!

Enterprise-friendly Want to know what network events are being detected? Or rules your users have added? LuLu provides simple mechanisms to subscribe to such events, and stores data such as rules in an open, easily digestible manner.