The problem

nmap stopped working with the following error message:

$ nmap XXX.YYY.ZZZ.0/24
dyld: Library not loaded: /usr/local/opt/openssl/lib/libssl.1.0.0.dylib
  Referenced from: /usr/local/bin/nmap
  Reason: image not found
Abort trap: 6

• Application: nmap (but could be any application)
• Platform: macOS Catalina
• Package manager: brew

A solution

Uninstall and reinstall nmap

$ brew uninstall nmap
$ brew install nmap

Background

This type of problem seems to happen regularly with brew. OpenSSL 1.0.0 has been removed from brew. Perhaps it does not correctly update all packages when a dependency has been removed.