Money quote: “It's not a vulnerability/security issue in Notepad++, but for remedying this issue, from this release (v7.3.3) forward, notepad++.exe checks the certificate validation in scilexer.dll before loading it. If the certificate is missing or invalid, then it just won't be loaded, and Notepad++ will fail to launch.
Checking the certificate of DLL makes it harder to hack. Note that once users’ PCs are compromised, the hackers can do anything on the PCs. This solution only prevents from Notepad++ loading a CIA homemade DLL. It doesn't prevent your original notepad++.exe from being replaced by modified notepad++.exe while the CIA is controlling your PC.”