Which Is More Secure? An Open Source Project Or A Proprietary Project?

The securities of these projects depends mainly on the size of the project, the total number of the developers who are working under this project and the one factor, which is most essential as well as important, is the control of the quality. Just the type of project won’t determine its quality, the inside matter of the corresponding projects will matter.

Proprietary software is more secure than open-source software.
Unlike proprietary software, open-source software is transparent about potential vulnerabilities.

Open source software is, almost by definition, more flexible but requires more effort to use, whereas the opposite is true for proprietary software in general. You can build a house by having all the raw materials dumped on the lot and then build whatever you like as you go along.