Am I plagiarising if I publish something that has been independently discovered since? My field is primarily connected with computer networks. Sometime in 2016, I discovered a vulnerability which at that time was not known (I searched a lot to make sure). I experimented with it and got exciting results. However, I couldn't publish it immediately due to some-other work my prof gave me.

When I was looking for good conferences on this subject I came across a GitHub repository exhibiting the same concept but it was created just months back. The work has not appeared in any conferences/journals.

I was wondering, since I thought of it and implemented it first, can I send it to  a conference or would that count as plagiarism?
Plagiarism is the explicit use of someone's work without attribution or permission, usually with the intent of passing it off as your own.

In the present case, the mere existence of the same work elsewhere does not make your work plagiarism. If you used their work and did not give them credit, that would be plagiarism.