How serious is plagiarism in a master’s thesis? I published my PhD thesis three years ago.

I have just come to know that a master’s student has copied three chapters from my PhD thesis half a year later. My PhD supervisor was also the supervisor of his master’s project. Each and every word and mathematical symbol is exactly the same. That person is now a PhD student himself. I’m obviously extremely shocked, angered and hurt by this. But before I take any action I wanted to ask the community.

How serious is plagiarism in a master’s thesis? I suspect that my PhD supervisor also had a role to play in this. He either tacitly ignored this blatant plagiarism or even actively suggested it. How should I deal with this given that my relations with my PhD supervisor will become strained if I report this?
Edit based on subsequent info by the OP...

This has to be raised with the relevant department / committee / Dean so that it can be addressed in a manner they see fit.

They could demand changes, or they could leave all alone.

If you don't report this, then if the Master's thesis gets published first YOU will be accused of plagiarism...

So you have to report this, but how you do that is going to be a difficult choice.


you could see your advisor first and get the master's student to change those chapters.
If that is not successful, then you have to go further, as the possible consequences of leaving it could be serious for you.