[tialaramex]

IIRC Microsoft is rewriting some of these backend services in Rust, although not because it will increase security but because it lets them get better perf than existing solutions without the safety tradeoff they'd have suffered to go to C++ which would have been their option 15-20 years ago. I don't know whether Sharepoint was on that list.

[p_ing]

SharePoint is primarily written in C# [.NET Framework 4.8] and leverages ASP.NET; there would be no reason to rewrite the majority in another language. There is some C++ in SharePoint Search (and a few other components here and there).

IIS which SharePoint runs atop of is written in presumably primarily C.

You can decompile most of SharePoint if you ever need to peek at the code. That's a huge advantage to figure out how it works.

[mynameisash]

You also can get better velocity than with other languages due to the compile-time checks.