Hello Andras,

From what I read on this page (http://www.mitsuba-renderer.org/download.html), it is open source :
"
Mitsuba is currently maintained and developed by Wenzel Jakob. It is free software and can be redistributed and modified under the terms of the GNU General Public License (Version 3) as provided by the Free Software Foundation.
If you used Mitsuba in your about-to-be published research project, a citation is kindly requested:

@misc{Mitsuba,
Author = {Wenzel Jakob},
Year = {2010},
Note = {http://www.mitsuba-renderer.org},
Title = {Mitsuba renderer}
}
"


And the source code is here
https://www.mitsuba-renderer.org/hg/

--Michel