Whenever a movie is set for release there is one copy made to be shared with some share holders. Censor Board is one of the share holders. Reviewers are the other group. These are prominent well known reviewers not some scoopwhoop bullshit reviewer. Another group is a selected group of people from the cast and the crew. No, a lightboy is not included. Assistant Producers do get a share though. And no, not every cast member gets to watch the finished product before the actual release. Most certainly the 3 or 4 names which you see on the cast list on a poster definitely get the same copy but they can watch it within the premises of the production house or the studio. They can't carry it out. The one with the reviewer and the censor board are the only ones which can go out of the studio.
To conclude... the leak was done by a reviewer most probably. Or someone who works with a prominent reviewer. CBFC is out of the question because they would be ripped apart publicly if caught. And it is very easy to get caught if you are in CBFC and are leaking something. It is damn easy if you are the reviewer. And easier if you are just working alongside him.
Don't ask me how I know it. Take it with a grain of salt.