We have them everywhere. And I'm looking for the term to refer to them.
These are the people who cannot keep any matter to themselves. Irrespective of the degree of seriousness of the matter, they'll simply spit it out in front of others. In short, they are not eligible to say the idiom I'll carry this to my grave!
In my school/college days, we had a very informal term for them. We used to call such people 'the BBC'! That's because if you have told anything to them, they'll spread the word for sure.
There could be more than one word. I'll be happy to have the closest term for it though.