What you call someone who leaks information from a team or group. Like Spy.
Can you call him/her a "Rat"?
The first noun that came to my mind is whistleblower, but if you look up the thesaurus on Dictionary.com, there are plenty more such as bigmouth, squealer (to name a few that I'm familiar with), and rat is also there!
You can browse some other possible words following the link below: http://www.thesaurus.com/browse/whistleblower
When several leaks take place over a period of time, then the individual doing the leaking is known as a mole.
The term was popularised in the works of John Le Carré. In Tinker Tailor Soldier Spy, much is made of the character Smiley searching for the mole in British Intelligence leaking information to the Soviets.
The term has become much more widespread in usage than as a spy and would be appropriate for any team.
If you are looking for a non-slang term, a good one might be informer, which Merriam-Webster's Eleventh Collegiate Dictionary (2003) defines as follows:
informer n (14c) 1 : one that imparts knowledge or news 2 : one that informs against another; specif : one who makes a practice esp. for a financial reward of informing against others for violations of penal laws
Of course, in the example you give, the "reward" that the informer gets is preferment by the superior, and the metaphorical "violations of penal laws" are nothing more than criticisms of those in authority, but informer still seems apt. Another option is tattletale (again as defined in the Eleventh Collegiate):
tattletale n (1888) one that tattles [in the sense of "to tell secrets"]: INFORMER
And yet another is talebearer:
talebearer n (15c) one that spreads gossip or rumors; also : TATTLETALE
Yet another way to describe such a person is with the phrase "betrayer of confidences," meaning someone who participates in a no-holds-barred, everything-on-the-table discussion that has as one of its ground rules that what is said in the room stays in the room, and then breaks faith with that understanding by acting as a conduit of what was said in confidence, to nonparticipants.
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