EXCLUSIVE

Courtney Love hates Nirvana's latest release because it's not theirs ... there's no way it's Kurt Cobain singing -- and she's siccing her lawyers on whoever's responsible for the fake song.

Sources tell TMZ ... Love caught wind of the tune -- which was uploaded to YouTube last week -- and with the help of a real Nirvana member ... confirmed it was fake. Courtney is part owner of Cobain's music rights, and got YouTube to take down the song.

The people behind the fake jam claimed to have hacked Love's computer to uncover a rare track ... which Cobain never planned to release. We're told that whole story is also fake -- Courtney never had the song ... nor was she ever hacked.