Benji Dunn: According to one of these e-mails, your Mr.Davian is going to attend a function in tomorrow afternoon at one Vatican City.
Luther: What the hell's Davian doing in Vatican?
Benji: Well, it's all got to do with "the Rabbit's foot".
Ethan: What is that?
Benji: I'm assuming it is a kind of code-word for a deadly weapon, something ...
It's a biohazard, that means
something that may cause harm to people or to the environment, especially a poisonous chemical or an infectious disease
Here, this picture confirms that Rabbit's foot is a biohazard.
Just to add on and add some sources and additional things to consider:
First we don't have enough information to know for certain what the true nature of Ethan's and Lindsay's relationship was, but given that Ethan Hunt is a traditional protagonist and is probably viewed as an honorable man, we may assume that he means what he says, when he tells Luther, ...