What is the hash function used by MongoDB to hash the database user passwords?

In MongoDb use can add user to a database for access control. A user can be inserted with:

db.addUser({ user: "aaaa", pwd: "1234", roles: [...]})

and the inserted user is:

{
    "user" : "aaa",
    "pwd" : "08ccdf34dbb3ca05dcc195e30994b628",
    "roles" : [
        "userAdminAnyDatabase"
    ],
    "_id" : ObjectId("52a88b45965c4e7ad46bb97b")
}

What function does MongoDB use to compute/hash the pwd key?