Security concerning MongoDB on ec2?
EC2 security policies by default block all incoming ports except ones you have sanctioned, as such the firewall will actually stop someone from getting directly to your MongoDB instance; as such yes it is secure enough.
Since the instances are physically isolated there is no chance of the problems you would get on shared hosting of someone being able to route through the back of their instances to yours (though some things are still shared like IO read head).
I'm using a flask on an ec2 instance as server, and on that same machine I'm using that flask talking to a MongoDB. 
For the ec2 instance I only leaves port 80 and 22 open, without leaving the mongo port (27017) because all the clients are supposed to talk to the flask server via http calls. Only in the flask I have code to insert or query the server.
What I'm wondering is
Is it secure enough? I'm using a key file to ssh to that ec2 machine, but I do need to be 99% sure that nobody else could query/insert into the mongodb
If not, what shall I do?
Thanks!