Invoking an asynchronous method inside a middleware in node-http-proxy

I'm trying to create a proxy with node-http-proxy in Node.js that checks whether a request is authorized in a mongodb.

Basically, I created a middleware module for the node-http-proxy that I use like this:

httpProxy.createServer(
require('./example-middleware')(),
9005, 'localhost'
).listen(8005)

What the middleware module does is using mongojs to connect to mongodb and run a query to see if the user is authorized to access the resource:

module.exports = function(){
// Do something when first loaded!
console.log("Middleware loaded!");

return function (req, res, next) {
var record = extractCredentials(req);
var query = -- Db query --

//Debug:
log("Query result", query);

db.authList.find(query).sort({
    "url.len": -1
}, function(err, docs){
    console.log(docs);

    // Return the creator for the longest matching path:
    if(docs.length > 0) {
        console.log("User confirmed!");
        next();
    } else {
        console.log("User not confirmed!");
        res.writeHead(403, {
            'Content-Type': 'text/plain'
        });
        res.write('You are not allowed to access this resource...');
        res.end();
    }

});

}
}

Now the problem is that as soon as I add the asynchronous call to mongodb using mongojs the proxy hangs and never send the response back.

To clarify: on a "User not confirmed" everything works fine and the 403 is returned. On a "user confirmed" however I see the log but the browser then hangs forever and the request isn't proxied.

Now, if I remove the "user confirmed" and next() part outside of a callback it does work:

module.exports = function(){
// Do something when first loaded!
console.log("Middleware loaded!");

return function (req, res, next) {
    var record = extractCredentials(req);
    var query = --- query ---


    console.log("User confirmed!");
    next();
}

but I can't do that since the mongojs query is meant (rightfully I guess) to be executed asynchronously, the callback being triggered only when the db replied...

I also tried the version without using a middleware:

http.createServer(function (req, res) {
  // run the async query here!
  proxy.proxyRequest(req, res, {
  host: 'localhost',
  port: 9000
});
}).listen(8001);

But that did not help either...

Any clue? Note that I'm new to node.js so I suspect a misunderstanding on my side...