NGINX routes to the wrong virtual host when Host header contains colon character

My nginx.conf has several "server" sections, and a catch-all server section. Here is an example nginx.conf to give you an idea:

user www-data;
worker_processes auto;
worker_cpu_affinity auto;
pid /run/nginx.pid;

events {
    worker_connections 4000;
    use epoll;
    accept_mutex off;
}

http {
    include /etc/nginx/mime.types;
    default_type application/octet-stream;
    error_log /var/log/nginx/error.log;

    server {
        listen 80;

        server_name foo.com;

        location / {
            default_type text/plain;
            return 200 "hello from foo.com";
        }

        error_page 500 502 503 504 /500.html;
    }

    server {
        listen 80 default_server;
        server_name _;

        location / {
            return 403 "sorry";
        }
    }

}

I'm expecting the server to return 403 if the "Host" header is anything but "foo.com".

Somebody's apparently running Burp Suite on my server, and I noticed and interesting behavior when they send a "Host: foo.com:more-stuff-here" header: NGINX routes the request to the first "server" section. It looks as if it ignores the colon and everything after it in the header value.

I can reproduce it locally with the above nginx.conf:

$ curl -H "Host: foo.com" http://127.0.0.1
hello from foo.com

$ curl -H "Host: foo.com:unexpected-content" http://127.0.0.1
hello from foo.com

$ curl -H "Host: bar.com" http://127.0.0.1
sorry

Why does NGINX do this? Is this an expected behavior? What should I change in nginx.conf to ensure requests with "Host: foo.com:more-stuff-here" header go to the default block?

Update: For anybody researching the same issue, I also created a ticket in NGINX issue tracker.

The following might work for default_server:

server {
    listen 80 default_server;

    server_name _ ~example\.com:;

    location / {
        return 403 "sorry";
    }
}

The important part is the tilde ~, which indicates a regular expression match.

The only alternative thought I could think of would be to code something in at the server side application level to check the host value and return the 403 response in code.

This however is not really the right way to do it, because the issue is a client issue to fix and not for your application.

for example, taking from this SO answer given a PHP application you could easily amend this logic to achieve the desired effect, note specifically the check for the SERVER_PORT as per Gerald's answer

$allowed_hosts = array('foo.example.com', 'bar.example.com');
$allowed_ports = array(80,443);
if (!isset($_SERVER['HTTP_HOST']) || !in_array($_SERVER['HTTP_HOST'], $allowed_hosts) || !in_array($_SERVER['SERVER_PORT'], $allowed_ports )) {
    header($_SERVER['SERVER_PROTOCOL'].' 400 Bad Request');
    exit;
}