I'm asking about an error I've got a few times in the past day, outside of the theoretical. I'm wondering what may be actually happening in my case.

The error comes from Sentry, on my ubuntu/nginx/uwsgi/Django via AWS Elastic Beanstalk stack:

Invalid HTTP_HOST header: ''. You may need to add u'' to ALLOWED_HOSTS. Exception while resolving variable 'exception_type' in template 'unknown'. is associated with scan-15.shadowserver.org. ALLOWED_HOSTS is there to prevent the HTTP host header attacks.

Should I be concerned? What is going on here in concrete terms?



Should I be concerned?

Not particularly. You're being scanned by a known non-malicious party called Shadowserver. They're very open about their activities and intent.

Bear in mind that on the Internet you're going to get scanned. A lot. By many people who are outright malicious. While some may have legitimate concerns about 'scan vigilantes' like Shadowserver, Shadowserver is certainly more benign than a lot of other traffic you can expect to receive.

What is going on here in concrete terms?

They are connecting to your web server and sending a request using an invalid Host: header - one that doesn't match your actual site, but actually matches their IP. There are a variety of ways the host header can be abused; Shadowserver is looking for vulnerable IPs. Given their mission statement, they're probably looking for web servers that can be tricked into sending or redirecting traffic to [the IP they used in the Host: header], which would indicate that those web servers can be abused in an amplification attack.

The fact that your server reports Invalid HTTP_HOST header leads me to believe it correctly rejects connections which don't contain a valid Host: header for your site. Good job.

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