How do I limit the size of a Net::HTTP request?

Question

I'm creating an API service which allows people to provide a URL of an image to the API call, and the the service downloads the image to process.

How do I ensure somebody does NOT give me the URL of, like, a 5MB image? Is there a way to limit the request?

This is what I have so far, which basically grabs everything.

  req = Net::HTTP::Get.new(url.path)
  res = Net::HTTP.start(url.host, url.port) { |http|
    http.request(req)
  }

Thanks, Conrad

Answer 1

Try running this first:

Net::HTTP.start(url.host, url.port) { |http|
  response = http.request_head(url.path)
  raise "File too big." if response['content-length'].to_i > 5*1024*1024
}

You still have a race condition (someone could swap out the file after you do the HEAD request), but in the simple case this asks the server for the headers it would send back from a GET request.

Answer 2

cwninja unfortunately gave you an answer that will only work for accidental attacks. An intelligent attacker will have no trouble at all defeating that check. There are two main reasons his method should not be used. First, nothing guarantees that the information in a HEAD response will match the corresponding GET response. A properly behaving server certainly will do this, but a malicious actor does not have to follow the spec. The attacker could simply send a HEAD response that says it has a Content-Length that's less than your threshold, but then hand you a huge file in the GET response. But that doesn't even cover the potential for a server to send back a response with the Transfer-Encoding: chunked header set. A chunked response could quite possibly never end. A few people pointing your server at never-ending responses could carry out a trivial resource-exhaustion attack, even if your HTTP client enforces a timeout.

To do this correctly, you need to use an HTTP library that allows you to count the bytes as they're received, and abort if it crosses the threshold. I would probably recommend Curb for this rather than Net::HTTP. (Can you even do this at all with Net::HTTP?) If you use the on_body and/or on_progress callbacks, you can count the incoming bytes and abort mid-response if you receive a file that's too large. Obviously, as cwninja already pointed out, if you receive a Content-Length header larger than your threshold, you want to abort for that too. Curb is also notably faster than Net::HTTP.