Why shouldn't we allow body in a GET or HEAD request?

Question

I'm coming to this from the InfoSec side, not the AppDev side, I just wanted to put that caveat in first. The issue is that my WAF is blocking certain images with the response, HTTP protocol compliance failed:Body in GET or HEAD requests. I need to justify keeping this rule active, so I'm asking, as a non-developer:

1. is this getting blocked because that's the rules of GET and HEAD requests, or can we allow Body in GET and HEAD requests, but it's really not a good idea ?
2. Why is it not a good idea? What are the potential problems that arise from allowing Body in a GET or HEAD request?

Thanks in advance for everyone's help.

Answer 1

GET and HEAD don't send request bodies, only POST and PUT do (see RFC 2616) You are blocked for another reason, as I saw myself often, one of the request headers Content-Length or Transfer-Encoding which can be tolerated if really there's no request body at all (Content-Length: 0). As the WAF finds these headers, it considers the request as having a body, even of size 0.

1. If you loosen the policy, you will allow legitimate traffic but also open the door to abnormal traffic on GET/PUT. To circumvent this, you can add an iRule or LTM policy to remove the headers on GET/PUT, until F5 releases a better version of the software to not block the traffic when the body is of size 0.
2. The potential problem comes when a bugged Web server would buffer the data sent in a GET/HEAD body instead of returning a 400 error, and ignoring the data. This data could lead to memory consumption, or to injecting hacker's data into legitimate users requests with unknown results at this time. If you are confident in your Web server, you may loosen the WAF policy.