Suricata

Hello, Team!
We've run into troubles with a few threats variants.
Let's imagine some sort of malware which tries to steal a user information.
There are several methods, but one of them is simply send a data via POST request:

POST /... HTTP/1.1
Host: ...
...
Content-Type: application/x-www-form-urlencoded
Content-Length: 22

login=foo&password=*bar*

Below is a simple rule example:

alert http any any -> any any ( \
msg:"TEST 'no Content-Length' vs http_client_body"; \
flow:to_server,established; \
content:"login=foo&password=bar"; http_client_body; \
classtype:trojan-activity; \
sid:1; rev:1;)

Let's imagine that a threat actor will manualy remove the 'Content-Length' option from HTTP header:

POST /... HTTP/1.1
Host: ...
...

login=foo&password=*bar4*

In this case according to HTTP specification a request will be wrong.
But from a network transport side it's not a problem and a packet will be successfully transferred.

The same can be done with GET method although this technique is not recommended.

I've attached two pcap dumps as example.
I've also played a bit with 'Content-Type' and 'Content-Length' options in each session.
As a result, Suricata doesn't detect the body content with 'http_client_body' modifier in cases when 'Content-Length' is absent.
We'll have the same result if just change the 'Content-Length' value to a smaller one: Content-Length: 22 -> Content-Length: 20

A default HTTP server will return an error because it will not resolve a 'login=foo&password=bar' combination as HTTP method of a new request.
But it's up to us how we'll configure our HTTP server.

I believe that removing or reducing the 'Content-Length' field value is a quite easy trick to bypass Suricata detection with http_client_body modifier for a pattern.
I suggest to improve a body length recognizing logic and not to be based on 'Content-Length' value only.

Thank you.
Sincerely yours, Alexey Vishnyakov