Bug #2655
Updated by Victor Julien about 5 years ago
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: <pre> POST _POST /... HTTP/1.1 Host: ... ... Content-Type: application/x-www-form-urlencoded Content-Length: 22 login=foo&password=*bar* </pre> login=foo&password=*bar*_ Below is a simple rule example: <pre> alert _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;) </pre> rev:1;)_ Let's imagine that a threat actor will manualy remove the 'Content-Length' option from HTTP header: <pre> POST _POST /... HTTP/1.1 Host: ... ... login=foo&password=*bar4* </pre> 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