WAF 390702

Rule 390702
Status	Active
Alert Message	Atomicorp.com WAF Rules: Multiple/Conflicting Connection Header Data Found

Description

This rules detects when multiple or conflicting connection headers are found. For example, multiple entrys such as below:

Connection: keep-alive, keep-alive

Or conflicting headers, for example:

Connection: keep-alive, close

Broken and/or malicous clients often have duplicate or conflicting headers. No client should submit an entry with multiple or conflicting entries. Unfortunately, some automated programs do not not obey the HTTP RFC, and malicious clients are known to use this method to try to trick intrusion detection systems and firewalls to allow the attack to pass through. A proper request should only contain one entry, for example:

Connection: keep-alive

The use of multiple, or conflicting behavior is not normal or common for actual clients, and is extremely rare in a legitimate client. If you see this rule being triggered you have either a malicious client connecting to your system, or a very broken application or client. In either case, we do not recommend you disable this rule as it will detect potentially unknown attacks associated with this condition.

This has also been known to happen when a malicious client is proxying its connection through a system to hide its real IP. Real non-malicious proxy servers do not cause this behavior, this behaviour is best thought of as a bug the client is causing as its manipulating the session to hide its real location.

Examples

Classic Example

This is what an HTTP request looks like to the server. This is what the client sends to the server to request a resource, to post data, etc. The headers below are set by the client, not the server.

GET /some/file HTTP/1.1
Connection: keep-alive, keep-alive
Accept: */*
Referer: http://www.example.com/someurl
Accept-Language: en
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 
Cookie: somecookie
Host: http://www.example.com

In the example above, the "Connection:" header has two entries "keep-alive, keep-alive". Per the HTTP RFC this header should only have one entry "keep-alive". This is used by some attackers to cause DOS attacks on servers, and is also an indicator that either a broken proxy or a broken client is attempting to connect to the server, both of which may indicate the client has malicious intent.

Malicious proxy example

GET /someurl/ HTTP/1.0
Host: example.com
Connection: close, close
Accept-Language: zh-cn,zh-tw, zh-cn,zh-tw
User-agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/535.2 (KHTML like Gecko) Chrome/15.0.860.0 Safari/535.2Chrome/15.0.860.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/533.20.25 (KHTML like Gecko) Version/15.0.860.0, Mozilla/5.0 (Windows NT 5.1) AppleWebKit/535.2 (KHTML like Gecko) Chrome/15.0.860.0 Safari/535.2Chrome/15.0.860.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/533.20.25 (KHTML like Gecko) Version/15.0.860.0
Accept: */*

The example above is a real attack that occurred on a honeypot. In the headers above there a number of errors and duplication caused by the attacker. For excample, the Connection header and Accept-Language headers are duplicated, a classic give away that the attacker is trying to proxy traffic through a compromised host. The attack has also sent a highly duplicated User-agent header, which contains numbers contradictory user agents (the client is reported to be using Chrome and Safari at the same time, which is completely impossible.

Troubleshooting

False Positives

None. This rule can not generate a false positive. If this rule triggers the client is sending duplicate Connection headers, which is both invalid and may indicate an attack is imminent or in progress.

The detection of bogus headers is an excellent way to detect new attacks and attack sources that may not otherwise be detectable. As this behavior is never normal, and does not occur with either a properly configured proxy server, or any web browser we do not recommend you disable this rule.

Tuning Guidance

None. This rule detects invalid connections. If clients are connecting in this manner this is a bug on the client side, and the connection is invalid.

Additional Information

Similar Rules

None.

Knowledge Base Articles

None.

Outside References

None.