The following problem (as discussed in this paper) has not yet been identified. Certain firewalls today, will not authenticate the validity of certain protocol fields, within the packet they are processing. The risk is exposure of information. What kind of information can be exposed? Mainly it will be unique patterns of behavior produced by the probed machines answering our crafted queries (or other kind of network traffic initiated in order to elicit a reply). Those patterns will help a malicious computer attacker to identify the operating systems in use.
Download the paper in PDF format here.