You are part of a security staff at a highly profitable bank and each day, all traffic on the network is logged for later review. Every Friday when major deposits are made youre seeing a series of bits placed in the -Urgent Pointer- field of a TCP packet. This is only 16 bits which isnt much but it concerns you because:
A. This could be a sign of covert channeling in bank network communications and should be investigated.
B. It could be a sign of a damaged network cable causing the issue.
C. It could be a symptom of malfunctioning network card or drivers and the source system should be checked for the problem.
D. It is normal traffic because sometimes the previous fields 16-bit checksum value can over run into the urgent pointers 16-bit field causing the condition.
Explanation:
The Urgent Pointer is used when some information has to reach the server ASAP. When the TCP/IP stack at the other end sees a packet using the Urgent Pointer set, it is duty bound to stop all ongoing activities and immediately send this packet up the stack for immediate processing. Since the packet is plucked out of the processing queue and acted upon immediately, it is known as an Out Of Band (OOB)packet and the data is called Out Of Band (OOB) data.
The Urgent Pointer is usually used in Telnet, where an immediate response (e.g. the echoing of characters) is desirable.
Covert Channels are not directly synonymous with backdoors. A covert channel is simply using a communication protocol in a way it was not intended to be used or sending data without going through the proper access control mechanisms or channels. For example, in a Mandatory Access Control systems a user at secret has found a way to communicate information to a user at Confidential without going through the normal channels.
In this case the Urgent bit could be used for a few reasons:
1. It could be to attempt a Denial of service where the host receiving a packet with the Urgent bit set will give immediate attention to the request and will be in wait state until the urgent message is receive, if the sender does not send the urgent message then it will simply sit there doing nothing until it times out. Some of the TCP/IP stacks used to have a 600 seconds time out, which means that for 10 minutes nobody could use the port. By sending thousands of packet with the URGENT flag set, it would create a very effective denial of service attack.
2. It could be used as a client server application to transmit data back and forward without going through the proper channels. It would be slow but it is possible to use reserved fields and bits to transmit data outside the normal communication channels.
The other answers are incorrect
The following reference(s) were/was used to create this question:
http://www.vijaymukhi.com/vmis/tcp.htm
and
http://www.fas.org/irp/nsa/rainbow/tg030.htm document covering the subject of covert channels
and also see:
http://gray-world.net/papers.shtml which is a large collection of documents on Covert Channels