A computer program located on a server that monitors the information circulating on the network for the purpose of extracting information.

Network cards are capable of checking whether or not the information received is addressed to their system as well as rejecting it if this is not the case. However, the sniffer manages to deactivate the address verification filter, making the network card accept all the packets reaching the computer where it is installed, whether or not these are addressed to the device. Hence, unencrypted traffic sent may be “listened” to by those who use sniffers.

Traffic analyses may also be used to determine the relations existing between several users (knowing what users or systems a specific person interacts with).

It is not easy to detect whether our network traffic is being monitored by a sniffer, and so it is always recommendable to use encryption in all communications.