WordNet (r) 3.0 (2006):

sniffer
    n 1: a person who sniffs

The Free On-line Dictionary of Computing (19 January 2023):

packet sniffer
packet sniffing
sniffer

    A network monitoring utility program
that captures data
   packets and decodes them using built-in knowledge of common
protocols.	Sniffers are used
   to help debug and monitor networking problems.  A sniffer installed
as malware might capture
   user names and passwords without the user's knowledge and transfer
them to a criminal.

   (2022-12-07)