For either clientside LAN, in transit on WAN, or serverside LAN:
Packets can be sniffed easily.
Firing up something like Wireshark (tcpdump) will let you see the source and destination IP of the packet (always in cleartext), and the contents (either in cleartext or encrypted).
Through LAN network manipulation you can modify network traffic to inject code or disable encryption with for example software like Nighthawk (windows) or MITMProxy.
manipulating traffic in transit on WAN will be harder but the resources of the likely threat there are also much more vast
for example codename QUANTUMINSERT:
The documents leaked by former National Security Agency (NSA)
contractor Edward Snowden mention dozens of hard- and software attacks
available to the NSA to gain and maintain access to target networks.