LoFP LoFP / iot (internet of things) devices and networks may use telnet and can be excluded if desired. some business work-flows may use telnet for administration of older devices. these often have a predictable behavior. telnet activity involving an unusual source or destination may be more suspicious. telnet activity involving a production server that has no known associated telnet work-flow or business requirement is often suspicious.

Techniques

Sample rules

Accepted Default Telnet Port Connection

Description

This rule detects network events that may indicate the use of Telnet traffic. Telnet is commonly used by system administrators to remotely control older or embedded systems using the command line shell. It should almost never be directly exposed to the Internet, as it is frequently targeted and exploited by threat actors as an initial access or backdoor vector. As a plain-text protocol, it may also expose usernames and passwords to anyone capable of observing the traffic.

Detection logic

(data_stream.dataset:(fortinet_fortigate.log or network_traffic.flow
        or panw.panos or pfsense.log or sonicwall_firewall.log or suricata.eve)
    or event.category:(network or network_traffic))
    and event.type:(connection and not (denied or end))
    and not event.action:(Reject or client-rst or connection-denied or
        connection-end or denied or deny or flow_denied or flow_dropped or
        flow_terminated or network_flow or server-rst or timeout)
    and not (event.action:netflow_flow and not network.packets > 1)
    and not network.application:(stretchoid-scanning or traceroute)
    and destination.port:23
    and (network.protocol:(telnet or not *) or data_stream.dataset:fortinet_fortigate.log)