Telnet is now mostly a device protocol, and devices ship with fixed default credentials that are documented, widely known, and seldom changed. So the first and highest-yield Telnet attack is trying defaults: generic pairs (root:root, admin:admin, root:<vendor>, blank passwords) and the specific documented credentials for the identified device. These accounts are usually administrative, an enable-capable network account or a root shell on embedded Linux, so a default-credential hit is typically full device control, and mass IoT compromises have been built on exactly this.
# identify the device (banner), then try its documented defaults + generics
nc <target> 23
hydra -C iot-default-combos.txt telnet://<target> # user:pass combo list
Exploitation notes#
- Fingerprint the device first (banner); defaults are vendor/model-specific, so identification turns guessing into a lookup, and generic pairs cover the rest.
- Use paired combo lists (documented user:pass) rather than full cross-products for speed and quiet; IoT default lists (as used by Telnet-spreading worms) are highly effective.
- The accounts are commonly administrative, so success is device control (config change, firmware, often a root shell), not a low-privilege foothold.
- Blank or empty passwords are a frequent device default; test them explicitly.