Cisco recently announced a vulnerability in 300 OF THEIR SWITCH MODELS in the recent Wikileaks Vault 7 dump. Apparently the CIA discovered the vulnerability and created an exploit for it for their own nefarious purposes, rather than informing Cisco so they could fix it.
Those of you who blindly trust your government to "keep you safe", there you go. There should be sanctions levied against the CIA for this clear violation of public trust. There won't be though.
If you're using Cisco switches, you should disable telnet immediately and keep it disabled until further notice. Cisco will be pushing out the updates as soon as possible.