The +++ that led to lawsuits and exploits
The Three-Plus Salute: Iconic, Infamous, and Occasionally Infuriating
The +++ escape sequence — affectionately known as the three-plus salute — is one of the most legendary commands in modem history. It allowed users to switch from data mode to command mode without hanging up, a critical feature that saved many a dial-up session.
But Hayes didn’t just invent it — they patented it. Their version included a one-second pause (called the guard time) before and after the +++ to prevent accidental triggers. If other modem vendors wanted to use this method, they had to pay up… or get sued.
Many budget modem makers chose not to license Hayes’ sequence. Instead, they created a workaround known as the Time Independent Escape Sequence — or TIES, for short. TIES dropped the guard time entirely. It simply triggered command mode if +++ was followed by a valid AT command (like +++ATH0<cr>).
This had consequences. While +++AT sequences were rare in normal data streams, they could occur — especially during binary transfers — causing modems to unexpectedly drop into command mode, or worse, hang up entirely. Cue chaos.
Naturally, hackers took notice. These vulnerabilities became an early form of remote disruption — what some now consider the first Denial of Service (DoS) attack. Further Reading: https://en.wikipedia.org/wiki/Time_Independent_Escape_Sequence https://seclists.org/bugtraq/1998/Sep/193