Call Back Security

Call back security, also known as dial back security, is a security mechanism used to authenticate users over dialup lines.

The way call back security works is the dialup user calls into the system and enters his username and password. The system then hangs up on the user and calls the user back at a preconfigured number.

Call back security can sometimes be defeated using this technique:

1. Configure your modem to auto-answer. On Hayes-compatible modems, the command to do this is ATS0=1.
2. Dial the remote system.
3. When the remote system hangs up on you, stay on the line.
4. Play dial tone into the phone line to fool the remote modem into dialing.

This technique will not work if the system uses a different set of lines for dialing out than for users dialing in, but that is not common.