Fun_People Archive
7 Oct
The + + + AT Modem Patch

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From: Peter Langston <psl>
Date: Wed,  7 Oct 98 17:19:18 -0700
To: Fun_People
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Subject: The + + + AT Modem Patch

X-Lib-of-Cong-ISSN: 1098-7649
From: Oliver Xymoron <>
Subject: Re: 1+2=3,  + + + A T H 0 =Old school DoS (fwd)

    Oliver Xymoron <> has put together a web page that uses  
an idea from Adrian Gonzalez <agonzale@NLAREDO.GLOBALPC.NET> to exploit a  
frailty of many existing modems.  Actually, it exploits the frailty in the  
process of fixing it -- it remotely sends and installs a modem "patch" that  
disallows further remote control of the modem.
    Readers of the bugtraq  
mailing list have discussed (at length) the observation that many modems use  
the Hayes command set (which includes gaining control of the modem by sending  
three plus signs followed by a pause) except they don't require the pause  
since Hayes has a patent on using a guard timer to detect the required pause.   
The upshot is that a sequence like "+ + + A T H 0" (without the space  
characters) will make such a modem hang up the phone line.
    Adrian's idea (see was to  
use the bug to patch the modem by sending "+ + + A T S 2 = 2 5 5 & W O 1 <CR>"  
which grabs control of the modem, disables the "+ + +" feature, and puts the  
modem back on-line (if the phone line hasn't dropped already).
    Oliver's idea was to make a web page to squirt this bug patch through  
your modem.  To do that, go to  
and click the "Ping me!" button.

 -=[ P. Langston - - - Seattle, WA ]=-

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