This article is from the Computer Viruses FAQ, by Nick FitzGerald firstname.lastname@example.org with numerous contributions by others.
An "ANSI bomb" is a sequence of characters, usually embedded in a text
file, that reprograms various keyboard functions of computers with ANSI
console (screen and keyboard) drivers. In theory a special sequence of
characters could have been included in this FAQ sheet to reprogram your
Enter key to issue the command "format c:" with a return character
tacked on the end.
Such a possibility however, need not translate into much of a threat.
It is rare for modern software to require the computer it runs on to
have an ANSI console, so few PCs or other machines should load ANSI
drivers. Also, few people use software that simply "types" output to
the terminal device, so such an ANSI bomb in an e-mail or News posting
would most likely not reprogram your keyboard anyway. Further, although
FORMAT C: may be catastrophic under certain versions of DOS, it won't
hurt Macintoshes and would probably have very unexpected, or no, effects
on other systems.
If you are at all worried about the possibility of having something
untoward happen on your PC due to an ANSI bomb *and* you have to load an
ANSI driver (some communications software still requires it), look for
one of the third-party ANSI drivers which abound on BBSes and FTP sites.
Most of these have improved performance over DOS's ANSI.SYS *and* either
do not support, or let you disable, keyboard re-mapping.