n. A hole in the security of a system deliberately left
in place by designers or maintainers. The motivation for this is
not always sinister; some operating systems, for example, come out
of the box with privileged accounts intended for use by field
service technicians or the vendor's maintenance programmers.
Historically, back doors have often lurked in systems longer than
anyone expected or planned, and a few have become widely known.
The infamous RTM
worm of late 1988, for example, used a back door
in the BSD
UNIX `sendmail(8)' utility.
Ken Thompson's 1983 Turing Award lecture to the ACM revealed the
existence of a back door in early UNIX versions that may have
qualified as the most fiendishly clever security hack of all time.
The C compiler contained code that would recognize when the
`login' command was being recompiled and insert some code
recognizing a password chosen by Thompson, giving him entry to the
system whether or not an account had been created for him.
Normally such a back door could be removed by removing it from the
source code for the compiler and recompiling the compiler. But to
recompile the compiler, you have to *use* the compiler --- so
Thompson also arranged that the compiler would *recognize when
it was compiling a version of itself*, and insert into the
recompiled compiler the code to insert into the recompiled `login'
the code to allow Thompson entry --- and, of course, the code to
recognize itself and do the whole thing again the next time around!
And having done this once, he was then able to recompile the
compiler from the original sources, leaving his back door in place
and active but with no trace in the sources.
The talk that revealed this truly moby hack was published as
"Reflections on Trusting Trust", `Communications of the
ACM 27', 8 (August 1984), pp. 761--763.
Syn. trap door
; may also be called a `wormhole'. See also
, logic bomb