[common] A hole in the security of a system deliberately left in
place by designers or maintainers. The motivation for such holes 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. Syn.
trap door; may also be called a wormhole. See also
iron box, cracker,
worm, logic bomb.
Historically, back doors have often lurked in systems longer than
anyone expected or planned, and a few have become widely known. Ken
Thompson's 1983 Turing Award lecture to the ACM admitted the existence of a
back door in early Unix versions that may have qualified as the most
fiendishly clever security hack of all time. In this scheme, 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; the hack
perpetuated itself invisibly, leaving the back door in place and active but
with no trace in the sources.
The Turing lecture that reported this truly moby hack was later
published as “Reflections on Trusting Trust”,
Communications of the ACM 27, 8 (August 1984),
pp. 761--763 (text available at http://www.acm.org/classics/).
Ken Thompson has since confirmed that this hack was implemented and that
the Trojan Horse code did appear in the login binary of a Unix Support
group machine. Ken says the crocked compiler was never distributed. Your
editor has heard two separate reports that suggest that the crocked login
did make it out of Bell Labs, notably to BBN, and that it enabled at least
one late-night login across the network by someone using the login name
“kt”.
