2014-04-26 10:52:28 -04:00
|
|
|
back door
|
|
|
|
|
|
2014-04-26 11:54:15 -04:00
|
|
|
n. [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.
|
|
|
|
|
|
