37 lines
2.3 KiB
Plaintext
37 lines
2.3 KiB
Plaintext
back door
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n. [common] A hole in the security of a system deliberately left in place by
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designers or maintainers. The motivation for such holes is not always
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sinister; some operating systems, for example, come out of the box with
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privileged accounts intended for use by field service technicians or the
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vendor's maintenance programmers. Syn. trap door ; may also be called a
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wormhole. See also iron box , cracker , worm , logic bomb. Historically,
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back doors have often lurked in systems longer than anyone expected or
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planned, and a few have become widely known. Ken Thompson's 1983 Turing
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Award lecture to the ACM admitted the existence of a back door in early Unix
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versions that may have qualified as the most fiendishly clever security hack
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of all time. In this scheme, the C compiler contained code that would
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recognize when the login command was being recompiled and insert some code
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recognizing a password chosen by Thompson, giving him entry to the system
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whether or not an account had been created for him. Normally such a back
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door could be removed by removing it from the source code for the compiler
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and recompiling the compiler. But to recompile the compiler, you have to use
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the compiler so Thompson also arranged that the compiler would recognize
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when it was compiling a version of itself , and insert into the recompiled
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compiler the code to insert into the recompiled login the code to allow
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Thompson entry and, of course, the code to recognize itself and do the whole
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thing again the next time around! And having done this once, he was then
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able to recompile the compiler from the original sources; the hack
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perpetuated itself invisibly, leaving the back door in place and active but
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with no trace in the sources. The Turing lecture that reported this truly
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moby hack was later published as Reflections on Trusting Trust ,
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Communications of the ACM 27 , 8 (August 1984), pp. 761--763 (text available
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at http://www.acm.org/classics/ ). Ken Thompson has since confirmed that
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this hack was implemented and that the Trojan Horse code did appear in the
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login binary of a Unix Support group machine. Ken says the crocked compiler
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was never distributed. Your editor has heard two separate reports that
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suggest that the crocked login did make it out of Bell Labs, notably to BBN,
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and that it enabled at least one late-night login across the network by
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someone using the login name kt.
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