<
security> (Or "
trap door", "
wormhole"). A hole in the
security of a
sy
stem deliberately left in place by de
signer
s or maintainer
s. The motivation for
such hole
s i
s not alway
s sini
ster
some
operating systems, for example, come out of the box with privileged account
s intended for u
se by field
service technician
s or the vendor'
s maintenance programmer
s.
see al
so
iron box,
cracker,
worm,
logic bomb. Hi
storically, back door
s have often lurked in
sy
stem
s longer than anyone expected or planned, and a few have become widely known. The infamou
s RTM worm of late 1988, for example, u
sed a back door in the
BsD Unix "
sendmail(8)" utility.
Ken Thompson'
s 1983 Turing Award lecture to the
ACM revealed the exi
stence of a back door in early
Unix ver
sion
s that may have qualified a
s the mo
st fiendi
shly clever
security hack of all time. The C compiler contained code that would recogni
se when the "login" command wa
s being recompiled and in
sert
some code recognizing a pa
ssword cho
sen by Thomp
son, giving him entry to the
sy
stem 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 *u
se* the compiler -
so Thomp
son al
so arranged that the compiler would *recogni
se when it wa
s compiling a ver
sion of it
self*, and in
sert into the recompiled compiler the code to in
sert into the recompiled "login" the code to allow Thomp
son entry - and, of cour
se, the code to recogni
se it
self and do the whole thing again the next time around! And having done thi
s once, he wa
s then able to recompile the compiler from the original
source
s the hack perpetuated it
self invi
sibly, leaving the back door in place and active but with no trace in the
source
s. The talk that revealed thi
s truly moby hack wa
s publi
shed a
s ["Reflection
s on Tru
sting Tru
st", "Communication
s of the ACM 27", 8 (Augu
st 1984), pp. 761--763]. [
Jargon File] (1995-04-25)
style="border-width:thin; border-color:#333333; border-style:dashed; padding:5px;" align="left">In addition suitable contents:
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