back door

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back door

(Or "trap door", "wormhole"). 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. 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. 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 recognise 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 *recognise 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 recognise 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 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].

back door

A secret way to take control of a computer. Also called "trap doors," back doors are built into software by the original programmer, who can gain access to the computer by entering a code locally or remotely. For example, a back door in an application would enable a person to activate either normal or hidden functions within the software. A back door in an operating system would provide access to all system functions in the computer. See Easter Egg and Back Orifice.
References in periodicals archive ?
Seconds earlier he had noticed five men through the kitchen window and opened the back door to be confronted by the gang.
The front door is always locked and you couldn't see from the gate in the garden that the back door was open.
The national carrier operates some of its flights with only one entry- exit point since the back door has been rendered useless due to staff shortage.
Strange as it sounds, probably coming through the back door is slightly easier in that you get up a momentum that can be very hard to stop," he said.
Then, they attempted to break through the back door, which turned out to be the wrong back door, belonging to another business.
But when she came downstairs the following morning she discovered Kevin Carl Jones, 33, had pulled the back door off its hinges.
The back door of the victim's Washington home went up in flames, causing PS27,000 of damage.
The raider ran out of the back door into the garden and down an alleyway to the side of the house with the angry occupant in hot pursuit.
At least three men, one armed with a machete, went to the back door of a home on Durham Road in Gateshead and spoke to the occupant.
He said, "the era of packing the Parliament through the back door by using the defunct Article 58 (2) (b) is over for all times and no back doors and side doors will be allowed to be reopened for sending the elected Parliaments home.
However, delegates clearly felt any suspension was the thin end of the wedge and an attempt by the Tory-led coalition to deregulate Sunday trading by the back door.
Talking to the BBC in Srinagar, Ali Geelani expressed serious concern over the back door negotiation between Pakistan and India, saying that the government of Pakistan has not informed him formally regarding the talks.