back door

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back door

(security)
(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, Back Orifice and one-way hash function.
References in classic literature ?
Three days later, however, the nurse and child only just escaped from the back door as you rushed in at the front one.
A few score yards down New Gaunt Street, and leading into Gaunt Mews indeed, is a little modest back door, which you would not remark from that of any of the other stables.
The affectionate kindness of the whole circle hustled him into his greatcoat, and shoved him out into the moonlight, as if he were a fugitive traitor with whom they sympathised, and a troop of horse were at the back door.
Having taken last night's milk from the ice-box, she went to the back door, and, having filled James's saucer, stood on the grass beside it, sniffing the morning air.
Nancy had seen her slip out with a letter in her hand, and stop the butcher's man who had just been delivering some meat at the back door.
Linton walked out by the back door into the yard, and from thence to the front entrance.
There's always a modus viviendi," Penfentenyou murmured, and tiptoed along the hall to a back door, which he opened quite silently.
Back doors of some Air India aircraft are non- functional due to shortage of cabin crew Commander of Mumbai- Delhi flight refuses to fly with only three hostesses on board Staff shortage delayed as many as 16 flights for hours at Delhi airport on Monday
Henderson, whose research created Open the Gates to the Ivy League, the ten best college back doors, including the primary reasons, are:
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.
The bungalows at Walker Road, Barry, have no back doors and the pensioners fear that should there be a fire in the front of their properties they would be trapped.
Back doors of home smashed and electrical items stolen.