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 ?
You may have felt safe when you signed up for Internet services or bought stuff online, but those online vendors have big back doors just waiting for us to walk through.
Yet, when a resident dies in a nursing home, typically the hallways are cleared and in an almost secretive way, the body is taken out the back doors to a waiting hearse, as if it were something to be ashamed of.
In addition to the expanded wiretapping provisions, near-future proposals have included the criminalization of unbreakable encryption, the reintroduction of the infamous Clipper Chip (a government-sanctioned, NSA-sponsored encryption scheme that would contain a back door for law-enforcement decoding), publically unaccountable funding for Digital Telephony (which would give the FBI authority over the design of telecom networks to ensure effective surveillance capabilities), and even a constitutional amendment to criminalize "bomb-making" information on the Internet (information that is both legal and available on the printed page).
Susie locks her back door and slams the screen door to make sure it latches.
Spooks and law-enforcement officials say this back door is needed so they can monitor the communications of terrorists, drag runners, and wire-fraud artists.
The couple retained an attorney, who filed a lawsuit seeking a court's permission for the couple to use their own back door.
She said: "I couldn't believe it, I walked back into the house and told Sonny there was a car at the back door and two bodies.
The immediate purpose of the back door in the My Door virus appears simply to use the infected computer to further spread the virus.
Bell, a 12-month-old collie/labrador cross took one look at the sinister figure lurking at the back door and let rip with everything in her canine armoury.
on April 29th, former police officer Jose Vazquez of East Chicago, Indiana, was watching television alone at the campaign headquarters of his favored mayoral candidate when someone knocked at the back door.
After the gunman leaped over the counter, his pistol fired, hitting Gram in the back, and he ran out the back door without taking any money.
When it rains, we can't get out of the back door because of a broken gutter, which is like a waterfall.