Trojan horse

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A subversive group or device placed within enemy ranks.
• noun The hollow wooden horse in which, according to legend, Greeks hid and gained entrance to Troy, later opening the gates to their army.
• noun Computers A program that appears to be legitimate but is designed to have destructive effects, as to data residing in the computer onto which the program was loaded.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Classical mythology) a large hollow wooden horse built by Greek soldiers besieging Troy during the Trojan War, and left as a “gift” when they pretended to abandon their seige. It was taken into the city by the Trojans, and Greek soldiers concealed inside came out and opened the gates to the city, enabling the capture of the city by the Greeks.
• noun Hence, any thing or person which appears harmless but is designed to destroy or attack from within. It may sometimes refer to a group; -- see also fifth column.
• noun (Computers) A computer program designed to evade the security precautions within a computer system and perform illicit operations, or to do malicious damage, and often designed to look like a different kind of program, such as a game, archiver, or directory lister. This term is not applied to a program that replicates itself, such as a virus.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a hollow wooden horse by which the Greeks gained access to Ilium or Troy.
• noun by extension a subversive person or device placed within the ranks of the enemy
• noun computing a malicious program that is disguised as legitimate software
• noun business an offer made to lure customers, seeming like a good deal, that has the ultimate effect of extorting large amounts of money from the customer
• noun politics a person, organization, social movement, legislation, or ideology with a negative agenda or evil intentions under the guise of positive values or good intentions