the.com/trojan horse

means Something that looks like a harmless or welcome gift but secretly conceals a hidden threat or hostile intent — used today especially for malware disguised as legitimate software.

from Straight out of Greek legend: unable to breach Troy's walls after years of siege, the Greeks built a giant hollow wooden horse, hid soldiers inside it, and left it as a supposed peace offering. The Trojans hauled it inside their own walls, and that night the hidden men slipped out and opened the gates to the waiting army. The tale comes down mainly through Virgil's Aeneid rather than Homer's Iliad, which ends before the horse appears. The phrase entered English as a byword for 'enemy within,' and computing simply borrowed it wholesale in the 20th century for programs that smuggle in malicious code under the guise of something useful.