What type of malware often inserts backdoors into legitimate programs?

The type of malware that often inserts backdoors into legitimate programs is Trojans. Trojans are designed to appear legitimate or harmless, leading users to execute them, believing they are safe applications. Once installed, they can create backdoors in the system, allowing unauthorized access to attackers. This functionality can persist even after the apparent host program has been removed, making Trojans particularly dangerous.

In contrast, worms replicate themselves and spread across networks without injecting backdoors into legitimate software. Ransomware encrypts files or locks users out of their systems until a ransom is paid, but it typically does not create hidden access points in other software. Adware primarily generates unwanted advertisements and is focused on displaying ads rather than compromising system security through backdoors.