Executables are just text files that are interpreted by the operating system, so what is preventing viruses to just insert malicious text into executables files that will do malicious actions once interpreted ? Are executables obfuscated so we can't find a place that we will be sure will be executed once the executable is running ?
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Executables are not text files. Apart from that, modifying executables is exactly what many viruses do or did. Nowadays, computers are better protected, for instance in DOS and older versions of Windows, you could just modify executables. In Windows Vista and up, most programs are inside the program files folder, and a normal user cannot modify files in those folders, until they acquired elevated rights. A malicious program ran by the user would also not be able to access those folders, unless it would know of a security breach in this protection system.