Additional information (me)
Memory only software / malware
This is a program that once loaded into memory stays there and can be accessed or reactivated whenever necessary without having to load another copy from disk. This type of software doesn't make any changes to the file-system of the registry and will be terminated after a reboot or memory-clean-up operation.
Memory-only malware generally does not bring many concerns with it, but that is not completely justified: system administrators should be complacent about memory-scraping malware. A reboot to restore solution offers the perfect remedy because it clears the malware out of memory and enables the PC to be reverted to a safe disk state. If users are fearful of reinfection, then custom configurations and reboot schedules can be set up to ensure that endpoints are restored and hardened against threats, because although not resident on the system: it can do a lot of harm and gather a lot of information while running from the memory (only).
Memory resident software / malware
This is a program that once loaded into memory stays there and can be accessed or reactivated whenever necessary without having to load another copy from disk. Normally, a computer does not have enough memory to hold all the programs you use. When you want to run a program, therefore, the operating system is obliged to free some memory by copying data or programs from main memory to a disk. This process is known as swapping. Certain programs can be marked as being memory resident, which means that the operating system is not permitted to swap them out to a storage device, they will always remain in memory.
Memory-resident malware is a type of fileless malware. It consists of malicious software that's stored in a computer's random access memory (RAM). It doesn't consist of any files. This behaviour leaves very few signs of infection, making it difficult for traditional tools and non-experts to identify. Where a system restart is performed, most memory-resident malware has the ability to re-execute itself in memory using a number of mechanisms such as registry entries. In instances where multiple domain controller systems are infected, re-execution after a system restart might not be necessary at all. To detect memory-resident malware, it is essential that traditional antivirus is supplemented by technologies that facilitate volatile system memory (RAM) capture and continuous behavioural monitoring, using f.e. Machine Learning techniques and A.I.
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