Everyone who’s familiar with operating systems theoretical structure, whether he attented a college course or he has just read a book on this subject, knows the concept of a system call i.e. how a user space application talks with the kernel asking it to perform various jobs such as opening a file, creating a memory mapped region, etc.
While studying Windows internals for my job, I had to deepen my knowledge of executable loading process, including their memory layout, address relocations and so on.
I came accross the PEB ( process environment block ), a data structure ( mostly undocumented ) that NT systems use internally to handle many aspects of a process, including a list of loaded libraries, environment variables, command line arguments, heap informations, TLS slots and so on.
The interesting fact about the PEB is that can be inspected to obtain those informations without the use of any standard API, thus resulting in an interesting technique to detect bad written virtual machines, emulators and any sort of sandboxing that could be used by a malware analyst or an anti virus product.
Moreover you could check the PEB to detect if a DLL has been injected into your process to perform api hooking.
API hooking softwares usually hooks API such as EnumProcessModules and patch them to hide the presence of the injected module. Inspecting the PEB you will be able to perform the same task only analyzing your address space, thus avoiding API patching.