164 lines
8.1 KiB
Markdown
164 lines
8.1 KiB
Markdown
# Analysing Malicious Windows Programs
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## The Windows API
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##### Types and Hungarian Notation
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`DWORD` - 32-bit unsigned integer
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`WORD` - 16-bit unsigned integer
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Hungarian notation is where variables are prefixed with their data type e.g. `dwSize` has prefix `dw` for `DWORD` indicating it is a 32-bit unsigned int
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| Type and Prefix | Description |
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| ------------------- | ------------------------------------------------------------ |
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| `WORD` (`w`) | A 16-bit unsigned value |
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| `DWORD` (`dw`) | A double word, 32-bit unsigned value |
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| Handles (`H`) | A reference to an object. The information stored in the handle is not documented, and the handle should be manipulated only by the Windows API |
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| Long Pointer (`LP`) | A pointer to another type e.g. `LPByte` is a pointer to a byte. Strings are usually prefixed with `LP` because they are actually pointers. |
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| Callback | Represents a function that will be called by the Windows API |
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##### Handles
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*Handles* are items that have been opened or created in the OS, such as a window, process, module, menu, file etc.
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- Handles are like pointers in that they refer to an object or memory location
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- Unlike pointers handles cannot be used in arithmetic operations
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- The only use case is storing it and using it later in a function call
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##### File System Functions
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Most malware will interact with the system by creating or modifying files. Microsoft provides several functions for accessing the file system:
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- `CreateFile` - used to create and open files. It can open existing files, pipes, streams and I/O devices.
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- `ReadFile` and `WriteFile` - used for reading and writing to the contents of files. Both operate on files as a stream.
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- `CreateFileMapping` and `MapViewOfFile` - *File mappings* are commonly used by malware writers because they allow a file to be loaded into memory and manipulated easily.
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- `CreateFileMapping` loads a file from disk into memory
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- `MapViewOfFile` returns a pointer to the base address of the mapping, this can be used to access the file in memory
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##### Special Files
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Windows has a number of file types that can be accessed much like regular files, but that are not accessed by their drive letter and folder (like `C:\docs`)
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###### Shared Files
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Shared files are special files with names that start with `\\serverName\share` or `\\?\serverName\share`
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- They access directories or files in a shared folder stored on a network.
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- `\\?\` prefix tells the OS to disable all string parsing and allows access to longer filenames
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###### Files Accessible via Namespaces
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*Namespaces* can be thought of as a fixed number of folders, each storing different types of objects
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- The lowest level namespace is `NT` with the prefix `\.`
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- The `NT` namespace has access to all devices, and all other namespaces exist within the `NT` namespace
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The `Win32` device namespace (prefix `\\.\`) is often used to access physical devices directly and read/write to them like a file.
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- `\\.\PhysicalDisk1` to directly access the disk while ignoring its file system
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- By doing this malware can read and write data to an unallocated sector in the drive without creating a file
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- This is very good for avoiding detection
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###### Alternate Data Streams
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ADS allows additional data to be added to an existing file within `NTFS`
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- The extra data doesn’t show up in a directory listing nor when displaying the contents of the file
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- It’s only visible when accessing the stream
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- ADS data is named `normalFile.txt:Stream:$DATA`
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## The Windows Registry
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The *Windows registry* is used to store OS and program configuration information, such as settings and options.
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In early versions of Windows the registry was just a hierarchy of `.ini` files to improve performance.
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Malware often uses the registry for *persistence* or configuration data. The malware adds entries into the registry that will allow it to run automatically when the computer boots.
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- **Root key** - The registry is divided into five top-level sections called *root keys* (sometimes called `HKEY`)
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- **Subkey** - Akin to a subfolder within a folder
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- **Key** - A key is a folder in the registry that can contain additional folders or values
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- The root key and subkey are both keys
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- **Value entry** - A *value entry* is an ordered pair with a name and value
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- **Value or data** - The data stored in a registry entry
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#### Registry Root Keys
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- `HKEY_LOCAL_MACHINE` (`HKLM`) - Stores settings that are global to the local machine
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- Contains ` HKEY_LOCAL_MACHINE\ SOFTWARE\Microsoft\Windows\CurrentVersion\Run`
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- This is the key that stores a list of executables that are run at start up
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- `HKEY_CURRENT_USER` (`HKCU`) - Stores settings specific to the current user
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- This is a virtual key, stored in `HKEY_USERS\SID`
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- Where `SID` is the security identifier of the user currently logged in
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- `HKEY_CLASSES ROOT` - Stores information defining types
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- `HKEY_CURRENT_CONFIG` - Stores settings about the current hardware configuration, specifically differences between the current and standard configuration
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- `HKEY_USERS` - Defines settings for the default user, new user and current user
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##### Common Registry Functions
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- `RegOpenKeyEx` - Opens a registry for editing and querying
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- `RegSetValueEx` - Adds a new value to the registry and sets its data
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- `RegGetValue` - Returns the data for a value entry in the registry
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You can use RegEdit to view and edit the registry.
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### Networking APIs
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## Following Malware Execution
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#### DLLs
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To store malicious code:
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- Malware often uses a `dll` to load itself into another process
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- This is because one process can only contain one `.exe`
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By using Windows `dll`s:
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- Windows dlls contain the functionality to interact with the OS
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- Looking at what dlls are used can help find the functionality of the malware
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By using third-party `dll`s
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- This can provide further insight to what the malware does
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- e.g. if it uses a Mozilla `dll` instead of the standard Windows API, it might be using functions not found in the Windows API such as encryption
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`DLL`s are similar to `EXE`s, there’s a flag in the PE to indicate the file is a dll.
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#### Processes
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- Malware can execute outside the current program by creating a new process or modifying an existing one.
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- A process is a program being executed by Windows
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- Each process manages its own resources such as open handles and memory
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- A process contains one or more threads that are executed by the CPU.
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- `CreateProcess` can be used to create a new process
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#### Threads
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Processes are the container for execution, but *threads* are what the Windows OS executes.
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- Threads are independent sequences of instructions that are executed by the CPU without waiting for other threads
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- A process contains one or more threads, which execute part of the code within a process.
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- Threads within a process all share a memory space but have separate registers and stacks
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`CreateThread` can be used to create new threads
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1. Malware can use `CreateThread` to load a new malicious library into a process with `CreateThread` called and the address of `LoadLibrary` as the start address
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2. Malware can create two new threads: one to listen on a socket or port and then output that to standard input of a process, and the other to read from standard output and send that to a socket.
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#### Services
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Another way for malware to execute additional code is by installing it as a *service*.
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- Windows allows tasks to run without their own processes or threads by using services that run as background applications
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- Code is scheduled and run by the Windows service manager without user input.
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- Services are normally run as `SYSTEM` or another privileged account
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- Key service functions:
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- `OpenSCManager` Returns a handle to the service control manager
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- `CreateService` - Adds a new service to the service control manager
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- Allows the caller to specify whether the service will start automatically at boot time or be started manually
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- `StartService` Starts the service, only used if service needs to be started manually
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