Analyze Windows API Call

Hi Analyst,

Today I would like to share information on analyzing the API call to gather detailed information. Malware analyst requires understanding each API call that affects the operating system to have the details of malware’s behavior being analyzed.

When we analyze the import table from a malware sample, We understand that all the API in the import table would be called at least once from the code segment. For example below, I would like to know the details of WriteFile call

to make the analysis easier, We can use an application called API Monitor, which you can download from

Once you have installed it, you can start the application API Monitor with administrator privilege. The first thing you need to do is to set what API you want to monitor

You need to check mark the API that you want to monitor

The following steps are to set the binary that you want to monitor when it is executed. Follow the steps below

When you click the button OK, the application will directly be executed and monitored for the API call. You can see the details below

So from the above details, we can quickly analyze each malware call that might impact the environment. We can also see each parameter that was passed to the API so that we can also see the content that has been written to the file.

The analyst is able to see the details of parameter being passed to the WriteFile API. as shown above as below

BOOL WriteFile(
  HANDLE       hFile,
  LPCVOID      lpBuffer,
  DWORD        nNumberOfBytesToWrite,
  LPDWORD      lpNumberOfBytesWritten,
  LPOVERLAPPED lpOverlapped


A handle to the file or I/O device (for example, a file, file stream, physical disk, volume, console buffer, tape drive, socket, communications resource, mailslot, or pipe).

The hFile parameter must have been created with the write access. For more information, see Generic Access Rights and File Security and Access Rights.

For asynchronous write operations, hFile can be any handle opened with the CreateFile function using the FILE_FLAG_OVERLAPPED flag or a socket handle returned by the socket or accept function.


A pointer to the buffer containing the data to be written to the file or device.

This buffer must remain valid for the duration of the write operation. The caller must not use this buffer until the write operation is completed.


The number of bytes to be written to the file or device.

A value of zero specifies a null write operation. The behavior of a null write operation depends on the underlying file system or communications technology.

Windows Server 2003 and Windows XP:  Pipe write operations across a network are limited in size per write. The amount varies per platform. For x86 platforms it’s 63.97 MB. For x64 platforms it’s 31.97 MB. For Itanium it’s 63.95 MB. For more information regarding pipes, see the Remarks section.


A pointer to the variable that receives the number of bytes written when using a synchronous hFile parameter. WriteFile sets this value to zero before doing any work or error checking. Use NULL for this parameter if this is an asynchronous operation to avoid potentially erroneous results.

This parameter can be NULL only when the lpOverlapped parameter is not NULL.

For more information, see the Remarks section.


A pointer to an OVERLAPPED structure is required if the hFile parameter was opened with FILE_FLAG_OVERLAPPED, otherwise this parameter can be NULL.

For an hFile that supports byte offsets, if you use this parameter you must specify a byte offset at which to start writing to the file or device. This offset is specified by setting the Offset and OffsetHigh members of the OVERLAPPED structure. For an hFile that does not support byte offsets, Offset and OffsetHigh are ignored.

To write to the end of file, specify both the Offset and OffsetHigh members of the OVERLAPPED structure as 0xFFFFFFFF. This is functionally equivalent to previously calling the CreateFile function to open hFile using FILE_APPEND_DATA access.

Return value

If the function succeeds, the return value is nonzero (TRUE).

If the function fails, or is completing asynchronously, the return value is zero (FALSE). To get extended error information, call the GetLastError function.

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