Creating a DACL

Creating a proper discretionary access control list (DACL) is a necessary and important part of application development. Because a NULL DACL permits all types of access to all users, do not use NULL DACLs.

The following example shows how to properly create a DACL. The example contains a function, CreateMyDACL, that uses the security descriptor definition language (SDDL) to define the granted and denied access control in a DACL. To provide different access for your application's objects, modify the CreateMyDACL function as needed.

In the example:

1. The main function passes an address of a SECURITY_ATTRIBUTES structure to the CreateMyDACL function.

2. The CreateMyDACL function uses SDDL strings to:

   • Deny access to guest and anonymous logon users.
   • Allow read/write/execute access to authenticated users.
   • Allow full control to administrators.

   For more information about the SDDL string formats, see Security Descriptor String Format.

3. The CreateMyDACL function calls the ConvertStringSecurityDescriptorToSecurityDescriptor function to convert the SDDL strings to a security descriptor. The security descriptor is pointed to by the lpSecurityDescriptor member of the SECURITY_ATTRIBUTES structure. CreateMyDACL sends the return value from ConvertStringSecurityDescriptorToSecurityDescriptor to the main function.

4. The main function uses the updated SECURITY_ATTRIBUTES structure to specify the DACL for a new folder that is created by the CreateDirectory function.

5. When the main function is finished using the SECURITY_ATTRIBUTES structure, the main function frees the memory allocated for the lpSecurityDescriptor member by calling the LocalFree function.

#define _WIN32_WINNT 0x0500

#include <windows.h>
#include <sddl.h>
#include <stdio.h>

#pragma comment(lib, "advapi32.lib")

BOOL CreateMyDACL(SECURITY_ATTRIBUTES *);

void main()
{
     SECURITY_ATTRIBUTES  sa;
      
     sa.nLength = sizeof(SECURITY_ATTRIBUTES);
     sa.bInheritHandle = FALSE;  

     // Call function to set the DACL. The DACL
     // is set in the SECURITY_ATTRIBUTES 
     // lpSecurityDescriptor member.
     if (!CreateMyDACL(&sa))
     {
         // Error encountered; generate message and exit.
         printf("Failed CreateMyDACL\n");
         exit(1);
     }

     // Use the updated SECURITY_ATTRIBUTES to specify
     // security attributes for securable objects.
     // This example uses security attributes during
     // creation of a new directory.
     if (0 == CreateDirectory(TEXT("C:\\MyFolder"), &sa))
     {
         // Error encountered; generate message and exit.
         printf("Failed CreateDirectory\n");
         exit(1);
     }

     // Free the memory allocated for the SECURITY_DESCRIPTOR.
     if (NULL != LocalFree(sa.lpSecurityDescriptor))
     {
         // Error encountered; generate message and exit.
         printf("Failed LocalFree\n");
         exit(1);
     }
}


// CreateMyDACL.
//    Create a security descriptor that contains the DACL 
//    you want.
//    This function uses SDDL to make Deny and Allow ACEs.
//
// Parameter:
//    SECURITY_ATTRIBUTES * pSA
//    Pointer to a SECURITY_ATTRIBUTES structure. It is your
//    responsibility to properly initialize the 
//    structure and to free the structure's 
//    lpSecurityDescriptor member when you have
//    finished using it. To free the structure's 
//    lpSecurityDescriptor member, call the 
//    LocalFree function.
// 
// Return value:
//    FALSE if the address to the structure is NULL. 
//    Otherwise, this function returns the value from the
//    ConvertStringSecurityDescriptorToSecurityDescriptor 
//    function.
BOOL CreateMyDACL(SECURITY_ATTRIBUTES * pSA)
{
     // Define the SDDL for the DACL. This example sets 
     // the following access:
     //     Built-in guests are denied all access.
     //     Anonymous logon is denied all access.
     //     Authenticated users are allowed 
     //     read/write/execute access.
     //     Administrators are allowed full control.
     // Modify these values as needed to generate the proper
     // DACL for your application. 
     TCHAR * szSD = TEXT("D:")       // Discretionary ACL
        TEXT("(D;OICI;GA;;;BG)")     // Deny access to 
                                     // built-in guests
        TEXT("(D;OICI;GA;;;AN)")     // Deny access to 
                                     // anonymous logon
        TEXT("(A;OICI;GRGWGX;;;AU)") // Allow 
                                     // read/write/execute 
                                     // to authenticated 
                                     // users
        TEXT("(A;OICI;GA;;;BA)");    // Allow full control 
                                     // to administrators

    if (NULL == pSA)
        return FALSE;

     return ConvertStringSecurityDescriptorToSecurityDescriptor(
                szSD,
                SDDL_REVISION_1,
                &(pSA->lpSecurityDescriptor),
                NULL);
}