求助：CAsyncSocket类套接字接收十六进制数据流出错

以下是服务器接收十六进制数据的函数：
void CTcpServer1Dlg::OnReceive()
{
    char* buff=new char[1000];
    int nReceived;
    CString strReceived,str1,str2,str3;
    int nFlag=0;
    nReceived=m_sConnectSocket.Receive(buff,1000);
    do
    {
       if(nReceived==0)
       {
            nFlag=0;
        }
       else if(nReceived==NULL)
       {
            AfxMessageBox("接收数据出错！",MB_OK|MB_ICONSTOP);
            return;
       }
       else if(nReceived>1000||nReceived<0)
       {
            AfxMessageBox("接收数据出错！",MB_OK|MB_ICONSTOP);
            return;
       }
       else
       {
        for(int i=0;i<nReceived;i++)
        {
            str1.Format("%02X ",buff[i]);
            str2+=str1;
        }
        nFlag=1;
       }
     }while(nFlag==0);
    str3.Format("From Client: %s",str2);
    m_listRecvData.AddString(str3);//接收数据列表框显示数据
    UpdateData(FALSE);
}
编译后显示数据如下：
00
00 06 00 FFFFFF1B 0B 1F 15 00 FFFFFF5B 00 07 2B 3B EB
正常情况不应该出现“FFFFFF”这样的字符串，请问各位高手，这是怎么回事？应该怎样改？