SMB / Named Pipes

Created the Sunday 27 August 2023. Updated 3 weeks, 6 days ago.

Named Pipes are a feature of the Windows operating system that allow for inter-process communication (IPC) using a pipe metaphor. These are particularly useful in networking scenarios as they can be made accessible over a network and facilitate a client-server model of communication. The SMB (Server Message Block) protocol is commonly used in Windows environments for shared access to files, printers, and other network services.

When malware uses Named Pipes over the SMB protocol for network evasion, it is essentially wrapping its communication in layers of legitimate-looking traffic. Named Pipes can be set up to pass data packets that may appear harmless or even normal to network monitoring tools. This makes detection harder because the malicious traffic is mixed with regular, authorized network activity.

Technique Identifier

U9011

Technique Tags

Named Pipes SMB Network Pivot Evasion

Featured Windows API's

Below, you will find a list of the most commonly used Windows API's that are currently utilized by malware authors for current evasion technique. This list is meant to provide an overview of the API's that are commonly used for this purpose. If there are any API's that you feel should be included on this list, please do not hesitate to contact us. We will be happy to update the list and provide any additional information or documentation that may be helpful.

Code Snippets

Description

This code snippet demonstrates how to use Named Pipes in .NET for sending commands and receiving their results. In this example, the named pipe client requests a list of running processes from the named pipe server, which then returns the requested list. It's important to note that this snippet combines both the client and server code for demonstration purposes. In a real-world scenario, the client and server should be separated into two distinct applications and preferably run on two remote systems.

// This example demonstrates how to use named pipes to route commands to a server and retrieve the corresponding responses.

using System.Diagnostics;
using System.IO.Pipes;
using System.Text;

class Program
{
    public enum Command
    {
        ProcessList,
        Exit,
    }

    const string pipeName = "NamedPipeExample";

    public static void Main(string[] args)
    {               
        Thread namedPipeServerThread = new(() =>
        {
            try
            {
                // https://learn.microsoft.com/en-us/dotnet/api/system.io.pipes.namedpipeserverstream?view=net-7.0?WT_mc_id=SEC-MVP-5005282
                using NamedPipeServerStream serverPipe = new(pipeName, PipeDirection.InOut);

                serverPipe.WaitForConnection();

                using StreamReader reader = new(serverPipe);
                using StreamWriter writer = new(serverPipe) { AutoFlush = true };
                ///

                while (true)
                {
                    switch(Enum.Parse(typeof(Command), reader.ReadLine() ?? ""))
                    {
                        case Command.ProcessList:
                            {
                                StringBuilder sb = new();

                                foreach (Process process in Process.GetProcesses())                                
                                    sb.AppendLine($"({process.Id.ToString().PadRight(5, ' ')}){process.ProcessName}");
                                
                                // Encode as Base64 to send to whole list in one single `WriteLine`
                                writer.WriteLine(Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(sb.ToString())));

                                break;
                            }
                        default:
                            {
                                // Exit or unknown or empty string.
                                break;
                            }
                    }
                }
            }
            catch { }
        });
        namedPipeServerThread.Start();

        Thread namedPipeClientThread = new(() =>
        {
            try
            {
                // `.` means local machine, it can be replaced by the network computer name hosting a the named pipe server.
                // https://learn.microsoft.com/en-us/dotnet/api/system.io.pipes.namedpipeclientstream?view=net-7.0?WT_mc_id=SEC-MVP-5005282
                using NamedPipeClientStream clientPipe = new(".", pipeName, PipeDirection.InOut);

                clientPipe.Connect();

                using StreamReader reader = new(clientPipe);
                using StreamWriter writer = new(clientPipe) { AutoFlush = true };
                ///

                // Ask server for running process
                writer.WriteLine(Command.ProcessList);

                // Receive response
                string? response = reader.ReadLine();  
                if (response != null)
                    Console.WriteLine(System.Text.Encoding.UTF8.GetString(Convert.FromBase64String(response)));                

                // Tell server, we finished our job
                writer.WriteLine(Command.Exit);
            }
            catch { }
        });
        namedPipeClientThread.Start();

        ///
        namedPipeServerThread.Join();
        namedPipeClientThread.Join();        
    }
}

Description

This code snippet demonstrates how to use named pipes on Windows through the WinAPI. The Proof of Concept (PoC) emphasizes sending Unicode string character by character until the CRLF, with each character taking up 2 bytes. While there are various methods to achieve a similar outcome, this particular approach has its own set of advantages and disadvantages. Feel free to modify the example to suit your specific needs.

// This PoC does not handle exceptions, consider handling exception if used it in production.
program NamedPipes;

uses Winapi.Windows,
     System.SysUtils,
     System.Classes;

const PIPE_NAME           = 'NamedPipeExample';
      SERVER_MACHINE_NAME = '.'; // `.` = Local Machine

var SERVER_LISTENING_EVENT : THandle;

Type
  TCommand = (
    cmdPing,
    cmdPong,
    cmdExit
  );

  TServer = class(TThread)
  protected
    {@M}
    procedure Execute(); override;
  end;

  TClient = class(TThread)
  protected
    {@M}
    procedure Execute(); override;
  end;

(* Local *)

{ _.PIPE_WriteInteger
  Write to named pipe a signed integer (4 bytes), since in our example, named pipe has
  a buffer of 2 bytes, we must split our signed integer to two words }
procedure PIPE_WriteInteger(const hPipe : THandle; const AValue : Integer);
var wLow, wHigh   : Word;
    ABytesWritten : Cardinal;
begin
  wLow  := Word(AValue and $FFFF);
  wHigh := Word(AValue shr 16);
  ///

  WriteFile(hPipe, wLow, SizeOf(Word), ABytesWritten, nil);
  WriteFile(hPipe, wHigh, SizeOf(Word), ABytesWritten, nil);
end;

{ _.PIPE_ReadInteger
 Reconstruct signed integer from two words }
function PIPE_ReadInteger(const hPipe : THandle) : Integer;
var wLow, wHigh : Word;
    dwBytesRead : Cardinal;
begin
  result := -1;
  ///

  ReadFile(hPipe, wLow, SizeOf(Word), dwBytesRead, nil);
  ReadFile(hPipe, wHigh, SizeOf(Word), dwBytesRead, nil);

  ///
  result := wLow or (wHigh shl 16);
end;

{ _.PIPE_WriteLine
Write to NamedPipe and append a CRLF to signify end of buffer }
procedure PIPE_WriteLine(const hPipe : THandle; AMessage : String);
var ABytesWritten : Cardinal;
    i             : Cardinal;
begin
  AMessage := Trim(AMessage) + #13#10;
  ///

  for I := 1 to Length(AMessage) do begin
    // https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-writefile?WT_mc_id=SEC-MVP-5005282
    if not WriteFile(
      hPipe,
      AMessage[I],
      SizeOf(WideChar),
      ABytesWritten,
      nil
    ) then
      break;
  end;
end;

{ _.PIPE_ReadLine
Read NamedPipe Buffer until CRLF is reached }
function PIPE_ReadLine(const hPipe : THandle) : String;
var ABuffer     : WideChar;
    dwBytesRead : Cardinal;
    CR          : Boolean;
    LF          : Boolean;
begin
  result := '';
  ///

  CR := False;
  LF := False;

  while True do begin
    // https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-readfile?WT_mc_id=SEC-MVP-5005282
    if not ReadFile(hPipe, ABuffer, SizeOf(ABuffer), dwBytesRead, nil) then
      break;

    case ABuffer of
      #13 : CR := True;
      #10 : LF := True;
    end;

    if CR and LF then
      break;

    ///
    result := result + ABuffer;
  end;
end;

(* TServer *)

{ TServer.Execute }
procedure TServer.Execute();
var hPipe : THandle;
begin
  hPipe := INVALID_HANDLE_VALUE;
  try
    // https://learn.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-createnamedpipew?WT_mc_id=SEC-MVP-5005282
    hPipe := CreateNamedPipeW(
      PWideChar(Format('\\.\pipe\%s', [PIPE_NAME])),
      PIPE_ACCESS_DUPLEX,
      PIPE_TYPE_MESSAGE or PIPE_READMODE_MESSAGE or PIPE_WAIT,
      1,
      SizeOf(WideChar),
      SizeOf(WideChar),
      NMPWAIT_USE_DEFAULT_WAIT,
      nil
    );

    if hPipe = INVALID_HANDLE_VALUE then
      Exit();

    SetEvent(SERVER_LISTENING_EVENT); // Signal we are listening for named pipe client

    while (not Terminated) do begin
      // https://learn.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-connectnamedpipe?WT_mc_id=SEC-MVP-5005282
      if not ConnectNamedPipe(hPipe, nil) then
        continue;
      try
        while (not Terminated) do begin
          case TCommand(PIPE_ReadInteger(hPipe)) of
            cmdPing : PIPE_WriteLine(hPIpe, Format('Pong: %d', [GetTickCount()]));

            else begin
              WriteLn('Bye!');

              break;
            end;
          end;
        end;

        WriteLn(PIPE_ReadLine(hPipe));
      finally
        // https://learn.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-disconnectnamedpipe?WT_mc_id=SEC-MVP-5005282
        DisconnectNamedPipe(hPipe);
      end;
    end;
  finally
    if hPipe <> INVALID_HANDLE_VALUE then
      // https://learn.microsoft.com/en-us/windows/win32/api/handleapi/nf-handleapi-closehandle?WT_mc_id=SEC-MVP-5005282
      CloseHandle(hPipe);

    ///
    ExitThread(0);
  end;
end;

(* TClient *)

{ TClient.Execute

  An alternative to CreateFileW + WriteFile would be to use:
    - https://learn.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-callnamedpipew?WT_mc_id=SEC-MVP-5005282
}
procedure TClient.Execute();
var hPipe : THandle;
begin
  hPipe := INVALID_HANDLE_VALUE;
  try
    // https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilew?WT_mc_id=SEC-MVP-5005282
    hPipe := CreateFileW(
      PWideChar(Format('\\%s\pipe\%s', [
        SERVER_MACHINE_NAME,
        PIPE_NAME
      ])),
      GENERIC_READ or GENERIC_WRITE,
      0,
      nil,
      OPEN_EXISTING,
      0,
      0
    );

    if hPipe = INVALID_HANDLE_VALUE then
      Exit();

    PIPE_WriteInteger(hPipe, Integer(TCommand.cmdPing));

    WriteLn(PIPE_ReadLine(hPipe));

    PIPE_WriteInteger(hPipe, Integer(TCommand.cmdExit));
  finally
    if hPipe <> INVALID_HANDLE_VALUE then
      // https://learn.microsoft.com/en-us/windows/win32/api/handleapi/nf-handleapi-closehandle?WT_mc_id=SEC-MVP-5005282
      CloseHandle(hPipe);

    ///
    ExitThread(0);
  end;
end;

(* _.EntryPoint *)

var Server : TServer;
    Client : TClient;

begin
  AllocConsole();
  ///

  // Create a event to signal when named pipe server is successfully listening for
  // Namedpipe clients.
  // When event is signaled, we can start our named pipe client thread.
  SERVER_LISTENING_EVENT := CreateEvent(nil, False, False, nil);
  if SERVER_LISTENING_EVENT = 0 then
    Exit();
  try
    // Launch NamedPipe Server
    Server := TServer.Create();

    ///
    WaitForSingleObject(SERVER_LISTENING_EVENT, INFINITE);
  finally
    CloseHandle(SERVER_LISTENING_EVENT);
  end;

  // Launch NamedPipe Client
  Client := TClient.Create();

  // Wait for Threads end
  Client.WaitFor();
  Server.WaitFor();

end.

Contributors

Jean-Pierre LESUEUR

Additional Resources

External Links

The resources provided below are associated links that will give you even more detailed information and research on current evasion technique. It is important to note that, while these resources may be helpful, it is important to exercise caution when following external links. As always, be careful when clicking on links from unknown sources, as they may lead to malicious content.