cat

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Summary

• Needs Admin: False
• Version: 2
• Author: @djhohnstein

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Arguments

• path (String) - Path to the file to read
  • CLI Name: Path
  • Display Name: Path to File
  • Description: File to read

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Usage

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Example 1: Basic File Reading

cat -Path C:\config.txt
cat C:\config.txt

[File contents displayed here...]

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Example 2: Reading with Quoted Paths

cat "C:\Program Files\app\config.txt"
cat 'C:\Users\user\Documents\file with spaces.txt'

[File contents displayed here...]

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Example 3: File Not Found

cat C:\nonexistent.txt

Error: File C:\nonexistent.txt does not exist.

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Example 4: Access Denied

cat C:\Windows\System32\config\SAM

Error: Access denied.

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Example 5: Large File Reading

cat C:\logs\large_log_file.txt

[First 256KB chunk...]
[Second 256KB chunk...]
[Continues until file is completely read...]

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Detailed Summary

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1. Parameter Processing and Validation

[DataContract]
internal struct CatParameters
{
    [DataMember(Name = "path")]
    public string Path;
}

• JSON Structure: Uses CatParameters structure with a single Path field
• Quote Handling: Python parser removes surrounding quotes from file paths
• Path Validation: Checks file existence using File.Exists() before attempting to read
• Error Handling: Returns appropriate error messages for missing files

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2. Asynchronous File Reading Architecture

private static int _chunkSize = 256000;
private byte[] _buffer = new byte[_chunkSize];
private long _bytesRemaining = 0;
private ThreadSafeList<string> _contents = new ThreadSafeList<string>();

• Main Thread: Handles file opening and coordination
• Flush Thread: Periodically sends accumulated content to Mythic (_flushContents action)
• Async Callbacks: File reading occurs through BeginRead/EndRead async pattern
• Thread Synchronization: Uses AutoResetEvent objects for coordination

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3. Chunked Reading Implementation

private void FileReadCallback(IAsyncResult result)
{
    FileStream fs = (FileStream)result.AsyncState;
    fs.EndRead(result);
    try
    {
        _contents.Add(System.Text.Encoding.UTF8.GetString(_buffer));
        _bytesRemaining = fs.Length - fs.Position;
        if (_bytesRemaining > 0 && !_cancellationToken.IsCancellationRequested)
        {
            _buffer = _bytesRemaining > _chunkSize ? new byte[_chunkSize] : new byte[_bytesRemaining];
            fs.BeginRead(_buffer, 0, _buffer.Length, FileReadCallback, fs);
        } else
        {
            _fileRead.Set();
        }
    } catch (Exception ex)
    {
        // Error handling
    }
}

• Fixed Chunk Size: 256,000 bytes (256KB) per chunk
• Dynamic Buffer Sizing: Last chunk uses exact remaining bytes
• UTF-8 Encoding: All content is converted to UTF-8 strings
• Recursive Reading: Each chunk completion triggers the next chunk read
• Progress Tracking: Uses _bytesRemaining to track read progress

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4. Real-Time Output Streaming

_flushContents = new Action(() =>
{
    string output = "";
    while(!_cancellationToken.IsCancellationRequested && !_completed)
    {
        WaitHandle.WaitAny(_timers, 1000);
        output = string.Join("", _contents.Flush());
        SendMessageToMythic(output);
    }
    output = string.Join("", _contents.Flush());
    SendMessageToMythic(output);
});

• Periodic Flushing: Sends output every 1000ms or when content is available
• Thread-Safe Collection: Uses ThreadSafeList<string> for safe access across threads
• Final Flush: Ensures all remaining content is sent after reading completes
• Cancellation Support: Respects cancellation tokens for operation termination

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5. File Opening and Initial Setup

public override void Start()
{
    CatParameters parameters = _jsonSerializer.Deserialize<CatParameters>(_data.Parameters);
    if (!File.Exists(parameters.Path))
    {
        resp = CreateTaskResponse($"File {parameters.Path} does not exist.", true, "error");
    }
    else
    {
        TT.Task.Factory.StartNew(_flushContents, _cancellationToken.Token);
        FileStream fs = null;
        FileInfo finfo = new FileInfo(parameters.Path);
        IMythicMessage[] artifacts = new IMythicMessage[]
        {
            Artifact.FileOpen(finfo.FullName)
        };
        // File reading logic...
    }
}

• Parameter Deserialization: Converts JSON parameters to CatParameters structure
• File Existence Check: Validates file exists before attempting to open
• Flush Thread Startup: Starts the output streaming thread
• FileInfo Creation: Gets file metadata for artifact generation
• Artifact Creation: Generates FileOpen artifact with full file path

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6. Error Handling and Exception Management

try
{
    fs = File.OpenRead(parameters.Path);
    // Reading logic...
}
catch (UnauthorizedAccessException ex)
{
    resp = CreateTaskResponse("Access denied.", true, "error", artifacts);
}
catch (Exception ex)
{
    resp = CreateTaskResponse($"Unable to read {parameters.Path}: {ex.Message}", true, "error", artifacts);
}

• File Not Found: Checked before file opening with File.Exists()
• Access Denied: Handles UnauthorizedAccessException with specific message
• General I/O Errors: Catches all other exceptions with detailed error messages
• Callback Errors: File reading callback includes exception handling for async operations

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7. Memory Management and Buffer Optimization

_bytesRemaining = fs.Length;
if (_bytesRemaining < _buffer.Length)
{
    _buffer = new byte[_bytesRemaining];
}

• Initial Buffer Sizing: Creates appropriately sized buffer for small files
• Dynamic Resizing: Adjusts buffer size for final chunk to avoid over-allocation
• Fixed Chunk Strategy: Uses consistent 256KB chunks for predictable memory usage
• Buffer Reuse: Reuses buffer array for each chunk to minimize allocations

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8. Cancellation and Cleanup

WaitHandle[] _timers = new WaitHandle[]
{
    _complete,
    _cancellationToken.Token.WaitHandle
};

• Token Monitoring: Checks cancellation token throughout the operation
• Early Termination: Can stop file reading mid-process if cancelled
• Resource Cleanup: Properly closes file streams and releases resources
• State Management: Uses completion flags to coordinate shutdown

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9. Artifact Generation and Tracking

FileInfo finfo = new FileInfo(parameters.Path);
IMythicMessage[] artifacts = new IMythicMessage[]
{
    Artifact.FileOpen(finfo.FullName)
};

• File Open Artifact: Records that the specified file was accessed
• Full Path Recording: Uses FileInfo.FullName to get the complete path
• Timeline Integration: Artifacts are timestamped and associated with the task
• Audit Trail: Provides forensic evidence of file access activities

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10. Command Line Parsing Flexibility

async def parse_arguments(self):
    if len(self.command_line) == 0:
        raise Exception("Require file path to retrieve contents for.\n\tUsage: {}".format(CatCommand.help_cmd))
    if self.command_line[0] == "{":
        self.load_args_from_json_string(self.command_line)
    else:
        if self.command_line[0] == '"' and self.command_line[-1] == '"':
            self.command_line = self.command_line[1:-1]
        elif self.command_line[0] == "'" and self.command_line[-1] == "'":
            self.command_line = self.command_line[1:-1]
        self.add_arg("path", self.command_line)

• Quote Removal: Automatically removes surrounding double or single quotes
• JSON Support: Accepts JSON-formatted parameters
• Error Messages: Provides helpful usage information for invalid syntax
• Direct Path Support: Accepts file path directly without parameter flags

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MITRE ATT&CK Mapping

• T1005 - Data from Local System
• T1039 - Data from Network Shared Drive
• T1025 - Data from Removable Media

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Technical Deep Dive

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Asynchronous I/O Implementation

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BeginRead/EndRead Pattern

fs.BeginRead(_buffer, 0, _buffer.Length, FileReadCallback, fs);

• Non-Blocking: Main agent thread is not blocked during file I/O operations
• Scalable: Can handle multiple concurrent file operations
• Responsive: Agent remains responsive to other commands during large file reads
• Resource Efficient: Uses I/O completion ports on Windows for optimal performance

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Callback-Based Processing

• Each completed read triggers the next read operation
• Allows for cancellation between chunks
• Provides progress feedback through the streaming mechanism
• Handles errors gracefully within the async context

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Threading Model

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Thread Roles

1. Main Thread: Handles file opening, validation, and coordination
2. Task Thread: Runs the _flushContents action for output streaming
3. I/O Thread: Handles asynchronous file read callbacks (system-managed)

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Synchronization Primitives

• AutoResetEvent: _complete and _fileRead for coordination
• CancellationToken: For graceful operation termination
• ThreadSafeList: For safe data sharing between threads

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Memory and Performance Characteristics

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Chunk Size Optimization

• Network Efficiency: Reasonable size for network transmission to Mythic
• Memory Usage: Prevents excessive memory consumption for large files
• Responsiveness: Provides regular output updates for user feedback
• I/O Efficiency: Balances between too many small reads and excessive memory usage

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String Handling

_contents.Add(System.Text.Encoding.UTF8.GetString(_buffer));

• UTF-8 Conversion: Assumes all files are UTF-8 encoded or compatible
• Buffer Conversion: Converts entire buffer to string (may include extra bytes for final chunk)
• Thread-Safe Storage: Uses specialized collection for cross-thread access
• Memory Impact: Creates string copies of all file content (memory usage = ~2x file size)

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Error Handling Patterns

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Pre-Read Validation

• File existence checking before resource allocation
• Early return for common error conditions
• Artifact generation even on errors for audit trail

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Runtime Error Handling

• Specific handling for UnauthorizedAccessException
• Generic exception handling for unexpected errors
• Callback-level error handling for async operations

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Resource Cleanup

• Proper disposal of file streams
• Thread coordination for clean shutdown
• Cancellation token respect throughout operation

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APIs Used and Their Purposes

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Security Considerations

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File Access Permissions

1. Access Control: Command respects Windows file permissions and ACLs
2. Privilege Requirements: May require elevated privileges for system files
3. Audit Logging: Generates file access artifacts for forensic tracking
4. Error Disclosure: Error messages may reveal file system structure

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Data Handling

1. Memory Exposure: File contents are stored in managed memory during processing
2. Network Transmission: File contents are transmitted to Mythic server
3. Encoding Assumptions: Assumes UTF-8 encoding which may corrupt binary files
4. Buffer Management: Multiple copies of data exist in memory during processing

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Operational Security

1. File Locking: Uses shared read access, allowing concurrent access by other processes
2. Large Files: May consume significant memory and network bandwidth
3. Detection Risk: File access may be logged by security software
4. Performance Impact: Large file reads may impact system performance

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Defensive Considerations

1. File System Monitoring: File access events may trigger security alerts
2. Data Loss Prevention: May trigger DLP systems if reading sensitive files
3. Behavioral Detection: Pattern of file access may indicate data exfiltration
4. Audit Trail: Leaves artifacts that can be tracked during incident response

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Limitations

1. Binary File Support: Not optimized for binary files due to UTF-8 string conversion
2. Memory Usage: Memory consumption approximately doubles file size during processing
3. Encoding Issues: May corrupt files with non-UTF-8 encoding
4. File Size Limits: No explicit file size limits, but constrained by available memory
5. Concurrent Access: No file locking mechanism to prevent modification during reading
6. Error Recovery: No retry mechanism for transient I/O errors
7. Progress Reporting: No progress indication for very large files
8. Partial Reads: Cannot resume interrupted reads from a specific position

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Troubleshooting

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Common Issues and Solutions

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Debugging Steps

1. Verify File Path: Use ls or dir commands to confirm file existence and path
2. Check Permissions: Verify read access to the file and parent directories
3. Test with Small Files: Start with small files to verify basic functionality
4. Monitor Memory Usage: Watch system memory during large file operations
5. Check File Encoding: Verify text files use UTF-8 or compatible encoding

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Best Practices

1. Path Quoting: Always quote paths with spaces or special characters
2. Size Awareness: Be mindful of file sizes and available memory
3. Permission Planning: Ensure appropriate privileges before attempting access
4. Binary File Alternatives: Use specialized tools for binary file analysis
5. Incremental Reading: For very large files, consider reading in sections

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References

• .NET FileStream Class
• Asynchronous File I/O
• Thread Synchronization
• File and Directory Access
• Apollo Agent Source Code