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Introduction

shadow-shell is a cyber lab designed for:

  • analyzing shellcode
  • exploring memory exploits
  • supporting malware analysis

Repository structure

The repository contains two main parts:

  • arsenal: command line tools to support malware analysis
    • shexec: a shellcode runner that can be combined with other tools like strace (Linux), ProcMon and TCPView (Windows) to analyze shellcode functionality
  • lab: experimental code snippets, some are documented while others are not

⚠️ You should never execute untrusted shellcode on your system. Use a hardened VM or container for this purpose.

arsenal/
├── linux/
│   ├── arm64/
│   │   ├── shexec.s: Linux ARM64 shellcode runner
│   │   ├── shcode_hello.s: Linux ARM64 shellcode that prints "Hello!"
│   │   └── shcode_shell.s: Linux ARM64 shellcode that opens a shell
│   ├── x64/
│   │   ├── shexec.s: Linux x64 shellcode runner
│   │   └── shcode_hello.s: Linux x64 shellcode that prints "Hello, World!"
│   └── shexec.c: Linux shellcode runner (used as a reference while implementing shexec.s for different architectures)
└── windows/
    └── shexec.c: Windows shellcode runner

lab/
├── linux/
│   └── buffer-overflow/: proof of concept for a buffer overflow with the goal of executing shellcode
└── windows/
    └── shellcode/: proof of concept for basic shellcode execution

Both POCs are documented in detail on my blog:

Workflow

Shellcode analysis

  1. Emulate the shellcode
    Use an emulator like Qiling to analyze the shellcode.

  2. Fallback to shexec + strace
    If you suspect the shellcode may detect the emulator (and behave differently as a result) switch to using shexec with strace.

  3. Fallback to eBPF tracing
    If the shellcode seems to detect that it is being traced by strace use eBPF tracing.

How to build

Prerequisites

$ sudo dnf install docker
$ sudo dnf install mingw64-gcc
$ sudo dnf install nasm
$ sudo dnf install llvm
$ pip install frida-tools
$ rustup target add x86_64-pc-windows-gnu

x64 codebase

$ make x64

ARM64 codebase

I have an x64 PC so to quickly build and run ARM64 binaries my preference is to use an ARM64v8 Docker container. This setup has some limitations though as it does not implement ptrace so strace and gdb cannot be used.

$ sudo docker build --platform=linux/arm64 -t arm64 .
$ sudo docker run --platform=linux/arm64 --user $(id -u):$(id -g) --rm -it -v "$(pwd)":/workspace arm64 /bin/bash
$ make arm64

The image is also available on Docker Hub:

$ docker pull gemesa/arm64:latest

Alternatively an ARM64 cross-compiler and QEMU could also be used (Docker is doing something similar under the hood).

$ dnf search *aarch64*
$ sudo dnf install gcc-aarch64-linux-gnu
$ sudo dnf install binutils-aarch64-linux-gnu
$ sudo dnf install qemu-system-aarch64
$ sudo dnf install qemu-system-aarch64-core
$ sudo dnf install qemu-user-static-aarch64
$ sudo dnf install sysroot-aarch64-fc41-glibc

$ aarch64-linux-gnu-gcc -L /usr/aarch64-redhat-linux/sys-root/fc41/lib64 -L /usr/aarch64-redhat-linux/sys-root/fc41/lib --sysroot=/usr/aarch64-redhat-linux/sys-root/fc41 arsenal/linux/arm64/shexec.s -o shexec
$ aarch64-linux-gnu-as arsenal/linux/arm64/shcode_hello.s -o shcode_hello.o
$ aarch64-linux-gnu-ld shcode_hello.o -o shcode_hello
$ llvm-objcopy -O binary --only-section=.text shcode_hello shcode_hello.bin
$ qemu-aarch64 -L /usr/aarch64-redhat-linux/sys-root/fc41/usr shexec shcode_hello.bin
file size: 52 bytes
Hello!

Simply use the following command (ensure that the ARM64 cross-compiler is installed):

$ make arm64x

If you have a Raspberry Pi you can use it with all the debugging tools including strace and gdb.

How to use

Linux ARM64

shexec

$ make arm64
$ ./build/linux/arm64/shexec build/linux/arm64/shcode_hello.bin
file size: 52 bytes
Hello!
$ echo $$
1918
$ ./build/linux/arm64/shexec build/linux/arm64/shcode_shell.bin
file size: 40 bytes
$ echo $$
19341
$ exit
$ strace ./build/linux/arm64/shexec build/linux/arm64/shcode_hello.bin
...
openat(AT_FDCWD, "build/linux/arm64/shcode_hello.bin", O_RDONLY) = 3
newfstatat(3, "", {st_mode=S_IFREG|0755, st_size=52, ...}, AT_EMPTY_PATH) = 0
newfstatat(1, "", {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0), ...}, AT_EMPTY_PATH) = 0
getrandom("\x20\xc3\xbe\x6a\x9d\x68\x76\xf9", 8, GRND_NONBLOCK) = 8
brk(NULL)                               = 0x55754e8000
brk(0x5575509000)                       = 0x5575509000
write(1, "file size: 52 bytes\n", 20file size: 52 bytes
)   = 20
mmap(NULL, 52, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7facfe6000
read(3, "\1\251\214\322\201\215\255\362\341-\304\362A\1\340\362\341\217\37\370 \0\200\322\341\3\0\221\342\0\200\322"..., 52) = 52
close(3)                                = 0
mprotect(0x7facfe6000, 52, PROT_READ|PROT_WRITE|PROT_EXEC) = 0
write(1, "Hello!\n", 7Hello!
)                 = 7
exit(0)                                 = ?

Linux x64

shexec

$ make x64
$ ./build/linux/x64/shexec build/linux/x64/shcode_hello.bin
file size: 57 bytes
Hello, World!
$ msfvenom -p linux/x64/exec CMD='echo \"Hello, World!\"' -f raw -o shellcode.bin
$ ./build/linux/x64/shexec shellcode.bin
file size: 57 bytes
Hello, World!
$ strace ./build/linux/x64/shexec build/linux/x64/shcode_hello.bin 
...
openat(AT_FDCWD, "build/linux/x64/shcode_hello.bin", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=57, ...}) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0x1), ...}) = 0
getrandom("\xae\x28\x5d\xfd\xd1\xae\x82\x68", 8, GRND_NONBLOCK) = 8
brk(NULL)                               = 0x55c7bbc4d000
brk(0x55c7bbc6e000)                     = 0x55c7bbc6e000
write(1, "file size: 57 bytes\n", 20file size: 57 bytes
)   = 20
mmap(NULL, 57, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f94405b3000
read(3, "H\307\300\1\0\0\0H\307\307\1\0\0\0H\2155\25\0\0\0H\307\302\17\0\0\0\17\5H\307"..., 57) = 57
close(3)                                = 0
mprotect(0x7f94405b3000, 57, PROT_READ|PROT_WRITE|PROT_EXEC) = 0
write(1, "Hello, World!\n\0", 15Hello, World!
)       = 15
exit(0)                                 = ?

Windows x64

shexec.exe

$ make x64
$ msfconsole
msf6 > info payload/windows/x64/shell_bind_tcp
...
Basic options:
Name      Current Setting  Required  Description
----      ---------------  --------  -----------
EXITFUNC  process          yes       Exit technique (Accepted: '', seh, thread, process, none)
LPORT     4444             yes       The listen port
RHOST                      no        The target address

Description:
  Listen for a connection and spawn a command shell (Windows x64)
...
msf6 > exit
$ msfvenom -p windows/x64/shell_bind_tcp -f raw -o shellcode.bin

Open ProcMon and TCPView then:

> .\shexec.exe shellcode.bin

TCPView

References