mirror of
http://git.nowherejezfoltodf4jiyl6r56jnzintap5vyjlia7fkirfsnfizflqd.onion/nihilist/hacking-blogposts.git
synced 2025-05-16 04:16:59 +00:00
406 lines
16 KiB
Markdown
406 lines
16 KiB
Markdown
---
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search:
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exclude: true
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---
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# CSAW 2019 Beleaf
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## Downloading the binary file
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[ 192.168.100.126/24 ] [ /dev/pts/1 ] [~/binexp/1]
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→ wget https://github.com/guyinatuxedo/nightmare/raw/master/modules/03-beginner_re/csaw19_beleaf/beleaf
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--2021-02-22 19:55:50-- https://github.com/guyinatuxedo/nightmare/raw/master/modules/03-beginner_re/csaw19_beleaf/beleaf
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Resolving github.com (github.com)... 140.82.121.4
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Connecting to github.com (github.com)|140.82.121.4|:443... connected.
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HTTP request sent, awaiting response... 302 Found
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Location: https://raw.githubusercontent.com/guyinatuxedo/nightmare/master/modules/03-beginner_re/csaw19_beleaf/beleaf [following]
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--2021-02-22 19:55:51-- https://raw.githubusercontent.com/guyinatuxedo/nightmare/master/modules/03-beginner_re/csaw19_beleaf/beleaf
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Resolving raw.githubusercontent.com (raw.githubusercontent.com)... 185.199.110.133, 185.199.108.133, 185.199.111.133, ...
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Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|185.199.110.133|:443... connected.
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HTTP request sent, awaiting response... 200 OK
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Length: 7624 (7.4K) [application/octet-stream]
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Saving to: ‘beleaf’
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beleaf 100%[===============================================================================>] 7.45K --.-KB/s in 0.01s
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2021-02-22 19:55:51 (676 KB/s) - ‘beleaf’ saved [7624/7624]
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[ 192.168.100.126/24 ] [ /dev/pts/1 ] [~/binexp/1]
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→ file beleaf
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beleaf: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=6d305eed7c9bebbaa60b67403a6c6f2b36de3ca4, stripped
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[ 192.168.100.126/24 ] [ /dev/pts/1 ] [~/binexp/1]
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→ chmod +x beleaf
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` ![]()
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## Solution
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Now, first things first, we are going to use pwntools' pwn tool to check the security of the binary file itself.
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[ 192.168.100.126/24 ] [ /dev/pts/1 ] [~/binexp/1]
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→ pwn checksec beleaf
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[*] '/home/nothing/binexp/1/beleaf'
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Arch: amd64-64-little
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RELRO: Full RELRO
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Stack: Canary found
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NX: NX enabled
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PIE: PIE enabled
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So we are dealing with a 64bit binary, that scans the input of the user and then checks it, very much like the previous challenge we solved, [helithumper](heli.md). So we're going to import the file into ghidra, and take a look at the main function
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Here the main function is not called 'main' like in the previous challenge, to do so i had to look for the 'Enter the flag >>>" print statement which happened to be in the FUN_001008a1 function as you can see in the screenshot above. The code that ghidra gives us says that our text input is called 'local_98' and then later on the length of our text input is passed into sVar1
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undefined8 FUN_001008a1(void)
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{
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size_t sVar1;
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long lVar2;
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long in_FS_OFFSET;
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ulong local_b0;
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char local_98 [136];
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long local_10;
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local_10 = *(long *)(in_FS_OFFSET + 0x28);
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printf("Enter the flag\n>>> ");
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__isoc99_scanf(&DAT;_00100a78,local_98);
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sVar1 = strlen(local_98);
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if (sVar1 < 0x21) {
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puts("Incorrect!");
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/* WARNING: Subroutine does not return */
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exit(1);
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}
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local_b0 = 0;
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while (local_b0 < sVar1) {
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lVar2 = FUN_001007fa((int)local_98[local_b0]);
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if (lVar2 != *(long *)(&DAT;_003014e0 + local_b0 * 8)) {
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puts("Incorrect!");
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/* WARNING: Subroutine does not return */
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exit(1);
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}
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local_b0 = local_b0 + 1;
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}
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puts("Correct!");
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if (local_10 != *(long *)(in_FS_OFFSET + 0x28)) {
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/* WARNING: Subroutine does not return */
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__stack_chk_fail();
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}
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return 0;
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}
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now let's look at what we need to get the 'correct' output, first of all if our input LENGTH (sVar1) is less than 0x21 or 33 bytes, we will get 'incorrect', so we need at least 33 characters:
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if (sVar1 < 0x21) {
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puts("Incorrect!");
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/* WARNING: Subroutine does not return */
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exit(1);
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Then we see that we enter a for loop (which is a while loop with a variable being incremented (here it is local_b0))
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while (local_b0 < sVar1) {
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lVar2 = FUN_001007fa((int)local_98[local_b0]);
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if (lVar2 != *(long *)(&DAT;_003014e0 + local_b0 * 8)) {
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puts("Incorrect!");
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/* WARNING: Subroutine does not return */
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exit(1);
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}
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local_b0 = local_b0 + 1;
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}
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puts("Correct!");
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in this for loop, each character of our text input (local_98 at the index 0,1,2,3 ... 32) gets passed into the 'FUN_001007fa' function the result of that function gets passed to the if statement as 'lVar2' to get checked against a certain '&DAT;_003014e0' which is basically an array, the if statement checks for the characters at offsets of 8. So let's double click it to see what it is:
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DAT_003014e0 XREF[2]: FUN_001008a1:0010096b(*),
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FUN_001008a1:00100972(R)
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003014e0 01 ?? 01h
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003014e1 00 ?? 00h
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003014e2 00 ?? 00h
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003014e3 00 ?? 00h
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003014e4 00 ?? 00h
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003014e5 00 ?? 00h
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003014e6 00 ?? 00h
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003014e7 00 ?? 00h
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003014e8 09 ?? 09h
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003014e9 00 ?? 00h
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003014ea 00 ?? 00h
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003014eb 00 ?? 00h
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003014ec 00 ?? 00h
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003014ed 00 ?? 00h
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003014ee 00 ?? 00h
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003014ef 00 ?? 00h
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003014f0 11 ?? 11h
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003014f1 00 ?? 00h
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003014f2 00 ?? 00h
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003014f3 00 ?? 00h
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003014f4 00 ?? 00h
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003014f5 00 ?? 00h
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003014f6 00 ?? 00h
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003014f7 00 ?? 00h
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003014f8 27 ?? 27h '
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003014f9 00 ?? 00h
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003014fa 00 ?? 00h
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003014fb 00 ?? 00h
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003014fc 00 ?? 00h
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003014fd 00 ?? 00h
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003014fe 00 ?? 00h
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003014ff 00 ?? 00h
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00301500 02 ?? 02h
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And here we see the bytes we need are at offsets of 8, so we have the following:
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0x1 0x9 0x11 0x27 0x2
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Now let's take a look at the 'FUN_001007fa' function that checks each of our input text characters:
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long FUN_001007fa(char param_1)
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{
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long local_10;
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local_10 = 0;
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while ((local_10 != -1 && ((int)param_1 != *(int *)(&DAT;_00301020 + local_10 * 4)))) {
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if ((int)param_1 < *(int *)(&DAT;_00301020 + local_10 * 4)) {
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local_10 = local_10 * 2 + 1;
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}
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else {
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if (*(int *)(&DAT;_00301020 + local_10 * 4) < (int)param_1) {
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local_10 = (local_10 + 1) * 2;
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}
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}
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}
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return local_10;
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}
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in here, each character of our input text gets passed as the param_1 charcater, and then the function basically looks at the 'DAT_003014e0' array with offsets of 4, the function tries to find at which index our input text characters are in this array, so let's see what is in that 'DAT_003014e0' array
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DAT_00301020 XREF[6]: FUN_001007fa:00100820(*),
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FUN_001007fa:00100827(R),
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FUN_001007fa:00100844(*),
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FUN_001007fa:0010084b(R),
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FUN_001007fa:00100873(*),
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FUN_001007fa:0010087a(R)
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00301020 77 ?? 77h w
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00301021 00 ?? 00h
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00301022 00 ?? 00h
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00301023 00 ?? 00h
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00301024 66 ?? 66h f
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00301025 00 ?? 00h
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00301026 00 ?? 00h
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00301027 00 ?? 00h
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00301028 7b ?? 7Bh {
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00301029 00 ?? 00h
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0030102a 00 ?? 00h
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0030102b 00 ?? 00h
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0030102c 5f ?? 5Fh _
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0030102d 00 ?? 00h
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0030102e 00 ?? 00h
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0030102f 00 ?? 00h
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00301030 6e ?? 6Eh n
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00301031 00 ?? 00h
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00301032 00 ?? 00h
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00301033 00 ?? 00h
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00301034 79 ?? 79h y
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00301035 00 ?? 00h
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00301036 00 ?? 00h
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00301037 00 ?? 00h
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00301038 7d ?? 7Dh }
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00301039 00 ?? 00h
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0030103a 00 ?? 00h
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0030103b 00 ?? 00h
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0030103c ff ?? FFh
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0030103d ff ?? FFh
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0030103e ff ?? FFh
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0030103f ff ?? FFh
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00301040 62 ?? 62h b
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00301041 00 ?? 00h
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00301042 00 ?? 00h
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00301043 00 ?? 00h
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00301044 6c ?? 6Ch l
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00301045 00 ?? 00h
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00301046 00 ?? 00h
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00301047 00 ?? 00h
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00301048 72 ?? 72h r
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00301049 00 ?? 00h
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0030104a 00 ?? 00h
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0030104b 00 ?? 00h
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0030104c ff ?? FFh
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0030104d ff ?? FFh
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0030104e ff ?? FFh
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0030104f ff ?? FFh
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00301050 ff ?? FFh
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00301051 ff ?? FFh
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00301052 ff ?? FFh
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00301053 ff ?? FFh
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00301054 ff ?? FFh
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00301055 ff ?? FFh
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00301056 ff ?? FFh
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00301057 ff ?? FFh
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00301058 ff ?? FFh
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00301059 ff ?? FFh
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0030105a ff ?? FFh
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0030105b ff ?? FFh
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0030105c ff ?? FFh
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0030105d ff ?? FFh
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0030105e ff ?? FFh
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0030105f ff ?? FFh
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00301060 ff ?? FFh
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00301061 ff ?? FFh
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00301062 ff ?? FFh
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00301063 ff ?? FFh
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00301064 61 ?? 61h a
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00301065 00 ?? 00h
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00301066 00 ?? 00h
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00301067 00 ?? 00h
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00301068 65 ?? 65h e
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00301069 00 ?? 00h
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0030106a 00 ?? 00h
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0030106b 00 ?? 00h
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0030106c 69 ?? 69h i
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[...]
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now when you look at the characters in this array, you can get the feeling that you might be able to type flag{something} with it, so let's follow what the code does with the 2 arrays we found:
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we know that the start of the 1020array is 00301020. The character f will output 1 because **((0x00301024 - 0x00301020) / 4) = 1** so this is equal to 1. This 1 also corresponds to the 14e0 array from earlier:
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DAT_003014e0 XREF[2]: FUN_001008a1:0010096b(*),
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FUN_001008a1:00100972(R)
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003014e0 01 ?? 01h
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003014e1 00 ?? 00h
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003014e2 00 ?? 00h
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003014e3 00 ?? 00h
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003014e4 00 ?? 00h
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003014e5 00 ?? 00h
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003014e6 00 ?? 00h
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003014e7 00 ?? 00h
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003014e8 09 ?? 09h
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003014e9 00 ?? 00h
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003014ea 00 ?? 00h
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003014eb 00 ?? 00h
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003014ec 00 ?? 00h
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003014ed 00 ?? 00h
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003014ee 00 ?? 00h
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003014ef 00 ?? 00h
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003014f0 11 ?? 11h
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003014f1 00 ?? 00h
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003014f2 00 ?? 00h
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003014f3 00 ?? 00h
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003014f4 00 ?? 00h
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003014f5 00 ?? 00h
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003014f6 00 ?? 00h
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003014f7 00 ?? 00h
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003014f8 27 ?? 27h '
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003014f9 00 ?? 00h
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003014fa 00 ?? 00h
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003014fb 00 ?? 00h
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003014fc 00 ?? 00h
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003014fd 00 ?? 00h
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003014fe 00 ?? 00h
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003014ff 00 ?? 00h
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00301500 02 ?? 02h
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[...]
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and here you have to continue with the 0x9 value,**(0x00301020 + (4*9)) = 0x301044** this address corresponds to the l character
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fl
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11 is the third character **(0x00301020 + (4*11)) = 0x301064** this corresponds to the a character
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fla
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27 is the fourth character **(0x00301020 + (4*27)) = 0x3010bc** this corresponds to the g character
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flag
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from here you keep going and you end up with the following:
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flag{we_beleaf_in_your_re_future}
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so just run the binary with the flag to verify it is correct:
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[ 192.168.100.126/24 ] [ /dev/pts/1 ] [~/binexp/1]
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→ ./beleaf
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Enter the flag
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>>> flag{we_beleaf_in_your_re_future}
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Correct!
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## Title
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text
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` ![]()
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## Title
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text
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` ![]()
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