Kamil Gierach-Pacanek
CyberEthical.Me: Hacking for the Security Awareness

CyberEthical.Me: Hacking for the Security Awareness

Cyber Apocalypse 2021: Passphrase

Cyber Apocalypse 2021: Passphrase

Complete write-up

Kamil Gierach-Pacanek
ยทMay 10, 2021ยท

5 min read

Subscribe to my newsletter and never miss my upcoming articles

Table of contents

  • Introduction
  • Basic Information
  • Target of Evaluation
  • Analysis
  • Debugging
  • Flag
  • Additional readings

Introduction

You found one of their space suits forgotten in a room. You wear it, but before you go away, a guard stops you and asks some questions

Complete write up for the Passphrase challenge at Cyber Apocalypse 2021 CTF hosted by HackTheBox.eu. This article is a part of a CTF: Cyber Apocalypse 2021 series. You can fork all my writeups directly from the GitHub.

Learn more from additional readings found at the end of the article. I would be thankful if you mention me when using parts of this article in your work. Enjoy!

Basic Information

#
TypeCTF / Reversing
NameCyber Apocalypse 2021 / Passphrase
Started2021/04/23
URLsctf.hackthebox.eu/ctf/82
ctftime.org/event/1304
AuthorAsentinn / OkabeRintaro
https://ctftime.org/team/152207

Target of Evaluation

We are given the passphrase file.

Analysis

First things first, analyze what we have with the file tool:

$ file passphrase 

passphrase: 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]=60f6b6064d2e34a2b6a24dda9feb943b0b8c360f, not stripped

Symbols are left within the executable. This is good as it makes working with the file easier.

Running strings. Between the output lines I've got the following:

Halt! 
You do not look familiar..
Tell me the secret passphrase: 
[31m
Intruder alert! 
[32m
Sorry for suspecting you, please transfer this important message to the chief: CHTB{%s}
;*3$"

It looks like it is going to do some string comparisons maybe..

Check for dynamically linked libraries:

$ ldd passphrase 

    linux-vdso.so.1 (0x00007ffc0d5b6000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007ff3f9750000)
    /lib64/ld-linux-x86-64.so.2 (0x00007ff3f9b32000)

Nothing out stands, just regular libraries. Also, none is missing.

Symbols are included in the executable (not stripped) so lets list them:

$ nm passphrase                                      

0000000000202010 B __bss_start
0000000000202028 b completed.7698
                 w __cxa_finalize@@GLIBC_2.2.5
0000000000202000 D __data_start
0000000000202000 W data_start
0000000000000890 t deregister_tm_clones
0000000000000920 t __do_global_dtors_aux
0000000000201d78 d __do_global_dtors_aux_fini_array_entry
0000000000202008 D __dso_handle
0000000000201d80 d _DYNAMIC
0000000000202010 D _edata
0000000000202030 B _end
                 U fgets@@GLIBC_2.2.5
0000000000000bb4 T _fini
0000000000000960 t frame_dummy
0000000000201d70 d __frame_dummy_init_array_entry
0000000000000dfc r __FRAME_END__
0000000000201f70 d _GLOBAL_OFFSET_TABLE_
                 w __gmon_start__
0000000000000c94 r __GNU_EH_FRAME_HDR
0000000000000780 T _init
0000000000201d78 d __init_array_end
0000000000201d70 d __init_array_start
0000000000000bc0 R _IO_stdin_used
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
0000000000000bb0 T __libc_csu_fini
0000000000000b40 T __libc_csu_init
                 U __libc_start_main@@GLIBC_2.2.5
00000000000009c6 T main
                 U printf@@GLIBC_2.2.5
000000000000096a T printstr
                 U putchar@@GLIBC_2.2.5
                 U puts@@GLIBC_2.2.5
00000000000008d0 t register_tm_clones
                 U setbuf@@GLIBC_2.2.5
                 U sleep@@GLIBC_2.2.5
                 U __stack_chk_fail@@GLIBC_2.4
0000000000000860 T _start
0000000000202020 B stdin@@GLIBC_2.2.5
0000000000202010 B stdout@@GLIBC_2.2.5
                 U strcmp@@GLIBC_2.2.5
                 U strlen@@GLIBC_2.2.5
0000000000202010 D __TMC_END__
                 U usleep@@GLIBC_2.2.5

We can see that at the address 0x00202010 it does string compare. So, it confirms the assumption from before.

Dump Procedure Linkage Table:

$ objdump -j .plt -S passphrase 

passphrase:     file format elf64-x86-64


Disassembly of section .plt:

00000000000007a0 <.plt>:
 7a0:    ff 35 d2 17 20 00        pushq  0x2017d2(%rip)        # 201f78 <_GLOBAL_OFFSET_TABLE_+0x8>
 7a6:    ff 25 d4 17 20 00        jmpq   *0x2017d4(%rip)        # 201f80 <_GLOBAL_OFFSET_TABLE_+0x10>
 7ac:    0f 1f 40 00              nopl   0x0(%rax)

00000000000007b0 <putchar@plt>:
 7b0:    ff 25 d2 17 20 00        jmpq   *0x2017d2(%rip)        # 201f88 <putchar@GLIBC_2.2.5>
 7b6:    68 00 00 00 00           pushq  $0x0
 7bb:    e9 e0 ff ff ff           jmpq   7a0 <.plt>

00000000000007c0 <puts@plt>:
 7c0:    ff 25 ca 17 20 00        jmpq   *0x2017ca(%rip)        # 201f90 <puts@GLIBC_2.2.5>
 7c6:    68 01 00 00 00           pushq  $0x1
 7cb:    e9 d0 ff ff ff           jmpq   7a0 <.plt>

00000000000007d0 <strlen@plt>:
 7d0:    ff 25 c2 17 20 00        jmpq   *0x2017c2(%rip)        # 201f98 <strlen@GLIBC_2.2.5>
 7d6:    68 02 00 00 00           pushq  $0x2
 7db:    e9 c0 ff ff ff           jmpq   7a0 <.plt>

00000000000007e0 <__stack_chk_fail@plt>:
 7e0:    ff 25 ba 17 20 00        jmpq   *0x2017ba(%rip)        # 201fa0 <__stack_chk_fail@GLIBC_2.4>
 7e6:    68 03 00 00 00           pushq  $0x3
 7eb:    e9 b0 ff ff ff           jmpq   7a0 <.plt>

00000000000007f0 <setbuf@plt>:
 7f0:    ff 25 b2 17 20 00        jmpq   *0x2017b2(%rip)        # 201fa8 <setbuf@GLIBC_2.2.5>
 7f6:    68 04 00 00 00           pushq  $0x4
 7fb:    e9 a0 ff ff ff           jmpq   7a0 <.plt>

0000000000000800 <printf@plt>:
 800:    ff 25 aa 17 20 00        jmpq   *0x2017aa(%rip)        # 201fb0 <printf@GLIBC_2.2.5>
 806:    68 05 00 00 00           pushq  $0x5
 80b:    e9 90 ff ff ff           jmpq   7a0 <.plt>

0000000000000810 <fgets@plt>:
 810:    ff 25 a2 17 20 00        jmpq   *0x2017a2(%rip)        # 201fb8 <fgets@GLIBC_2.2.5>
 816:    68 06 00 00 00           pushq  $0x6
 81b:    e9 80 ff ff ff           jmpq   7a0 <.plt>

0000000000000820 <strcmp@plt>:
 820:    ff 25 9a 17 20 00        jmpq   *0x20179a(%rip)        # 201fc0 <strcmp@GLIBC_2.2.5>
 826:    68 07 00 00 00           pushq  $0x7
 82b:    e9 70 ff ff ff           jmpq   7a0 <.plt>

0000000000000830 <sleep@plt>:
 830:    ff 25 92 17 20 00        jmpq   *0x201792(%rip)        # 201fc8 <sleep@GLIBC_2.2.5>
 836:    68 08 00 00 00           pushq  $0x8
 83b:    e9 60 ff ff ff           jmpq   7a0 <.plt>

0000000000000840 <usleep@plt>:
 840:    ff 25 8a 17 20 00        jmpq   *0x20178a(%rip)        # 201fd0 <usleep@GLIBC_2.2.5>
 846:    68 09 00 00 00           pushq  $0x9
 84b:    e9 50 ff ff ff           jmpq   7a0 <.plt>

Nothing to see here.

Debugging

Now that we know what to look for, let's try strace ./passphrase and ltrace ./passphrase.

strace didn't show what we look for (and we look for string comparison R-value)

clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=0, tv_nsec=30000000}, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7fffeba20570) = 0
fstat(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0x2), ...}) = 0
brk(NULL)                               = 0x559459cdd000
brk(0x559459cfe000)                     = 0x559459cfe000
read(0, fdfdf
"fdfdf\n", 1024)                = 6
write(1, "\33[31m", 5)                  = 5
write(1, "\n", 1
)                       = 1
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=0, tv_nsec=30000000}, NULL) = 0

But ltrace - voilร :

strlen("\nTell me the secret passphrase: "...)                   = 32
sleep(1)                                                         = 0
fgets(dsada
"dsada\n", 40, 0x7f8568531980)                             = 0x7ffc986e2da0
strlen("dsada\n")                                                = 6
strcmp("3xtr4t3rR3stR14L5_VS_hum4n5", "dsada")                   = -49
printf("\033[31m")                                               = 5
strlen("\nIntruder alert! \360\237\232\250\n")                   = 22

So after presenting a secret passphrase, the executable gives us the flag:

strlen("\nTell me the secret passphrase: "...)                   = 32
sleep(1)                                                         = 0
fgets(3xtr4t3rR3stR14L5_VS_hum4n5
"3xtr4t3rR3stR14L5_VS_hum4n5\n", 40, 0x7f461a61c980)       = 0x7ffd79ab8ec0
strlen("3xtr4t3rR3stR14L5_VS_hum4n5\n")                          = 28
strcmp("3xtr4t3rR3stR14L5_VS_hum4n5", "3xtr4t3rR3stR14L5_VS_hum4n5") = 0
puts("\342\234\224"โœ”
)                                             = 4
printf("\033[32m")                                               = 5
printf("\nSorry for suspecting you, pleas"..., "3xtr4t3rR3stR14L5_VS_hum4n5"
Sorry for suspecting you, please transfer this important message to the chief: CHTB{3xtr4t3rR3stR14L5_VS_hum4n5}

Flag

CHTB{3xtr4t3rR3stR14L5_VS_hum4n5}

Additional readings

Cover photo by Ales Nesetril on Unsplash

Did you find this article valuable?

Support Kamil Gierach-Pacanek by becoming a sponsor. Any amount is appreciated!

See recent sponsors |ย Learn more about Hashnode Sponsors
ย 
Share this