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!
| # | |
| Type | CTF / Reversing |
| Name | Cyber Apocalypse 2021 / Passphrase |
| Started | 2021/04/23 |
| URLs | https://ctf.hackthebox.eu/ctf/82 |
| https://ctftime.org/event/1304 |
| Author | Asentinn / 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)
7a6: ff 25 d4 17 20 00 jmpq *0x2017d4(%rip)
7ac: 0f 1f 40 00 nopl 0x0(%rax)
00000000000007b0 <putchar@plt>:
7b0: ff 25 d2 17 20 00 jmpq *0x2017d2(%rip)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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 - voilà:
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"✔
) = 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
