1467 words
7 minutes
PicoCTF 2025 Writes up
2025-05-01
Cyber Security
CTF

Before#

這一篇是統測前打ㄉ，現在每天都過得好開心，終於可以做自己想做的事ㄌ，這場是跟 Remote Computer Explosion 一起打的 ，而我主要打ㄉ領域是 Forensics，大概解了4.5題，.5 的部分是因為那一題是和 p23一起解ㄉ至於 Misc 我開賽隔一天上去看之後就被打光光ㄌ，都沒有摸到 QQ

這邊附上一個 Score Board

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Forensics#

Ph4nt0m 1ntrud3r#

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  • hint1 : Filter your packets to narrow down your search.
  • hint2 : Attacks were done in timely manner.
  • hint3 : Time is essential

題目給了一個封包打開來之後會發現 content 的部分有一些 base64 的編碼

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然後題目的提示說跟時間有關，所以就用 Time 讓他排序好

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大概從 Time 0.002212 開始會出現 Flag 經過編碼之後的東東

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flag: picoCTF{1t_w4snt_th4t_34sy_tbh_4r_e5e8c78d}

RED#

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  • hint1 : The picture seems pure, but is it though?
  • hint2 : Red?Ged?Bed?Aed?
  • hint3 : Check whatever Facebook is called now.

這題一看就是 Steganography ，於是就把他丟去萬用工具 Aperi’Solve 上

接著就看到一串經過 base64 編碼的東西，把它拿去解碼就拿到 Flag 了

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picoCTF{r3d_1s_th3_ult1m4t3_cur3_f0r_54dn355_}

flags are stepic#

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  • hint1: in the country that doesn’t exist, the flag persists

這一題是隊友 hokak 解出來ㄉ，好像是 LSB 題，隊友根本通靈大師 >_<

至於圖片的部分是這張

image

這邊附上他的解題腳本

from PIL import Image
from Crypto.Util.number import *

img = Image.open('upz.png')


cutted = img.crop((0, 0, 90, 1))


pixels = cutted.load()

flag = ''
for i in range(90):
    pixel = pixels[i, 0]
    flag += "".join(str(pixel[x] & 1) for x in range(3))



for i in range(0, len(flag), 9):
    # print(flag[i:i+8])
    print(chr(int(flag[i:i+8], 2)), end="")

Bitlocker-1#

image

  • hint: Hash cracking

下載之後會有一個 .dd 的檔案，題目說要拿去 hash crack，所以我就跑去 Github 上找到了這個 工具 可以把 bitlocker 的值 dump 下來

┌──(kali㉿kali)-[~/picoctf/bitcracker/build]
└─$ ./bitcracker_hash -o a -i ../../bitlocker-1.dd

---------> BitCracker Hash Extractor <---------
Encrypted device ../../bitlocker-1.dd opened, size  100.00 MB

************ Signature #1 found at 0x3 ************
Version: 8
Invalid version, looking for a signature with valid version...

************ Signature #2 found at 0x2195000 ************
Version: 2 (Windows 7 or later)

=====> VMK entry found at 0x21950c5
Encrypted with Recovery Password (0x21950e6)
Searching for AES-CCM (0x2195102)...
        Offset 0x2195195.... found! :)
======== RP VMK #0 ========
RP Salt: 2b71884a0ef66f0b9de049a82a39d15b
RP Nonce: 00be8a46ead6da0106000000
RP MAC: a28f1a60db3e3fe4049a821c3aea5e4b
RP VMK: a1957baea68cd29488c0f3f6efcd4689e43f8ba3120a33048b2ef2c9702e298e4c260743126ec8bd29bc6d58

=====> VMK entry found at 0x2195241
Encrypted with User Password (0x2195262)
VMK encrypted with AES-CCM
======== UP VMK ========
UP Salt: cb4809fe9628471a411f8380e0f668db
UP Nonce: d04d9c58eed6da010a000000
UP MAC: 68156e51e53f0a01c076a32ba2b2999a
UP VMK: fffce8530fbe5d84b4c19ac71f6c79375b87d40c2d871ed2b7b5559d71ba31b6779c6f41412fd6869442d66d

************ Signature #3 found at 0x2c1d000 ************
Version: 2 (Windows 7 or later)

=====> VMK entry found at 0x2c1d0c5
Encrypted with Recovery Password (0x2c1d0e6)
Searching for AES-CCM (0x2c1d102)...
        Offset 0x2c1d195.... found! :)

This VMK has been already stored...quitting to avoid infinite loop!

User Password hash:
$bitlocker$0$16$cb4809fe9628471a411f8380e0f668db$1048576$12$d04d9c58eed6da010a000000$60$68156e51e53f0a01c076a32ba2b2999afffce8530fbe5d84b4c19ac71f6c79375b87d40c2d871ed2b7b5559d71ba31b6779c6f41412fd6869442d66d

Recovery Key hash #0:
$bitlocker$2$16$2b71884a0ef66f0b9de049a82a39d15b$1048576$12$00be8a46ead6da0106000000$60$a28f1a60db3e3fe4049a821c3aea5e4ba1957baea68cd29488c0f3f6efcd4689e43f8ba3120a33048b2ef2c9702e298e4c260743126ec8bd29bc6d58

Output file for user password attack: "a/hash_user_pass.txt"

Output file for recovery password attack: "a/hash_recv_pass.txt"

dump 出來之後可以用 hashcat 去解他，那這邊因為我解過不給我解ㄌ，所以我就直接丟結果

┌──(kali㉿kali)-[~/picoctf/bitcracker/build/a]
└─$ hashcat -m 22100 hash_user_pass.txt /usr/share/wordlists/rockyou.txt  --show
$bitlocker$0$16$cb4809fe9628471a411f8380e0f668db$1048576$12$d04d9c58eed6da010a000000$60$68156e51e53f0a01c076a32ba2b2999afffce8530fbe5d84b4c19ac71f6c79375b87d40c2d871ed2b7b5559d71ba31b6779c6f41412fd6869442d66d:jacqueline

搞出密碼之後我們要做的就是想辦法把那一個硬碟掛上去 Windows，這邊我用了 osfmount 當作掛載的工具

image

選完檔案之後就一直按 next

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到這個步驟之後就選 physical disk Emulation 然後 mount

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掛載成功ㄌ，最後就輸入剛剛的密碼 jacqueline ，就會解開檔案然後拿到 flag ㄌ uwu

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Flag: picoCTF{us3_b3tt3r_p4ssw0rd5_p15!_3242adb1}

Event-Viewing#

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  • hint1: Try to filter the logs with the right event ID
  • hint2: What could the software have done when it was ran that causes the shutdowns every time the system starts up?

敘述說到受害者下載了一個惡意檔案，於是我們就去看 Installer 的 EventID，會發現說 EventID=1033 的部分會是 Installer，然後就找到其中一段 flag

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知道 Totally_Legit_Software 是 Malware 之後，就直接用 ctrl+f 去找其他有關的紀錄，最後在 EventID=4657 的地方找到一串第二段 flag

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接下來找一下題目說的關機，通常會在 EventID = 1074 的地方

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會發現註解上有一個的東西，拿去解會拿到最後一段 flag

Flag: picoCTF{Ev3nt_vi3wv3r_1s_a_pr3tty_us3ful_t00l_81ba3fe9}

Bitlocker 2#

這一題是我跟 p23 一起解出來ㄉ，雷點在我用的 volatility plugin 雖然有噴出 fvek 的東西，但是並不是題目要ㄉ= =

首先準備volatility2(透過下載原始碼，用裡面的setup.py安裝)，並且安裝以下的插件 https://github.com/breppo/Volatility-BitLocker/tree/master

分析memdump.mem，先找到要用哪個Profile

┌──(kali㉿kali)-[~/picoctf/bitlocker2]
└─$ sudo vol.py -f memdump.mem kdbgscan
Volatility Foundation Volatility Framework 2.6.1
**************************************************
Instantiating KDBG using: Unnamed AS Win10x64_19041 (6.4.19041 64bit)
Offset (V)                    : 0xf80251217b20
Offset (P)                    : 0x2c00b20
KdCopyDataBlock (V)           : 0xf80250b28898
Block encoded                 : Yes
Wait never                    : 0x9254cf55c00f1d99
Wait always                   : 0x3c76799783d65800
KDBG owner tag check          : True
Profile suggestion (KDBGHeader): Win10x64_19041
Service Pack (CmNtCSDVersion) : 0
Build string (NtBuildLab)     : 19041.1.amd64fre.vb_release.1912
PsActiveProcessHead           : 0xfffff80251235130 (95 processes)
PsLoadedModuleList            : 0xfffff80251241820 (186 modules)
KernelBase                    : 0xfffff80250617000 (Matches MZ: True)
Major (OptionalHeader)        : 10
Minor (OptionalHeader)        : 0
KPCR                          : 0xfffff8024f52f000 (CPU 0)
KPCR                          : 0xffffc301a8c67000 (CPU 1)

確定Profile使用Win10x64_19041

然後使用插件的bitlocker指令，找到memory dump中的FVEK

┌──(kali㉿kali)-[~/picoctf/bitlocker2]
└─$ sudo vol.py -f memdump.mem --profile=Win10x64_19041 bitlocker
Volatility Foundation Volatility Framework 2.6.1

[FVEK] Address : 0x9e8879926a50
[FVEK] Cipher  : AES 128-bit (Win 8+)
[FVEK] FVEK: 5b6ff64e4a0ee8f89050b7ba532f6256

[FVEK] Address : 0x9e887496fb30
[FVEK] Cipher  : AES 256-bit (Win 8+)
[FVEK] FVEK: 60be5ce2a190dfb760bea1ece40e4223c8982aecfd03221a5a43d8fdd302eaee

[FVEK] Address : 0x9e8874cb5c70
[FVEK] Cipher  : AES 128-bit (Win 8+)
[FVEK] FVEK: 1ed2a4b8dd0290f646ded074fbcff8bd

[FVEK] Address : 0x9e88779f1a10
[FVEK] Cipher  : AES 128-bit (Win 8+)
[FVEK] FVEK: bccaf1d4ea09e91f976bf94569761654

使用--dislocker選項把這些FVEK都dump出來

┌──(kali㉿kali)-[~/picoctf/bitlocker2]
└─$ mkdir fvek

┌──(kali㉿kali)-[~/picoctf/bitlocker2]
└─$ sudo vol.py -f memdump.mem --profile=Win10x64_19041 bitlocker --dislocker ./fvek
Volatility Foundation Volatility Framework 2.6.1

[FVEK] Address : 0x9e8879926a50
[FVEK] Cipher  : AES 128-bit (Win 8+)
[FVEK] FVEK: 5b6ff64e4a0ee8f89050b7ba532f6256
[DISL] FVEK for Dislocker dumped to file: ./fvek/0x9e8879926a50-Dislocker.fvek



[FVEK] Address : 0x9e887496fb30
[FVEK] Cipher  : AES 256-bit (Win 8+)
[FVEK] FVEK: 60be5ce2a190dfb760bea1ece40e4223c8982aecfd03221a5a43d8fdd302eaee
[DISL] FVEK for Dislocker dumped to file: ./fvek/0x9e887496fb30-Dislocker.fvek



[FVEK] Address : 0x9e8874cb5c70
[FVEK] Cipher  : AES 128-bit (Win 8+)
[FVEK] FVEK: 1ed2a4b8dd0290f646ded074fbcff8bd
[DISL] FVEK for Dislocker dumped to file: ./fvek/0x9e8874cb5c70-Dislocker.fvek



[FVEK] Address : 0x9e88779f1a10
[FVEK] Cipher  : AES 128-bit (Win 8+)
[FVEK] FVEK: bccaf1d4ea09e91f976bf94569761654
[DISL] FVEK for Dislocker dumped to file: ./fvek/0x9e88779f1a10-Dislocker.fvek



┌──(kali㉿kali)-[~/picoctf/bitlocker2]
└─$ ls
bitlocker-2.dd  fvek  memdump.mem  volatility

┌──(kali㉿kali)-[~/picoctf/bitlocker2]
└─$ ls fvek
0x9e887496fb30-Dislocker.fvek  0x9e8874cb5c70-Dislocker.fvek  0x9e88779f1a10-Dislocker.fvek  0x9e8879926a50-Dislocker.fvek

安裝dislocker

sudo apt update && sudo apt install dislocker -y

使用dislocker，用這些FVEK解密bitlocker-2.dd 不是每一個FVEK下去解密都可以變正常的，我這邊試到5b6ff64e4a0ee8f89050b7ba532f6256(./fvek/0x9e8879926a50-Dislocker.fvek)才是可以正常解密的那一個FVEK

sudo mkdir /mnt/test
sudo dislocker -V bitlocker-2.dd --fvek keys/0x9e8879926a50-Dislocker.fvek -- /mnt/test
sudo cp /mnt/test/dislocker-file ./6a50.dd

找一台Windows電腦，安裝OSFMount，用來掛載解密出來的映像檔 開啟OSFMount，點選Mount new... image

在這裡選擇我們剛剛解密出來的映像檔，然後按下面的Next image

按Next image

這裡要確定Drive emulation一定要是Logical Drive Emulation，然後就可以按Mount image

掛載成功 image

Flag: picoCTF{B1tl0ck3r_dr1v3_d3crypt3d_9029ae5b}

PicoCTF 2025 Writes up
https://dkri3c1.github.io/posts/picoctf-2025/
Author
dkri3c1
Published at
2025-05-01
114資工系特殊選才心得