Path Traversal in Flask with Cockroachdb

Path Traversal occurs when an application allows user-supplied input to control file paths used in filesystem operations, enabling access to files outside the intended directory. In a Flask application using Cockroachdb, the risk typically arises at the application layer rather than directly within Cockroachdb, but the database interaction can amplify impact if sensitive data is involved. Flask routes that accept file identifiers or construct filesystem paths from request parameters can become entry points. For example, a route like /download/ that concatenates user input into a filesystem path without validation can allow an attacker to traverse directories using sequences like ../../../etc/passwd.

When Cockroachdb is used as the backend, Path Traversal becomes more concerning if the application stores or references sensitive data (e.g., configuration files, credentials, or logs) on the filesystem and those files contain database connection details or exported data. An attacker who exploits Path Traversal might read files that include Cockroachdb connection strings, SSL certificates, or backup scripts, leading to further compromise. Additionally, if the application dynamically generates or serves files based on database queries (for example, exporting query results to CSV), insufficient validation of file paths can expose the filesystem structure and indirectly reveal database metadata or operational details.

The combination does not change how Path Traversal works at the OS level, but it does expand the attack surface: Flask routes interact with Cockroachdb, and unchecked input can lead to unauthorized file access that exposes database-related artifacts. Because Cockroachdb is often deployed in distributed environments, configuration files or logs may contain node-specific information that can aid lateral movement. Therefore, securing file path handling in Flask becomes critical to prevent indirect exposure of Cockroachdb infrastructure details.

Remediation centers on strict input validation, path normalization, and avoiding direct filesystem operations with user input. When your Flask app interacts with Cockroachdb, ensure that file paths used for database backups, logs, or configuration are never derived from untrusted sources. Below are concrete code examples demonstrating secure patterns.

1. Secure file path handling with allowlist validation

Use an allowlist of permitted filenames or identifiers rather than trying to block dangerous sequences. This is especially important when generating filenames for Cockroachdb-related exports.

from flask import Flask, send_from_directory, abort
import os

app = Flask(__name__)

# Allowed file mapping for database exports
ALLOWED_EXPORTS = {
    'daily_report': 'report_2024_10_01.csv',
    'schema_dump': 'schema.sql',
}

@app.route('/export/')
def export_file(file_id):
    if file_id not in ALLOWED_EXPORTS:
        abort(403, 'File not allowed')
    filename = ALLOWED_EXPORTS[file_id]
    return send_from_directory(directory='/secure/exports', path=filename)

2. Using os.path.abspath and chroot-like base restriction

Always resolve paths to absolute locations and ensure they remain within a designated base directory. This prevents directory traversal even if validation is bypassed.

import os
from flask import Flask, request

app = Flask(__name__)
BASE_DIR = os.path.abspath('/var/app/data')

@app.route('/files/')
def get_file(subpath):
    # Normalize and join safely
    requested_path = os.path.normpath(os.path.join('/', subpath))
    full_path = os.path.abspath(os.path.join(BASE_DIR, requested_path.lstrip('/')))
    if not full_path.startswith(BASE_DIR):
        return 'Forbidden', 403
    # Proceed with safe filesystem operation, e.g., read a Cockroachdb backup
    if os.path.isfile(full_path):
        return open(full_path).read()
    return 'Not found', 404

3. Cockroachdb connection security

When your Flask app connects to Cockroachdb, avoid storing credentials in files that could be exposed via Path Traversal. Use environment variables and ensure file permissions are restrictive. Example secure connection setup:

import os
from cockroachdb import connect

# Use environment variables, never hardcode or load from user-controlled paths
db_user = os.environ.get('COCKROACH_USER')
db_password = os.environ.get('COCKROACH_PASSWORD')
db_host = os.environ.get('COCKROACH_HOST', 'localhost')

def get_db_connection():
    return connect(
        host=db_host,
        user=db_user,
        password=db_password,
        database='secure_app',
        sslmode='verify-full',
        sslrootcert='/secure/certs/ca.pem'
    )

4. Logging and operational files

Ensure logs and backup scripts referenced by Flask are stored outside the web-accessible directory and have strict filesystem permissions. Never allow user input to influence log file paths.

# Good: fixed log path, never derived from request
LOG_PATH = '/var/log/cockroach_integration/app.log'

@app.route('/health')
def health():
    with open(LOG_PATH, 'a') as log:
        log.write('Health check passed\n')
    return 'OK'