Symlink Attack in Flask with Cockroachdb

A symlink attack in a Flask application that uses CockroachDB occurs when an attacker manipulates file system references to gain unauthorized access to database files or configuration that the application relies on. Flask applications often store database connection parameters, certificates, or temporary data on disk. If these files are written to user-controlled locations without proper validation, an attacker can create or replace a symbolic link so that a write operation (e.g., a file upload or a configuration save) targets a sensitive CockroachDB file such as a certificate, a credentials file, or even a data directory.

When the application later reads from or writes to that path, it interacts with the attacker-controlled target. In a CockroachDB context, this can expose sensitive cluster metadata or allow manipulation of files used by CockroachDB clients. Because CockroachDB often uses TLS with client certificates, replacing a certificate file with a symlink can lead to credential theft or allow an attacker to redirect traffic. In a Flask application, this typically maps to the BFLA/Privilege Escalation and Unsafe Consumption checks in middleBrick’s 12 security checks, as the application may expose file operations without verifying the destination’s integrity.

Moreover, if the Flask app exposes an endpoint that downloads or serves files (e.g., a configuration or log retrieval endpoint), and the filename is derived from user input without canonicalization, an attacker can supply a path traversal combined with a symlink to reach arbitrary files on the host. This becomes especially relevant when the host runs CockroachDB tooling or stores sensitive artifacts on the same filesystem. middleBrick’s BOLA/IDOR and Property Authorization checks help surface such path traversal risks by correlating runtime behavior with the OpenAPI specification, identifying endpoints that allow unsafe file references.

Because the attack relies on the application’s file handling logic rather than a flaw in CockroachDB itself, the scanner must test unauthenticated endpoints that accept file inputs or dynamically construct filesystem paths. middleBrick runs parallel checks including Input Validation and Data Exposure to detect endpoints that could be leveraged in a symlink attack. The scanner does not fix the logic, but its findings include remediation guidance, such as validating file paths, avoiding user-controlled paths, and storing sensitive CockroachDB artifacts outside the web-accessible directory tree.

To mitigate symlink risks in a Flask application that interacts with CockroachDB, ensure that file operations never follow symbolic links derived from user input and that paths are resolved to their canonical absolute forms. Use os.path.realpath to resolve symlinks before any file operation, and enforce strict allowlists for directories where CockroachDB-related files may reside.

Below are concrete code examples for a Flask route that safely handles configuration or certificate files intended for CockroachDB clients.

import os
from flask import Flask, request, send_file

app = Flask(__name__)

# Define an absolute, application-controlled base directory for CockroachDB assets
COCKROACH_BASE = "/opt/app/cockroach-assets"

def safe_path(user_segment):
    # Join and resolve to eliminate any symlinks or path traversal components
    resolved = os.path.realpath(os.path.join(COCKROACH_BASE, user_segment.lstrip("/")))
    # Ensure the resolved path stays within the allowed base
    if not resolved.startswith(COCKROACH_BASE):
        raise ValueError("Path traversal detected")
    return resolved

@app.route("/download-cert")
def download_cert():
    filename = request.args.get("file", "")
    target = safe_path(filename)
    # Only allow known files; do not rely on the filesystem to enforce permissions
    if os.path.basename(target) not in ["ca.pem", "client.pem"]:
        return "Forbidden", 403
    return send_file(target, as_attachment=True)

For CockroachDB connections, avoid storing sensitive credentials as files that can be symlinked. Instead, use environment variables or a secrets manager, and ensure that any file-based configuration is created with secure permissions and without following symlinks. The following snippet demonstrates initializing a CockroachDB client using a certificate read from a secure, resolved path.

import ssl
from cockroachdb import client as crdb_client
from flask import current_app
import os

def get_secure_ssl_context():
    ca_path = os.path.realpath(os.path.join(current_app.root_path, "certs", "ca.pem"))
    # Ensure the path is within the app’s controlled directory
    if not ca_path.startswith(os.path.realpath(current_app.root_path + "/certs")):
        raise RuntimeError("Invalid certificate path")
    ctx = ssl.create_default_context(cafile=ca_path)
    return ctx

# Example client creation (pseudocode; adapt to your driver)
ssl_ctx = get_secure_ssl_context()
db = crdb_client.Client(
    hosts=os.getenv("COCKROACH_HOSTS"),
    ssl_context=ssl_ctx,
)

In production, restrict filesystem permissions for directories containing CockroachDB artifacts and audit file access patterns. middleBrick’s Continuous Monitoring and GitHub Action integrations can be used to enforce that new endpoints do not introduce unsafe file handling patterns, and the MCP Server can flag risky path operations during development within AI coding assistants.