Server Side Template Injection in Flask with Cockroachdb

Server Side Template Injection (SSTI) in Flask becomes particularly concerning when the application uses Cockroachdb as the backend database. Flask's default configuration often relies on Jinja2 for rendering dynamic content. If user-controlled input is directly embedded into templates without proper sanitization or isolation, an attacker can inject template code that executes on the server. When the injected template interacts with data queries directed at Cockroachdb, the risk shifts from simple reflection to potential data exfiltration or logic manipulation.

Consider a Flask route that accepts a user identifier and fetches profile information from Cockroachdb using an ORM or raw SQL. If the retrieved data is passed into a Jinja2 template and concatenated into a string that is later rendered, an attacker might supply a payload such as {{ config }} or {{ ''.__class__.__mro__[1].__subclasses__() }}. In a typical SSTI scenario, the injected code executes in the template context. With Cockroachdb involved, the injected template may invoke database queries via injected methods or variables, enabling the attacker to probe database structure or extract sensitive records through error messages or side channels. For example, an injected expression that attempts to access a database session or engine object can reveal connection configurations or query patterns, especially if the application stores database handles in the Flask app context or global configuration objects.

The interaction between Flask's template engine and Cockroachdb can amplify the impact of SSTI when the application uses dynamic query building. If a developer passes unsanitized template variables into raw SQL strings executed against Cockroachdb, the injected template code might alter query logic or expose stack traces that include database credentials or query details. Even when using an ORM like SQLAlchemy with Cockroachdb, improper exposure of model attributes or query APIs to the template layer can allow an attacker to traverse relationships or invoke methods that trigger unintended database operations. The key vulnerability arises when the template context includes objects that provide access to the database layer, either directly through an engine/session or indirectly through configuration variables that contain connection strings or query hints.

middleBrick detects this class of issue through its 12 parallel security checks, including Input Validation and Property Authorization, which analyze how user data flows into template rendering and backend database calls. The scanner does not rely on internal architecture, so it observes the behavior of the unauthenticated endpoint when supplied with malicious template patterns. If the application returns differences in timing, error messages, or reflected content when template injection payloads are sent, middleBrick flags the endpoint with a risk score and provides remediation guidance tied to OWASP API Top 10 and common coding practices for Flask and database integrations.

To mitigate SSTI when using Cockroachdb with Flask, isolate user input from template rendering and enforce strict separation between data and presentation logic. The following patterns demonstrate secure approaches for database interactions and template usage.

Use parameterized queries with SQLAlchemy

When querying Cockroachdb, avoid string interpolation in SQL statements. Instead, use SQLAlchemy's parameterized queries to ensure that user input is treated strictly as data. Example:

from sqlalchemy import text

# Safe query using bound parameters
query = text("SELECT id, email FROM users WHERE id = :user_id")
result = db.session.execute(query, {"user_id": user_id})
for row in result:
    print(row.email)

Disable dangerous object access in Jinja2

Configure the Flask app to restrict Jinja2's ability to access Python objects and system attributes. Create a custom environment that disables finalize hooks that could expose internal objects:

from jinja2 import Environment, select_autoescape

env = Environment(
    autoescape=select_autoescape(['html', 'xml']),
    enable_async=True
)
# Avoid adding global objects that expose database or config internals
env.globals.pop('config', None)

Validate and encode all template variables

Explicitly validate and encode any data passed to templates. Use Flask's escape function or autoescaping environments to neutralize injected template syntax:

from flask import escape

@app.route('/profile')
def profile():
    user_id = request.args.get('id')
    # Validate user_id before using it in a query
    if not user_id or not user_id.isdigit():
        return "Invalid user identifier", 400
    user_data = db.session.execute(
        text("SELECT name FROM users WHERE id = :uid"),
        {"uid": int(user_id)}
    ).fetchone()
    return f"Name: {escape(user_data.name) if user_data else 'Unknown'}"

Avoid exposing database objects to templates

Ensure that Flask's app config or global objects do not contain direct references to Cockroachdb engine or session instances. If configuration is needed, pass only non-sensitive values such as feature flags or timeouts, and retrieve database connections within view functions using controlled patterns:

from flask import g

def get_db():
    if 'db' not in g:
        g.db = create_cockroachdb_engine()
    return g.db

@app.teardown_appcontext
def close_db(error):
    db = g.pop('db', None)
    if db is not None:
        db.dispose()

These practices reduce the attack surface by preventing template code from reaching the database layer and by ensuring that any data returned from Cockroachdb is safely encoded before inclusion in rendered output.