Server Side Template Injection in Flask with Firestore

Server Side Template Injection (SSTI) occurs when an attacker can inject template code that is executed on the server. In Flask, this typically arises when a developer uses a templating engine such as Jinja2 and incorporates untrusted input into the template layer without proper validation or escaping. When the backend also interacts with Google Cloud Firestore to fetch or store data, the combination can amplify impact because sensitive data retrieved from Firestore may be rendered into the template, and malicious template code can potentially influence how Firestore queries are constructed or how their results are displayed.

Consider a Flask route that takes a user-supplied user_id and renders a profile page using Jinja2 while also reading user data from Firestore:

from flask import Flask, request, render_template_string
from google.cloud import firestore

app = Flask(__name__)
db = firestore.Client()

@app.route('/profile')
def profile():
    user_id = request.args.get('user_id', 'default')
    doc_ref = db.collection('users').document(user_id)
    doc = doc_ref.get()
    user_data = doc.to_dict() if doc.exists else {}
    template = '{{ user.name }} — {{ user.email }}'
    return render_template_string(template, user=user_data)

If user.name or user.email were attacker-controlled and the template string were built using untrusted input, an attacker could supply a payload such as {{ config }} or {% for c in [].__class__.__base__.__subclasses__() %}{% if c.__name__ == 'catch_warnings' %}{{ c.__init__.__globals__['os'].popen('id').read() }}{% endif %}{% endfor %} depending on the Flask and Jinja2 versions. Because the data is merged into the template at render time, the injected code executes in the server context. This can lead to information disclosure, remote code execution, or access to other Firestore documents if the application’s service account permissions are broad.

Additionally, SSTI can expose Firestore query construction patterns if the attacker is able to influence which fields are projected or which filters are applied through template logic. For example, if the template decides which document fields to include based on user input, an attacker may manipulate the output to reveal sensitive fields not intended for the client. The risk is compounded when developers mistakenly believe that Firestore access rules alone fully protect data; server-side template logic must also be hardened because the application process itself holds elevated permissions needed to read and write data.

Flask’s default configuration does not prevent developers from passing untrusted strings to render_template_string. Without strict input validation, allowlisting, or output encoding, SSTI becomes a practical threat. Security checks that test authentication, input validation, and data exposure are particularly important for this stack to detect unsafe usage of templates and overprivileged service accounts.

Remediation focuses on removing any direct concatenation of user input into template logic and ensuring Firestore access is performed safely with least privilege. Do not use render_template_string with unsanitized input; prefer render_template with explicit template files. If dynamic templates are required, validate and sanitize all variables before they reach the template layer, and apply output escaping appropriate for the context (HTML, JavaScript URL, style).

Below is a secure pattern for reading from Firestore in Flask without exposing SSTI:

from flask import Flask, request, render_template
from google.cloud import firestore
from markupsafe import escape

app = Flask(__name__)
db = firestore.Client()

@app.route('/profile')
def profile():
    user_id = request.args.get('user_id')
    if not user_id or not isinstance(user_id, str) or not user_id.isalnum():
        return 'Invalid user identifier', 400
    # Use a strict allowlist for document IDs if possible
    doc_ref = db.collection('users').document(user_id)
    doc = doc_ref.get()
    if not doc.exists:
        return 'User not found', 404
    user_data = doc.to_dict()
    # Explicitly pick only safe fields to pass to the template
    safe_data = {
        'name': escape(user_data.get('name', '')),
        'email': escape(user_data.get('email', ''))
    }
    return render_template('profile.html', user=safe_data)

Key remediation steps:

• Validate and restrict user_id to expected formats (e.g., alphanumeric) before using it as a Firestore document key.
• Use render_template with static template files instead of render_template_string with user-influenced strings.
• Explicitly select and escape only the fields you intend to render; avoid passing raw document dictionaries to templates.
• Apply the principle of least privilege to the Firestore service account used by the Flask app so that it cannot read or write unrelated collections.
• Add logging and monitoring around access patterns to detect abuse or anomalous queries.

These measures reduce the attack surface by ensuring that template rendering cannot be influenced by untrusted data and that Firestore interactions are bounded by validated inputs and minimal permissions. Regular scans that include authentication checks, input validation, and data exposure assessments can help verify that such safeguards are effective in production.