Nosql Injection in Flask with Api Keys

Nosql Injection occurs when an attacker can manipulate a NoSQL query by injecting malicious input. In Flask applications that rely on API keys for access control, insecure handling of these keys can amplify injection risks. When API keys are passed as query parameters, headers, or request bodies without proper validation, they may be concatenated into NoSQL queries, effectively extending the attack surface to the database layer.

Consider a Flask endpoint that retrieves user data based on a provided API key and a user identifier. If the API key is used directly in a PyMongo query without sanitization, an attacker who discovers or guesses a valid key might also inject crafted input into the identifier field. For example, the identifier could be manipulated to change the query logic, such as using operators like $ne or $where to bypass intended filters or extract unintended documents. Because API keys often grant broad read permissions, successful Nosql Injection can lead to unauthorized data access, data exposure, or enumeration of sensitive collections.

Another scenario involves using API keys to authenticate requests that include JSON payloads. If the server deserializes JSON input and embests values directly into NoSQL operations, special characters or nested operators can alter query structure. This can bypass authentication checks tied to the API key or reveal data belonging to other users. The combination of API keys and unsanitized NoSQL queries creates a chain where authentication controls fail due to injection, undermining the intended security model.

Additionally, logging or error messages that include the API key or query fragments may aid attackers in refining injection patterns. Flask applications that expose stack traces or verbose logs can inadvertently disclose information that facilitates further exploitation. Because NoSQL databases often lack the strict schema constraints of relational databases, injected payloads can have more varied and unexpected effects, making detection harder without comprehensive scanning.

To mitigate Nosql Injection risks related to API keys in Flask, enforce strict input validation, avoid direct concatenation of user-controlled values into database queries, and use parameterized or whitelisted patterns. Below are concrete, secure coding examples demonstrating these principles.

1. Validate and sanitize API keys and identifiers

Use allowlists for expected formats and reject unexpected characters early in the request lifecycle.

import re
from flask import Flask, request, jsonify

app = Flask(__name__)

def is_valid_api_key(key: str) -> bool:
    # Allow only alphanumeric keys of fixed length
    return bool(re.fullmatch(r'[A-Za-z0-9]{32}', key))

def is_valid_user_id(uid: str) -> bool:
    # Allow only alphanumeric user IDs
    return bool(re.fullmatch(r'[a-zA-Z0-9_-]{1,64}', uid))

@app.route('/user')
def get_user():
    api_key = request.args.get('api_key', '')
    user_id = request.args.get('user_id', '')
    if not is_valid_api_key(api_key):
        return jsonify({'error': 'Invalid API key'}), 400
    if not is_valid_user_id(user_id):
        return jsonify({'error': 'Invalid user identifier'}), 400
    # Proceed to query with validated inputs
    return jsonify({'status': 'ok'})

2. Use safe query building with PyMongo (parameterized patterns)

Avoid string interpolation. Use dictionary-based queries and restrict fields explicitly.

from flask import Flask, request, jsonify
from pymongo import MongoClient

app = Flask(__name__)
client = MongoClient('mongodb://localhost:27017')
db = client['mydatabase']

@app.route('/user')
def get_user():
    api_key = request.args.get('api_key', '')
    user_id = request.args.get('user_id', '')

    if not is_valid_api_key(api_key):
        return jsonify({'error': 'Invalid API key'}), 400
    if not is_valid_user_id(user_id):
        return jsonify({'error': 'Invalid user identifier'}), 400

    # Safe: use dictionary query with exact field matches
    user = db.users.find_one({
        'api_key': api_key,
        'user_id': user_id
    }, {
        'profile': 1,
        'preferences': 1
    })
    if user is None:
        return jsonify({'error': 'Not found'}), 404
    return jsonify(user)

3. Separate authentication from query logic

Use API keys strictly for access control at the middleware or gateway level, and avoid embedding them in database queries. Instead, map the key to a user or scope before constructing queries.

from flask import Flask, request, jsonify, g
from functools import wraps

app = Flask(__name__)

def require_api_key(f):
    @wraps(f)
    def decorated(*args, **kwargs):
        api_key = request.headers.get('x-api-key', '')
        if not is_valid_api_key(api_key):
            return jsonify({'error': 'Invalid API key'}), 401
        # Perform lightweight lookup to confirm key scope without querying user data
        if not key_has_permission(api_key):
            return jsonify({'error': 'Forbidden'}), 403
        g.api_key = api_key
        return f(*args, **kwargs)
    return decorated

def key_has_permission(key: str) -> bool:
    # Check against a short-lived cache or configuration
    allowed_keys = {'abc123...', 'def456...'}
    return key in allowed_keys

@app.route('/data')
@require_api_key
def get_data():
    # Query without including the API key in the NoSQL filter
    results = db.data.find({}, {'sensitive': 0})
    return jsonify(list(results))

4. Disable MongoDB operator injection via input validation

Ensure that user-controlled values cannot be interpreted as operators by rejecting inputs that start with $ in relevant fields.

@app.route('/search')
def search_items():
    term = request.args.get('q', '')
    if term.startswith('$'):
        return jsonify({'error': 'Invalid query'}), 400
    # Safe: term is used as a literal string, not as a key
    results = db.items.find({
        '$or': [
            {'name': {'$regex': term, '$options': 'i'}},
            {'description': {'$regex': term, '$options': 'i'}}
        ]
    })
    return jsonify([{'name': r['name']} for r in results])