Nosql Injection in Flask with Hmac Signatures

NoSQL injection in Flask applications that rely on HMAC signatures can occur when user-controlled data is used to build database queries without proper validation or escaping, while the integrity check on the signature is bypassed or misinterpreted. In Flask, developers often use HMAC signatures to verify that incoming requests have not been tampered with, for example to protect webhook payloads or API parameters. If the code validates the HMAC and then directly uses the request data in a NoSQL query, an attacker who does not need to forge a valid signature can still inject if the application trusts data that should have been sanitized. For example, an attacker may supply a JSON key such as username with a value like {'$regex': '.*', '$options': ''} in a login route, and if the backend uses pymongo to perform a query like db.users.find_one({'username': request.json['username']}), the dictionary is interpreted by MongoDB as an operator injection rather than a literal string value.

The combination of Flask routing, HMAC-based integrity checks, and NoSQL backends can create subtle trust boundaries. A developer might assume that because the HMAC is valid, the payload is safe, but HMAC does not sanitize data—it only ensures the payload has not been altered after signing. If the signing key is exposed or the verification logic has a bug (e.g., timing attacks or incorrect comparison), an attacker may bypass the signature check. Even when the signature is verified correctly, using raw user input in query filters can lead to injection. Common patterns that are risky include concatenating JSON strings into queries or using bson.json_util without strict schema validation. The OWASP API Security Top 10 categorizes this under API1:2023 Broken Object Level Authorization and API10:2023 Injection, because the API exposes an unauthenticated or weakly authenticated attack surface where malformed data reaches the database layer.

Real-world examples include endpoints that accept an id parameter for fetching user records, where the parameter is signed to prevent tampering but then passed directly to a MongoDB find call. An attacker could supply id as {'$ne': null} to retrieve unintended records if the query is not constructed with strict type and schema checks. Another scenario involves update operations where user input is used in the update document without serialization safeguards, enabling operators like $set or $inc to be injected. These patterns highlight why NoSQL injection remains relevant even when integrity protection like HMAC signatures is present: signatures guard integrity, not input safety.

To remediate NoSQL injection risks in Flask while using HMAC signatures, treat the signature as integrity assurance only and apply strict input validation and type-aware query construction. Always deserialize and validate incoming data against a defined schema before using it in database operations. For MongoDB with PyMongo, prefer dictionary-based queries with explicitly typed values, and avoid passing raw JSON objects directly into query or update documents.

Below are concrete, safe patterns in Flask using HMAC verification and PyMongo.

import hmac
import hashlib
import json
from flask import Flask, request, jsonify
from pymongo import MongoClient

app = Flask(__name__)
client = MongoClient('mongodb://localhost:27017')
db = client['mydb']
SECRET_KEY = b'super-secret-key'

def verify_hmac(data: bytes, signature: str) -> bool:
    expected = hmac.new(SECRET_KEY, data, hashlib.sha256).hexdigest()
    return hmac.compare_digest(expected, signature)

@app.route('/user', methods=['POST'])
def create_user():
    payload = request.get_data()
    signature = request.headers.get('X-Signature')
    if not signature or not verify_hmac(payload, signature):
        return jsonify({'error': 'invalid signature'}), 401

    try:
        body = json.loads(payload)
    except json.JSONDecodeError:
        return jsonify({'error': 'invalid json'}), 400

    # Validate and sanitize each field explicitly
    username = body.get('username')
    if not isinstance(username, str) or not (3 <= len(username) <= 64):
        return jsonify({'error': 'invalid username'}), 400

    # Use typed, schema-aware insertion instead of raw user input in query context
    user_doc = {
        'username': username,
        'email': body.get('email', ''),
        'created_at': datetime.utcnow()
    }
    result = db.users.insert_one(user_doc)
    return jsonify({'id': str(result.inserted_id)}), 201

@app.route('/user/', methods=['GET'])
def get_user(user_id):
    payload = request.get_data()
    signature = request.headers.get('X-Signature')
    if not signature or not verify_hmac(payload, signature):
        return jsonify({'error': 'invalid signature'}), 401

    # Validate user_id as a string or ObjectId, never concatenate
    if not isinstance(user_id, str):
        return jsonify({'error': 'invalid user id'}), 400

    # Safe query: use _id with proper type conversion if needed
    from bson.objectid import ObjectId
    try:
        obj_id = ObjectId(user_id)
    except:
        return jsonify({'error': 'invalid user id format'}), 400

    user = db.users.find_one({'_id': obj_id}, {'username': 1, 'email': 1})
    if not user:
        return jsonify({'error': 'not found'}), 404
    return jsonify(user), 200

Key takeaways: validate and type-check all inputs, use schema-aware operations, and never build query filters by string interpolation. HMAC signatures protect integrity but do not replace sanitization. These practices align with findings that map to OWASP API Top 10 categories and can reduce the risk of NoSQL injection in Flask-based APIs scanned by tools such as middleBrick.