Bola Idor in Flask with Hmac Signatures

Broken Object Level Authorization (BOLA/IDOR) occurs when an API exposes object references (IDs, keys, slugs) without verifying that the requesting actor is authorized to access that specific object. In Flask, combining predictable resource identifiers with HMAC signatures for integrity can still lead to BOLA/IDOR when the signature does not bind authorization to the object being accessed.

Consider a Flask endpoint that uses an HMAC signature to validate that a URL parameter has not been tampered with, but does not check whether the authenticated (or unauthenticated) subject has permission to view the resource. For example, an endpoint accepting user_id and a signature may verify the signature is valid, yet proceed to fetch and return data for any user_id provided. Because the signature only ensures the parameter has not been modified, it does not enforce ownership or access control, BOLA/IDOR is exposed.

An attacker can iterate over valid resource identifiers (e.g., numeric user IDs or UUIDs), observe which ones return 200 OK versus 404 or 403, and retrieve other users’ data. If the HMAC secret is static and the signature does not include a scope or permission claim, replaying or re-signing different identifiers becomes straightforward. The vulnerability is amplified when endpoints expose sensitive fields (PII, internal references) and rely solely on signature integrity rather than object-level authorization checks.

In OpenAPI terms, if path parameters like {user_id} are not coupled with runtime authorization checks and the spec does not describe scope-based constraints, the unauthenticated or low-privilege attacker surface remains large. Even with HMAC signatures ensuring data integrity, without explicit ownership or tenant checks the API returns data for objects the caller should not access, which is the classic BOLA/IDOR pattern.

To mitigate BOLA/IDOR when using HMAC signatures in Flask, ensure the signature binds not only data integrity but also the context of the operation and the caller’s authorization. Below are concrete patterns that combine HMAC verification with object-level authorization checks.

1. Include resource ownership in the signed payload

Sign a compound payload that includes the resource identifier and the user identifier (or tenant). Verify the signature, then confirm that the resource belongs to the subject derived from the payload or session.

import hash_hmac
import time
import json
from flask import Flask, request, jsonify, g

app = Flask(__name__)
SECRET = b'super-secret-key'  # store securely, e.g., env var

def verify_hmac(data: dict, received_sig: str) -> bool:
    payload = json.dumps(data, separators=(',', ':'), sort_keys=True).encode()
    expected = hmac.new(SECRET, payload, 'sha256').hexdigest()
    return hmac.compare_digest(expected, received_sig)

@app.route('/users/<user_id>')
def get_user(user_id):
    sig = request.args.get('sig')
    timestamp = request.args.get('ts')
    if not sig or not timestamp:
        return jsonify({'error': 'missing signature or timestamp'}), 400
    # reject old requests to mitigate replay
    if abs(time.time() - int(timestamp)) > 300:
        return jsonify({'error': 'request expired'}), 400
    # payload includes both the resource and the subject
    payload = {'user_id': user_id, 'actor_id': g.authenticated_user_id, 'ts': timestamp}
    if not verify_hmac(payload, sig):
        return jsonify({'error': 'invalid signature'}), 403
    # BOLA check: ensure actor is allowed to view this user_id
    if payload['actor_id'] != payload['user_id'] and not g.is_admin:
        return jsonify({'error': 'forbidden'}), 403
    # proceed to fetch and return data
    user_data = fetch_user_from_db(payload['user_id'])
    return jsonify(user_data)

2. Use scoped, short-lived nonces to prevent replay and enforce context

Include a nonce or a one-time token within the signed data and store used nonces for the request window. This prevents replay attacks and ties the signature to a specific operation scope.

import os
import hmac
import hashlib
import time
import json
from flask import Flask, request, jsonify, g

app = Flask(__name__)
SECRET = os.environb[b'HMAC_SECRET']
NONCE_STORE = set()  # in production use a bounded cache or Redis with TTL

def sign_payload(payload: dict) -> str:
    payload_bytes = json.dumps(payload, separators=(',', ':'), sort_keys=True).encode()
    return hmac.new(SECRET, payload_bytes, 'sha256').hexdigest()

def verify_and_store_nonce(nonce: str) -> bool:
    if nonce in NONCE_STORE:
        return False
    NONCE_STORE.add(nonce)
    # prune store periodically in real implementation
    return True

@app.route('/data/<record_id>')
def get_record(record_id):
    sig = request.args.get('sig')
    nonce = request.args.get('nonce')
    actor = g.authenticated_user_id
    if not verify_and_store_nonce(nonce):
        return jsonify({'error': 'invalid or replayed nonce'}), 403
    payload = {
        'record_id': record_id,
        'actor': actor,
        'nonce': nonce,
        'ts': int(time.time())
    }
    expected_sig = sign_payload(payload)
    if not hmac.compare_digest(expected_sig, sig):
        return jsonify({'error': 'signature mismatch'}), 403
    # BOLA enforcement: actor must own or be scoped for record_id
    if not user_can_access(actor, record_id):
        return jsonify({'error': 'forbidden - insufficient permissions'}), 403
    return jsonify(fetch_record(record_id))

3. Combine signature verification with explicit access control checks

Treat HMAC as integrity assurance only, and always enforce a separate authorization layer (e.g., role/ownership checks, ABAC/ACL). Never trust the decoded subject in the signature alone; validate against a policy store.

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

def require_scope_for_resource(scope: str):
    def decorator(f):
        @wraps(f)
        def wrapped(*args, **kwargs):
            sig = request.args.get('sig')
            # verify integrity
            payload = verify_hmac_and_get_payload(sig)
            if not payload:
                return jsonify({'error': 'invalid signature'}), 403
            g.auth_subject = payload.get('sub')
            # BOLA enforcement: compare resource ownership or roles
            resource_id = kwargs.get('resource_id')
            if not is_authorized(g.auth_subject, scope, resource_id):
                return jsonify({'error': 'forbidden - object level authorization failed'}), 403
            return f(*args, **kwargs)
        return wrapped
    return decorator

@app.route('/records/<resource_id>')
@require_scope_for_resource(scope='records:read')
def get_record(resource_id):
    return jsonify(fetch_record(resource_id))

These patterns ensure HMAC signatures are part of a broader security model where object-level authorization is explicitly enforced. This prevents BOLA/IDOR by binding the signed context to the actor and the resource, and by validating permissions on every request regardless of signature validity.