Bola Idor in Flask with Dynamodb

Broken Object Level Authorization (BOLA) occurs when an API exposes object identifiers (IDs) without verifying that the requesting user has permission to access the corresponding resource. In a Flask application that uses DynamoDB as the persistence layer, BOLA commonly arises when a route accepts a user-controlled ID (e.g., a record or item key) and queries DynamoDB without confirming the authenticated subject owns or is authorized to view that item.

DynamoDB’s key design influences the risk. Items are often keyed by user identifiers (e.g., user_id as a partition key) or by globally unique strings (e.g., UUIDs). If a Flask route uses a value such as item_id directly in a GetItem or Query request, and does not re-bind that value to the requester’s identity, an attacker can manipulate the ID to access other users’ data. For example, changing /records/123 to /records/124 might return another user’s record if the backend does not enforce ownership checks.

Consider a Flask route that retrieves a “note” by ID:

from flask import Flask, request
import boto3
from boto3.dynamodb.conditions import Key

app = Flask(__name__)
dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
table = dynamodb.Table('NotesTable')

@app.route('/notes/', methods=['GET'])
def get_note(note_id):
    # Missing authorization check: does the current user own this note_id?
    response = table.get_item(Key={'note_id': note_id})
    item = response.get('Item')
    if item:
        return {'note': item}, 200
    return {'error': 'Not found'}, 404

In this example, note_id is taken directly from the URL. If the item’s primary key is simply a UUID or an integer, and the application does not ensure that the authenticated user’s ID is part of the key or validated against it, the endpoint is vulnerable to BOLA. An authenticated user can enumerate or access notes belonging to others by iterating IDs or guessing valid identifiers.

DynamoDB’s lack of native row-level permissions means authorization must be enforced in application logic. A common insecure pattern is to use a Global Secondary Index (GSI) for querying without scoping to the user. For instance, querying a GSI for a status or tag without including the user identifier in the key condition can expose data across users.

To tie this to real-world risks, consider the OWASP API Top 10 2023 category Broken Object Level Authorization. BOLA is pervasive in APIs that expose numeric or predictable IDs. In DynamoDB, the partition key design determines the attack surface; if the key does not incorporate tenant or user context, it’s easy for an attacker to pivot across records. Additionally, insufficient logging and monitoring can delay detection of unauthorized object access, which is a related control in many compliance frameworks such as SOC 2 and PCI-DSS.

Remediation centers on ensuring that every data access operation includes the user’s identity as part of the key expression and that the application validates ownership before returning data. Below are concrete, DynamoDB-aware patterns for Flask.

1. Design keys to include user context

Model your DynamoDB table so that the partition key embeds the user identifier. This makes it difficult to access another user’s items unless the application explicitly uses the target user’s key.

# Example item structure:
# PK: USER#user_id
# SK: NOTE#note_id
# GSI1PK: NOTE#status (if needed for queries)

2. Scoped GetItem with ownership validation

Always include the authenticated user’s ID in the key and avoid exposing raw item IDs in URLs. If you must expose IDs, encode them with user context (e.g., base64(user_id:note_id)) and decode before querying.

from flask import Flask, g
import boto3
from botocore.exceptions import ClientError
import base64
import json

app = Flask(__name__)
dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
table = dynamodb.Table('NotesTable')

def get_current_user_id():
    # Assume g.user_id is set by authentication middleware
    return g.user_id

@app.route('/notes/', methods=['GET'])
def get_note(encoded_id):
    try:
        decoded = base64.urlsafe_b64decode(encoded_id + '==').decode('utf-8')
        user_id, note_id = decoded.split(':', 1)
    except (ValueError, binascii.Error):
        return {'error': 'Invalid identifier'}, 400

    if user_id != get_current_user_id():
        return {'error': 'Forbidden'}, 403

    response = table.get_item(
        Key={
            'PK': f'USER#{user_id}',
            'SK': f'NOTE#{note_id}'
        }
    )
    item = response.get('Item')
    if item:
        return {'note': item}, 200
    return {'error': 'Not found'}, 404

3. Query with user-scoped partition key

If you use a GSI for querying, include the user identifier in the partition key of the index or in the key condition expression.

@app.route('/notes', methods=['GET'])
def list_notes_by_status():
    user_id = get_current_user_id()
    status = request.args.get('status', 'active')
    try:
        response = table.query(
            IndexName='StatusIndex',
            KeyConditionExpression=Key('GSI1PK').eq(f'USER#{user_id}') & Key('status').eq(status)
        )
        items = response.get('Items', [])
        return {'notes': items}, 200
    except ClientError as e:
        return {'error': str(e)}, 500

4. Enforce authorization before data access

Even with scoped keys, add explicit checks when relationships are many-to-many or when using references. Use DynamoDB transactions or conditional writes where appropriate to prevent tampering.

@app.route('/notes/', methods=['DELETE'])
def delete_note(encoded_id):
    user_id = get_current_user_id()
    # decode and validate as above
    response = table.delete_item(
        Key={
            'PK': f'USER#{user_id}',
            'SK': f'NOTE#{note_id}'
        },
        ConditionExpression='attribute_exists(#pk)',
        ExpressionAttributeNames={'#pk': 'PK'}
    )
    return {'status': 'deleted'}, 200

These patterns ensure that DynamoDB operations are bound to the authenticated subject, mitigating BOLA. Remember that middleBrick scans can help identify such authorization gaps by testing unauthenticated and authenticated scenarios, mapping findings to frameworks like OWASP API Top 10 and providing remediation guidance.