Bola Idor in Koa with Dynamodb

Broken Object Level Authorization (BOLA) occurs when an API exposes one user’s resource to another by failing to enforce ownership or tenant checks at the object level. Using Koa with Amazon DynamoDB can inadvertently create BOLA when route parameters such as :id or query values are passed directly to DynamoDB queries without verifying that the requesting user has permission to access the targeted item.

Consider a Koa route like /users/:userId/profile. If the handler builds a DynamoDB GetItem request using userId from the URL without cross-checking it against the authenticated subject’s identifier, any authenticated user can change the ID in the URL and read or act on other users’ profiles. This is a classic BOLA/IDOR pattern: authorization is missing at the object level even though authentication may be present.

DynamoDB’s key-based model amplifies the risk. Because primary keys are often used directly as data access boundaries, omitting ownership validation means the API relies only on the client-supplied key. For example, a query like Key: { PK: { S: `USER#${userId}` } } that uses the route parameter without confirming that the authenticated subject owns that PK enables horizontal privilege escalation. Additionally, if secondary indexes are used without similar checks, BOLA can manifest across different query paths. In a Node.js Koa service, this typically happens when authorization logic is limited to route-level middleware and does not re-validate object ownership within the DynamoDB request itself.

Real-world attack patterns include changing numeric or UUID route parameters, manipulating URL paths, or leveraging predictable keys to enumerate resources. These map to OWASP API Top 10 A01:2023 broken object level authorization and commonly appear in APIs that store user data in DynamoDB without a consistent authorization model. For instance, an attacker authenticated as user A could fetch or modify user B’s data by simply altering the userId segment in the URL, provided the backend does not enforce per-object ownership checks against the authenticated identity.

To summarize, the combination of Koa routing, DynamoDB key-based access, and missing or inconsistent ownership checks creates a BOLA/IDOR surface. The vulnerability is not inherent to Koa or DynamoDB, but arises when route parameters are used as the sole determinant of access without validating that the authenticated subject is authorized to interact with the referenced object.

Remediation centers on ensuring that every DynamoDB request includes a server-side check that the authenticated subject is authorized to access the target item. This typically means deriving the resource’s owner from the data itself (e.g., a PK or userId attribute) and comparing it to the authenticated subject, rather than trusting the client-supplied identifier alone.

Below are concrete patterns for Koa with the AWS SDK for JavaScript v3. Prefer parameterized queries and avoid concatenating route parameters directly into keys.

Example: Safe GetItem with ownership check

import { DynamoDBClient, GetItemCommand } from '@aws-sdk/client-dynamodb';
import { unmarshall } from '@aws-sdk/util-dynamodb';

const client = new DynamoDBClient({ region: 'us-east-1' });

async function getUserProfile(ctx) {
  const { userId } = ctx.params; // route param
  const subjectId = ctx.state.user?.sub; // authenticated subject from auth middleware

  if (!subjectId) {
    ctx.status = 401;
    return;
  }

  // Ensure the requested userId matches the authenticated subject
  if (userId !== subjectId) {
    ctx.status = 403;
    return;
  }

  const command = new GetItemCommand({
    TableName: process.env.PROFILES_TABLE,
    Key: {
      PK: { S: `USER#${userId}` },
      SK: { S: 'PROFILE' }
    }
  });

  const response = await client.send(command);
  if (!response.Item) {
    ctx.status = 404;
    return;
  }

  ctx.body = unmarshall(response.Item);
}

Example: Query with partition key ownership

When using a composite key design, always include the authenticated subject in the partition key and enforce it server-side.

import { DynamoDBClient, QueryCommand } from '@aws-sdk/client-dynamodb';
import { unmarshall } from '@aws-sdk/util-dynamodb';

const client = new DynamoDBClient({ region: 'us-east-1' });

async function listUserPosts(ctx) {
  const subjectId = ctx.state.user?.sub;
  if (!subjectId) {
    ctx.status = 401;
    return;
  }

  // Partition key includes the subject to ensure isolation
  const command = new QueryCommand({
    TableName: process.env.POSTS_TABLE,
    KeyConditionExpression: 'PK = :pk AND begins_with(SK, :skPrefix)',
    ExpressionAttributeValues: {
      ':pk': { S: `USER#${subjectId}` },
      ':skPrefix': { S: 'POST#' }
    }
  });

  const response = await client.send(command);
  ctx.body = response.Items?.map(unmarshall) || [];
}

General practices

• Never trust route parameters as the sole access control mechanism; re-validate ownership against the authenticated subject on every request.
• Design your DynamoDB key schema so that data isolation is enforced by the key structure (e.g., prefixing keys with the user ID), reducing the chance of accidental cross-object access.
• Use middleware to attach the authenticated subject to ctx.state and consistently apply checks across routes.
• For broader protection, combine these patterns with DynamoDB condition expressions where appropriate to enforce ownership at the database level.

These fixes ensure that even if an attacker manipulates URL parameters, the server will not perform unauthorized DynamoDB operations, effectively mitigating BOLA/IDOR in a Koa + DynamoDB stack.