Broken Access Control in Koa with Dynamodb

Broken Access Control in a Koa application that uses DynamoDB typically arises when authorization checks are missing, incomplete, or bypassed before constructing and sending requests to DynamoDB. Because DynamoDB is a managed database service, the service itself does not enforce per-request user-level authorization; it processes requests exactly as the caller instructs. If your Koa middleware does not enforce attribute-level constraints (such as ensuring a user can only access their own records), the API surface becomes vulnerable to IDOR and BOLA-style attacks.

In this stack, a common pattern is to authenticate a request (e.g., via JWT) to identify a user, but then omit or incorrectly apply authorization logic before issuing DynamoDB operations. For example, a route like /api/users/:userId/profile might extract userId from the URL and directly use it in a GetItem or Query request without verifying that the authenticated subject matches that userId. Because DynamoDB will return the requested item if the caller has permission to read any item in the table (or a broad IAM policy grants access), the absence of a server-side ownership check results in unauthorized data exposure.

DynamoDB’s low-level request model makes it especially important to validate and scope every query. If your Koa code builds a KeyConditionExpression or FilterExpression using unchecked user input, an attacker can manipulate parameters to access other users’ data or to perform operations outside intended scope. For example, a missing or weak check on a projectId path or query parameter may allow an attacker to substitute another project identifier and enumerate or modify records they should not see.

Another contributing factor is over-permissive IAM policies attached to the runtime identity (such as an EC2 instance profile or container task role). If the policy allows broad dynamodb:GetItem or dynamodb:Query actions on a table without resource-level constraints, the backend effectively trusts any code running in that context. Combined with missing authorization logic in Koa, this enables vertical or horizontal privilege escalation via crafted requests that reference arbitrary item keys.

To detect this pattern with middleBrick, a scan will exercise unauthenticated and authenticated paths that include user-controlled identifiers, validating whether responses differ based on authorization context. Findings will highlight missing attribute-level checks and overly permissive configurations, providing remediation guidance specific to Koa and DynamoDB patterns. Note that middleBrick detects and reports these issues; it does not fix or block requests.

Remediation focuses on ensuring every DynamoDB request is scoped to the authenticated subject and that authorization is enforced before any database operation. Below are concrete Koa examples that demonstrate secure patterns.

1) Enforce ownership before querying

Always resolve the authenticated subject (e.g., from a JWT) and use it to constrain DynamoDB requests. Do not trust URL parameters for ownership checks.

const jwt = require('koa-jwt');
const AWS = require('aws-sdk');
const dynamodb = new AWS.DynamoDB.DocumentClient();

const authenticate = jwt({ secret: process.env.JWT_SECRET, algorithms: ['HS256'] });

router.get('/api/profile', authenticate, async (ctx) => {
  const subject = ctx.state.user.sub; // authenticated subject from JWT
  const userId = ctx.query.userId; // do NOT trust this for ownership

  // Enforce: only allow access to the authenticated user's own profile
  if (subject !== userId) {
    ctx.status = 403;
    ctx.body = { error: 'forbidden' };
    return;
  }

  const params = {
    TableName: process.env.PROFILES_TABLE,
    Key: { userId: subject }
  };

  try {
    const { Item } = await dynamodb.get(params).promise();
    ctx.body = Item || { message: 'not found' };
  } catch (err) {
    ctx.status = 500;
    ctx.body = { error: 'server error' };
  }
});

2) Scope queries by tenant or project using a verified mapping

When modeling multi-tenant data, store a mapping (e.g., in a separate table or as a verified attribute) and validate it before issuing DynamoDB operations.

router.get('/api/projects/:projectId/data', authenticate, async (ctx) => {
  const subject = ctx.state.user.sub;
  const requestedProjectId = ctx.params.projectId;

  // Verify that the subject has access to the requested project
  const membershipParams = {
    TableName: process.env.MEMBERSHIPS_TABLE,
    Key: { userId: subject, projectId: requestedProjectId }
  };

  const { Item } = await dynamodb.get(membershipParams).promise();
  if (!Item) {
    ctx.status = 403;
    ctx.body = { error: 'access denied to project' };
    return;
  }

  // Safe to query because membership was verified
  const dataParams = {
    TableName: process.env.PROJECT_DATA_TABLE,
    KeyConditionExpression: 'projectId = :pid',
    ExpressionAttributeValues: { ':pid': requestedProjectId }
  };

  const { Items } = await dynamodb.query(dataParams).promise();
  ctx.body = Items;
});

3) Avoid over-permissive IAM by scoping at runtime

While IAM policy adjustments are outside the application code, your Koa app should assume least privilege and include checks that enforce what the policy alone cannot. For example, do not perform operations on item identifiers supplied directly by the client without validation against the authenticated subject.

These patterns ensure that authorization is enforced in Koa before any DynamoDB request, preventing IDOR and BOLA issues. middleBrick’s scans can validate these controls by checking authenticated and unauthentiated paths and reporting findings aligned with frameworks such as OWASP API Top 10.