Auth Bypass in Koa with Dynamodb

An Auth Bypass in a Koa application using DynamoDB typically arises from incorrect authorization checks, overly permissive IAM policies, or insecure direct object references (IDOR) when looking up resources by key. In this stack, Koa handles HTTP routing and middleware, while DynamoDB serves as the data store. If authorization is applied only at the route level or omitted for "internal" endpoints, an authenticated user may be able to access or modify another user's data by supplying a different record key (e.g., a different user ID) in the request.

DynamoDB does not enforce row-level permissions by default. Authorization must be implemented in application code or via fine-grained IAM roles with conditions. A common pattern is to pass a user identifier (such as a Cognito identity ID or session subject) into the application and include it as part of the DynamoDB key or as a filter condition. If the Koa route does not validate that the item being accessed belongs to the requesting user, the API may inadvertently expose data across users, resulting in an IDOR/BOLA (Insecure Direct Object Reference/Logic of Association) issue.

For example, a Koa route that retrieves user profile data using a path parameter like /profile/:userId and forms a DynamoDB query without verifying that the authenticated subject matches userId can allow horizontal privilege escalation. An attacker who is logged in as user A can change the request to user B's ID and read user B's profile if the backend does not enforce ownership. Similarly, DynamoDB queries that use a generic table scan or secondary index without scoping to the requesting user can expose more data than intended.

The use of unauthenticated or misconfigured Cognito identities can further widen the attack surface. If the Koa backend trusts caller identity from the request without re-verifying permissions, and DynamoDB resource policies or IAM roles allow broad read or write access, an unauthenticated or low-privilege caller might perform actions such as GetItem or UpdateItem on sensitive records. Middleware that fails to validate scope, session origin, or token claims in the context of DynamoDB access can lead to auth bypass even when the framework enforces authentication.

Because DynamoDB is a managed NoSQL service, developers must enforce access controls at the application layer. Middleware in Koa should resolve the correct subject, construct parameterized queries with condition expressions (e.g., ensuring the partition key includes the user identifier), and avoid using elevated or shared credentials for routine data access. Combining route protection in Koa with tightly scoped IAM policies for DynamoDB reduces the likelihood of auth bypass by ensuring that even if an identifier is supplied, the operation is denied unless the caller is explicitly authorized for that specific item.

Remediation centers on scoping every DynamoDB operation to the authenticated subject and validating authorization before performing any database call. In Koa, this is typically implemented as middleware that attaches the user identity to the context and ensures that all downstream handlers enforce ownership or role-based checks.

First, ensure that your Koa routes do not rely solely on route parameters for access control. Instead, derive the subject from the authenticated session or token and use it in a filter or key condition. Below is an example of a secure Koa route that retrieves a user's own profile from DynamoDB using the AWS SDK for JavaScript v3, with the subject derived from the session and enforced in the query.

const { DynamoDBClient, GetCommand } = require("@aws-sdk/client-dynamodb");
const { marshall, unmarshall } = require("@aws-sdk/util-dynamodb");

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

async function getUserProfile(ctx) {
  // Assume ctx.state.user contains validated identity from auth middleware
  const subject = ctx.state.user.sub; // e.g., Cognito identity ID or user UUID
  const requestedUserId = ctx.params.userId;

  if (subject !== requestedUserId) {
    ctx.status = 403;
    ctx.body = { error: "Forbidden: cannot access other user data" };
    return;
  }

  const command = new GetCommand({
    TableName: process.env.PROFILE_TABLE_NAME,
    Key: marshall({ userId: subject }), // partition key must match subject
  });

  const response = await client.send(command);
  if (!response.Item) {
    ctx.status = 404;
    ctx.body = { error: "Not found" };
    return;
  }

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

module.exports = { getUserProfile };

This pattern enforces that the authenticated subject must match the requested user ID before a DynamoDB GetItem equivalent is issued. The key is constructed using the subject rather than the user-provided ID, preventing IDOR via path manipulation.

For operations that involve querying multiple items (e.g., a user’s orders), use a Key Condition Expression that includes the subject as the partition key value. This ensures that DynamoDB returns only items belonging to the caller.

const { DynamoDBClient, QueryCommand } = require("@aws-sdk/client-dynamodb");
const { marshall } = require("@aws-sdk/util-dynamodb");

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

async function listUserOrders(ctx) {
  const subject = ctx.state.user.sub;

  const command = new QueryCommand({
    TableName: process.env.ORDERS_TABLE_NAME,
    IndexName: "userId-index",
    KeyConditionExpression: "userId = :uid",
    ExpressionAttributeValues: marshall({ ":uid": subject }),
  });

  const response = await client.send(command);
  ctx.body = response.Items.map((item) => unmarshall(item));
}

module.exports = { listUserOrders };

In addition to scoping queries, apply defense in depth by validating and sanitizing all inputs used in DynamoDB expressions to mitigate injection risks. Avoid constructing command parameters by string concatenation. Use condition expressions for write operations to enforce constraints (e.g., version checks or status conditions) and prevent race conditions or unintended updates.

Finally, review IAM policies associated with the Koa service to ensure least privilege: allow only the specific DynamoDB actions on the specific resources required, and use conditions (such as restricting by dynamodb:LeadingKeys when applicable) to tighten access. This combination of Koa middleware checks and tightly scoped DynamoDB access patterns significantly reduces the chance of auth bypass in this stack.