Broken Access Control in Chi with Dynamodb

Broken Access Control (BOLA/IDOR) in a Chi application that uses Amazon DynamoDB often arises when route parameters or user input are directly mapped to DynamoDB keys without verifying that the requesting user is authorized to access the targeted item. In Chi, this typically occurs when a route like /users/:userId/resources/:resourceId accepts dynamic segments and uses them to construct a DynamoDB query or get request without confirming that the authenticated subject owns or is permitted to interact with that specific resourceId.

DynamoDB’s access patterns rely heavily on primary keys (partition key and optional sort key). If the application derives these key values directly from user-supplied parameters—such as resourceId or userId—and does not cross-check the authenticated identity, an attacker can manipulate these parameters to access or modify other users’ data. For example, changing :userId in the URL to another valid user ID might allow an attacker to read or update records that should be isolated to the original user. This is a classic IDOR pattern, and because DynamoDB does not enforce row-level ownership by itself, the application must enforce authorization before issuing any operation.

Chi is a lightweight routing library for Deno, and it does not provide built-in authorization. If the developer wires Chi routes directly to DynamoDB clients, the responsibility to enforce permissions falls entirely on the application code. A common insecure pattern is to perform a GetItem or Query using user-supplied keys without ensuring the requesting user’s identity matches the item’s owning user. Additionally, DynamoDB responses can leak sensitive attributes if the application returns the full item to the client after insufficient authorization checks, leading to data exposure.

Another subtlety involves queries that use an incomplete key condition. For instance, if a query uses only a sort key without properly scoping it to the authenticated user’s partition key, it might return items from other partitions or users. Because DynamoDB requires both components for precise access control, omitting the partition key derived from the authenticated subject creates an authorization bypass. This becomes critical when the application indexes data in a way that allows enumeration across users, which can be exploited through IDOR.

middleBrick’s BOLA/IDOR checks specifically test these scenarios by probing endpoints with modified identifiers to see whether authorization is enforced. For API security, this highlights the need to validate ownership on every DynamoDB operation and to treat user input as untrusted, regardless of whether the request is authenticated.

To remediate Broken Access Control when using DynamoDB in Chi, always enforce ownership checks before performing any database operation. This means deriving the authenticated user’s identifier from the session or token and using it as the partition key (or part of the key) in every DynamoDB call, rather than trusting route parameters alone.

Principle: Scoped Queries with User Context

Instead of using user-supplied IDs directly, construct the key using the authenticated subject. Below is a secure Chi route example in Deno that combines Chi routing with the AWS SDK for DynamoDB, ensuring that the userId in the path matches the authenticated user and is used as the partition key.

import { Application, Router } from "https://deno.land/x/[email protected]/mod.ts";
import { DynamoDBClient, GetItemCommand } = "https://deno.land/x/[email protected]/mod.ts";

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

// Assume a helper that extracts authenticated user identity from a session or token
async function getAuthenticatedUserId(request: Request): Promise {
  // Example: extract from auth header or session
  const auth = request.headers.get("authorization");
  if (!auth?.startsWith("Bearer ")) return null;
  const token = auth.substring(7);
  // Validate token and return user ID (pseudo-implementation)
  return validateTokenAndGetUserId(token);
}

router.get("/resources/:resourceId", async (context) => {
  const authenticatedUserId = await getAuthenticatedUserId(context.request);
  if (!authenticatedUserId) {
    context.response.status = 401;
    return;
  }

  const resourceId = context.params.resourceId;
  // Enforce ownership: use authenticated user as partition key
  const command = new GetItemCommand({
    TableName: "UserResources",
    Key: {
      userId: { S: authenticatedUserId },
      resourceId: { S: resourceId },
    },
  });

  try {
    const { Item } = await client.send(command);
    if (!Item) {
      context.response.status = 404;
      return;
    }
    context.response.body = Item;
    context.response.status = 200;
  } catch (err) {
    context.response.status = 500;
    context.response.body = { error: err.message };
  }
});

export default router;

This pattern ensures that even if an attacker changes resourceId in the URL, they cannot access items belonging to another user because the partition key is derived from the authenticated identity, not from user input.

Batch and Query Safeguards

When using Query or Scan operations, always include the authenticated user’s identifier in the filter and key condition expressions. For example, a Query that lists a user’s resources should include a condition on the partition key equal to the authenticated user ID:

const command = new QueryCommand({
  TableName: "UserResources",
  KeyConditionExpression: "userId = :uid AND begins_with(resourceId, :rid)",
  ExpressionAttributeValues: {
    ":uid": { S: authenticatedUserId },
    ":rid": { S: "prefix-" },
  },
});

Additionally, avoid returning the full DynamoDB attribute values directly to the client. Instead, project only necessary fields and sanitize output to prevent accidental data exposure. middleBrick’s Data Exposure checks can help identify endpoints that return sensitive attributes without proper filtering.

Continuous Monitoring

For ongoing protection, consider using the middleBrick Pro plan to enable continuous monitoring and integrate scans into your CI/CD pipeline with the GitHub Action. This helps catch regressions where authorization checks might be accidentally omitted or misaligned with new route parameters.