Sandbox Escape in Chi with Dynamodb

A sandbox escape in the context of Chi and Amazon DynamoDB occurs when an attacker who has limited execution capability within the Chi runtime (for example, through a serverless function or a restricted plugin environment) is able to perform unauthorized operations against DynamoDB resources that should be isolated. This typically maps to the BOLA/IDOR and Property Authorization checks in middleBrick scans, and it can also be surfaced by the Inventory Management and Unsafe Consumption checks when the API surface exposes DynamoDB-related endpoints without proper authorization boundaries.

DynamoDB-specific risks arise when APIs in Chi expose operations that assume client-supplied keys are safe to use directly. For example, if an endpoint accepts a userId parameter and uses it to construct a DynamoDB Key without verifying that the authenticated subject is allowed to access that specific partition key, an attacker can iterate or manipulate identifiers to reach other users’ data. This is a classic Insecure Direct Object Reference (IDOR) pattern, and middleBrick’s BOLA/IDOR check is designed to detect such authorization flaws by correlating spec definitions with runtime behavior.

Additionally, if Chi configurations inadvertently expose DynamoDB SDK clients or helper utilities in a way that allows dynamic expression evaluation, an attacker might leverage crafted input to cause the service to issue unintended operations. This can be part of an Unsafe Consumption check finding, where untrusted input influences which table or key is accessed. MiddleBrick’s Input Validation and Property Authorization checks look for these patterns by comparing the declared schema against observed requests that attempt to reference or mutate unexpected resources.

Another angle involves Inventory Management findings: if the API returns configuration or endpoint details that reference DynamoDB tables and do not enforce strict access controls, an attacker can learn internal resource names and use them to probe for misconfigured permissions. middleBrick cross-references OpenAPI/Swagger specs with runtime findings, so if the spec describes a DynamoDB-related parameter but the implementation lacks authorization checks, the scan will surface this discrepancy as a high-priority item with remediation guidance.

Because middleBrick scans the unauthenticated attack surface and runs checks in parallel, it can identify whether DynamoDB-related endpoints are reachable without proper authentication or authorization. The LLM/AI Security checks do not directly test DynamoDB, but they ensure that prompt injection or jailbreak probes do not lead to unintended data exposure that could include DynamoDB connection strings or table names in verbose error messages.

To remediate sandbox escape risks involving DynamoDB in Chi, enforce strict ownership checks at the data-access layer and validate all identifiers against the authenticated subject. Below are concrete examples using the AWS SDK for JavaScript in a Chi-based serverless function.

First, ensure that every DynamoDB operation uses a composite key that includes the authenticated user’s ID, and never trust client-provided IDs alone:

import { DynamoDBClient, GetItemCommand } from "@aws-sdk/client-dynamodb";

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

export async function handler(event, context) {
  const authenticatedUserId = context.identity?.userId;
  const suppliedUserId = event.pathParameters?.userId;

  if (!authenticatedUserId || suppliedUserId !== authenticatedUserId) {
    return { statusCode: 403, body: "Forbidden: insufficient permissions" };
  }

  const params = {
    TableName: process.env.USER_TABLE,
    Key: {
      userId: { S: authenticatedUserId },
      sortKey: { S: "profile" }
    }
  };

  const command = new GetItemCommand(params);
  const response = await client.send(command);

  return {
    statusCode: 200,
    body: JSON.stringify(response.Item)
  };
}

This pattern ensures that the user ID from the runtime context (e.g., from a JWT or an authenticated session) is used rather than the value provided in the request. It directly addresses BOLA/IDOR findings by enforcing ownership at the item level.

Second, apply fine-grained authorization using condition expressions and avoid scanning entire tables:

import { DynamoDBClient, UpdateItemCommand } from "@aws-sdk/client-dynamodb";

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

export async function handler(event, context) {
  const authenticatedUserId = context.identity?.userId;
  const targetUserId = event.pathParameters?.userId;

  if (authenticatedUserId !== targetUserId) {
    return { statusCode: 403, body: "Forbidden" };
  }

  const params = {
    TableName: process.env.PREFERENCES_TABLE,
    Key: {
      userId: { S: authenticatedUserId }
    },
    UpdateExpression: "set #pref = :val",
    ExpressionAttributeNames: {
      "#pref": "theme"
    },
    ExpressionAttributeValues: {
      ":val": { S: event.body.theme }
    },
    ConditionExpression: "attribute_exists(userId)"
  };

  const command = new UpdateItemCommand(params);
  await client.send(command);

  return { statusCode: 200, body: "Preferences updated" };
}

Using a ConditionExpression with attribute_exists adds an extra safeguard so that updates fail if the expected item does not exist under the authenticated user’s key. This reduces the impact of misconfigurations and aligns with Property Authorization checks that middleBrick reports.

Third, sanitize and validate any input that influences table or index names. Never concatenate user input into resource identifiers:

import { DynamoDBClient, QueryCommand } from "@aws-sdk/client-dynamodb";

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

export async function handler(event, context) {
  const authenticatedUserId = context.identity?.userId;
  const tableSuffix = "UserData"; // hardcoded or selected from a strict allowlist
  const tableName = `app_${tableSuffix}`;

  const params = {
    TableName: tableName,
    IndexName: "gsi-user-timestamp",
    KeyConditionExpression: "userId = :uid",
    ExpressionAttributeValues: {
      ":uid": { S: authenticatedUserId }
    }
  };

  const command = new QueryCommand(params);
  const response = await client.send(command);

  return {
    statusCode: 200,
    body: JSON.stringify(response.Items)
  };
}

Hardcoding or strictly allowing table name segments prevents attackers from injecting arbitrary table references. Combined with middleware that verifies the authenticated subject, this approach mitigates risks highlighted by middleBrick’s Inventory Management and Unsafe Consumption checks.