Sandbox Escape in Fiber with Dynamodb

A sandbox escape in the context of a Fiber application that interacts with DynamoDB occurs when user-supplied data influences DynamoDB operations in a way that allows an attacker to bypass intended isolation boundaries. Because DynamoDB is a managed NoSQL service, the escape typically does not involve breaking out of a compute sandbox, but rather bypassing authorization checks, elevating privileges, or accessing unintended data through malformed requests.

Consider a scenario where a Fiber handler builds a DynamoDB query using path parameters without strict validation. If the route includes a user ID that is concatenated into a KeyConditionExpression or used as a filter expression input, an attacker may manipulate the parameter to reference other users’ data. For example, an endpoint defined as GET /users/:userId/profile might construct a GetItem or Query request using :userId directly. Without proper validation, an attacker could supply a crafted ID such as otherUser#123 or exploit reserved words and injection-prone patterns to cause the query to match unintended items, effectively reading data outside the intended scope.

DynamoDB’s attribute values can also be leveraged in an escape if the application deserializes untrusted input into condition expressions or uses untrusted data in UpdateItem expressions. For instance, if a handler uses UpdateItem with an expression attribute value built from user input, an attacker may provide values such as 1; DROP TABLE users-style syntax. While DynamoDB does not support SQL, expression syntax allows specifying operators and paths. A malicious payload could modify the update behavior to affect different items or attributes, or to expose sensitive fields in the response when combined with other misconfigurations.

Another vector involves pagination and scan operations. If a Fiber endpoint allows clients to control the Limit or FilterExpression parameters for Query or Scan, an attacker may set extremely broad filters or disable filtering entirely. This can result in data exposure across partitions. Additionally, if the application uses IAM roles associated with the host to sign requests, misconfigured trust policies or overly broad permissions can amplify the impact. The scanner checks for such authorization flaws under BOLA/IDOR and Property Authorization checks, flagging endpoints where user-controlled input directly influences resource selection without proper ownership verification.

Input validation and type checking are especially important when the API consumes JSON payloads that are later translated into DynamoDB expression attribute values. Without strict schema enforcement, an attacker may embed nested objects or reserved expression names that change the semantics of the operation. The scanner tests for injection via the Input Validation check, examining whether the endpoint tolerates malformed or unexpected input that could shift the query’s intent. These checks are run in parallel across the 12 security assessments, including LLM/AI Security, which probes for prompt injection and data exfiltration patterns even in API-driven workflows that include AI components.

SSRF findings may also relate to DynamoDB if the application fetches endpoint configurations or user-supplied URLs that point to internal AWS metadata services. The scanner flags risky patterns where user input directly controls HTTP destinations used to retrieve credentials or configuration. In the context of compliance, findings map to OWASP API Top 10 (2023) categories such as Broken Object Level Authorization and Injection, and align with relevant controls in SOC2 and GDPR for data segregation and confidentiality.

Remediation centers on strict input validation, canonical resource identification, and safe construction of DynamoDB expressions. Always treat path and query parameters as untrusted. Use explicit allowlists for IDs and enforce format constraints before they are used in DynamoDB operations.

Example 1: Safe GetItem with user ID validation

import { Router } from 'express';
import { DynamoDBDocumentClient, GetCommand } from '@aws-sdk/lib-dynamodb';
import { dbClient } from './dynamodb-client';

const router = Router();
const ddbDocClient = DynamoDBDocumentClient.from(dbClient);

router.get('/users/:userId/profile', async (req, res) => {
  const { userId } = req.params;

  // Strict validation: allow alphanumeric and underscore, 3..36 chars
  if (!/^[a-zA-Z0-9_]{3,36}$/.test(userId)) {
    return res.status(400).json({ error: 'Invalid user ID format' });
  }

  const command = new GetCommand({
    TableName: process.env.USER_TABLE_NAME,
    Key: { userId: userId } // direct scalar value, no concatenation
  });

  try {
    const { Item } = await ddbDocClient.send(command);
    if (!Item) {
      return res.status(404).json({ error: 'Not found' });
    }
    // Return only intended public fields
    res.json({ userId: Item.userId, name: Item.name, avatarUrl: Item.avatarUrl });
  } catch (err) {
    console.error(err);
    res.status(500).json({ error: 'Internal server error' });
  }
});

Example 2: Safe Query with partition key and allowlist filter

import { DynamoDBDocumentClient, QueryCommand } from '@aws-sdk/lib-dynamodb';

router.get('/users/:userId/posts', async (req, res) => {
  const { userId } = req.params;
  const { limit, sort } = req.query;

  if (!/^[a-zA-Z0-9_]{3,36}$/.test(userId)) {
    return res.status(400).json({ error: 'Invalid user ID format' });
  }

  const limitNum = Number(limit);
  if (!Number.isInteger(limitNum) || limitNum < 1 || limitNum > 50) {
    return res.status(400).json({ error: 'Invalid limit' });
  }

  const command = new QueryCommand({
    TableName: process.env.POSTS_TABLE_NAME,
    KeyConditionExpression: 'userId = :uid',
    ExpressionAttributeValues: {
      ':uid': userId
    },
    Limit: limitNum,
    ScanIndexForward: sort !== 'desc' // default ascending
  });

  const { Items } = await ddbDocClient.send(command);
  res.json({ items: Items || [] });
});

Example 3: Safe UpdateItem with expression attribute names/values

import { DynamoDBDocumentClient, UpdateCommand } from '@aws-sdk/lib-dynamodb';

router.patch('/users/:userId/metadata', async (req, res) => {
  const { userId } = req.params;
  const { preferredTheme, notificationsEnabled } = req.body;

  if (!/^[a-zA-Z0-9_]{3,36}$/.test(userId)) {
    return res.status(400).json({ error: 'Invalid user ID format' });
  }

  const updateExpressionParts = [];
  const expressionAttributeValues = {};
  const expressionAttributeNames = {};

  if (typeof preferredTheme === 'string' && ['light', 'dark', 'system'].includes(preferredTheme)) {
    expressionAttributeNames['#theme'] = 'preferredTheme';
    expressionAttributeValues[':theme'] = preferredTheme;
    updateExpressionParts.push('#theme = :theme');
  }

  if (typeof notificationsEnabled === 'boolean') {
    expressionAttributeNames['#notif'] = 'notificationsEnabled';
    expressionAttributeValues[':notif'] = notificationsEnabled;
    updateExpressionParts.push('#notif = :notif');
  }

  if (updateExpressionParts.length === 0) {
    return res.status(400).json({ error: 'No valid fields to update' });
  }

  const command = new UpdateCommand({
    TableName: process.env.USER_TABLE_NAME,
    Key: { userId },
    UpdateExpression: `SET ${updateExpressionParts.join(', ')}`,
    ExpressionAttributeNames: expressionAttributeNames,
    ExpressionAttributeValues: expressionAttributeValues,
    ReturnValues: 'UPDATED_NEW'
  });

  try {
    const { Attributes } = await ddbDocClient.send(command);
    res.json({ updated: Attributes });
  } catch (err) {
    console.error(err);
    res.status(500).json({ error: 'Internal server error' });
  }
});

Operational and configuration guidance

• Enforce ownership checks on every request by comparing the authenticated subject (from session or token) with the resource owner ID, rather than relying on obscurity or ID patterns alone.
• Apply schema validation on incoming JSON payloads and use parameterized expression attribute names/values to prevent injection-style manipulation of DynamoDB semantics.
• Restrict IAM permissions to the least privilege required for each operation and avoid broad wildcard permissions on table resources.
• Use the middleBrick CLI to scan endpoints from the terminal: middlebrick scan <url>. For CI/CD, add the GitHub Action to fail builds if the security score drops below your chosen threshold, and consider the Pro plan for continuous monitoring and Slack/Teams alerts.