Credential Stuffing in Fiber with Dynamodb

Credential stuffing is a brute-force technique where attackers use lists of known username and password pairs to gain unauthorized access. When an API built with Fiber uses DynamoDB as its user store without adequate protections, the combination can expose authentication surfaces to this abuse. DynamoDB itself is a managed database service; the risk arises from how the application implements authentication logic and interacts with the database.

In a typical Fiber authentication flow, the application receives a POST to a login endpoint, reads a username from the request body, queries DynamoDB for the corresponding user record, and compares the provided password with a stored hash. If the endpoint does not enforce rate limiting or account lockout, attackers can send many requests per second using credential stuffing tools, cycling through username and password combinations. Because DynamoDB queries are fast and the scan is unauthenticated, the API may return distinct responses for existing versus non-existent users, enabling attackers to enumerate valid accounts.

Additional risk patterns include missing multi-factor authentication, weak password policies, and insufficient monitoring of authentication events. In a DynamoDB context, a lack of fine-grained access controls on the table can allow overly permissive IAM roles, increasing the impact of compromised credentials. The API’s error handling may inadvertently leak information in responses, such as distinguishing between ConditionalCheckFailedException and a successful read, which attackers can exploit to infer account existence.

Consider a Fiber handler structured like the following example. If this endpoint is exposed without rate limiting or request validation, it becomes susceptible to credential stuffing:

const { DynamoDBClient, GetItemCommand } = require("@aws-sdk/client-dynamodb");
const { unmarshall } = require("@aws-sdk/util-dynamodb");
const bcrypt = require("bcryptjs");
const express = require("express");
const app = express();
app.use(express.json());

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

app.post("/login", async (req, res) => {
  const { email, password } = req.body;
  if (!email || !password) {
    return res.status(400).json({ message: "email and password required" });
  }
  const command = new GetItemCommand({
    TableName: process.env.USERS_TABLE,
    Key: { email: { S: email } },
  });
  try {
    const data = await client.send(command);
    if (!data.Item) {
      return res.status(401).json({ message: "invalid credentials" });
    }
    const user = unmarshall(data.Item);
    const match = await bcrypt.compare(password, user.passwordHash);
    if (!match) {
      return res.status(401).json({ message: "invalid credentials" });
    }
    // session or token creation omitted for brevity
    res.json({ message: "ok" });
  } catch (err) {
    console.error(err);
    res.status(500).json({ message: "internal error" });
  }
});

This handler illustrates several issues common in the Fiber + DynamoDB pattern: the absence of rate limiting on the login route, uniform error messages that still reveal processing steps, and the potential for timing differences in bcrypt comparisons. Attackers can iterate through email lists, observing response times or status code nuances to refine their stuffing campaigns. Without additional controls such as account lockout, CAPTCHA, or anomaly detection, the API remains vulnerable.

middleBrick scans can surface these risks by analyzing the unauthenticated attack surface of such endpoints and identifying missing rate limiting, weak input validation, and inconsistent error handling. The scanner checks align with the OWASP API Top 10 and can map findings to compliance frameworks relevant to authentication and data protection.

To mitigate credential stuffing in a Fiber application backed by DynamoDB, implement layered defenses that address authentication logic, request throttling, and secure data handling. Below are concrete code examples that you can adopt to harden your endpoints.

1. Add rate limiting and request validation

Use a rate-limiting middleware to restrict the number of login attempts per IP or identifier within a time window. Validate input format strictly to prevent malformed requests from reaching DynamoDB:

const rateLimit = require("express-rate-limit");
const loginLimiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 5, // limit each IP to 5 login requests per window
  message: { message: "too many attempts, try again later" },
  standardHeaders: true,
  legacyHeaders: false,
});
app.post("/login", loginLimiter, async (req, res) => {
  // handler logic
});

2. Use consistent error responses and avoid user enumeration

Return the same generic error message and status code for both missing users and incorrect passwords. Add a small, constant-time delay on failed checks to reduce timing differences:

app.post("/login", async (req, res) => {
  const { email, password } = req.body;
  if (!email || !password) {
    await new Promise((r) => setTimeout(r, 300)); // constant delay
    return res.status(401).json({ message: "invalid credentials" });
  }
  const command = new GetItemCommand({
    TableName: process.env.USERS_TABLE,
    Key: { email: { S: email.toLowerCase().trim() } },
  });
  let user = null;
  try {
    const data = await client.send(command);
    user = data.Item ? unmarshall(data.Item) : null;
  } catch (err) {
    console.error(err);
    await new Promise((r) => setTimeout(r, 300));
    return res.status(500).json({ message: "internal error" });
  }
  if (!user) {
    await new Promise((r) => setTimeout(r, 300));
    return res.status(401).json({ message: "invalid credentials" });
  }
  const match = await bcrypt.compare(password, user.passwordHash);
  if (!match) {
    await new Promise((r) => setTimeout(r, 300));
    return res.status(401).json({ message: "invalid credentials" });
  }
  // proceed with authentication
  res.json({ message: "ok" });
});

3. Enforce secure password storage and account policies

Ensure passwords are hashed with a strong adaptive function and enforce minimum complexity. Consider adding multi-factor authentication support in your authentication flow:

const saltRounds = 12;
async function createUser(email, plainPassword) {
  const hashed = await bcrypt.hash(plainPassword, saltRounds);
  const command = new PutItemCommand({
    TableName: process.env.USERS_TABLE,
    Item: {
      email: { S: email.toLowerCase().trim() },
      passwordHash: { S: hashed },
      createdAt: { S: new Date().toISOString() },
    },
  });
  await client.send(command);
}

4. Apply least-privilege IAM and encryption

Configure DynamoDB table policies and IAM roles so the API only has the necessary permissions. Enable encryption at rest and in transit, and monitor suspicious authentication patterns through CloudWatch metrics integrated with your alerting system.

By combining these measures—rate limiting, consistent error handling, secure hashing, and strict IAM—you reduce the effectiveness of credential stuffing attacks against a Fiber API using DynamoDB. middleBrick can validate that these controls are reflected in your API’s behavior and surface related findings within your security risk assessment.