Padding Oracle in Fiber with Dynamodb

A padding oracle in a Fiber application that uses Dynamodb for session or credential storage can arise when encrypted data is processed without proper integrity verification. In this stack, an attacker can supply manipulated ciphertext (for example, a session token or API key stored as an encrypted item in Dynamodb) to a Fiber endpoint, and the server’s error responses inadvertently reveal whether the padding is valid before decryption fails. Because Fiber is a fast, expressive web framework for Node.js, developers may assume that performance and simplicity reduce risk, but the framework does not automatically enforce authenticated encryption or constant-time comparison for encrypted payloads stored in Dynamodb.

Consider a scenario where user-specific data is encrypted using a block cipher in CBC mode and the ciphertext is saved in a Dynamodb item. If the decryption routine compares padding bytes and returns distinct errors for padding failures versus MAC or signature failures, an attacker can iteratively send modified ciphertexts and observe timing differences or specific HTTP status messages. Over many requests, this adaptive chosen-ciphertext attack can recover plaintext or escalate privileges, such as modifying the user_id in the Dynamodb item to impersonate another account. The vulnerability is not in Dynamodb itself, but in how the application reads data from Dynamodb and processes it within Fiber routes without authenticated encryption or explicit integrity checks.

In practice, this can surface in endpoints that accept an identifier or token referencing a Dynamodb record. For example, a route like GET /profile/:token might fetch an encrypted profile from Dynamodb, decrypt it, and use the content to set local session state. If error handling leaks padding validity—such as returning 400 versus 401 based on padding correctness—an attacker can craft requests that probe the decryption behavior. Because the scan categories in middleBrick include Input Validation and Authentication, such a misconfiguration would be surfaced as a finding, emphasizing the need to treat data from Dynamodb as potentially untrusted until verified with strong cryptographic integrity checks.

To remediate padding oracle risks when using Dynamodb with Fiber, adopt authenticated encryption and avoid branching on sensitive data during decryption. Below are concrete, minimal examples showing how to integrate AWS SDK for Dynamodb with secure decryption patterns in a Fiber application.

1. Use authenticated encryption (AES-GCM) and verify before use

Instead of raw CBC with separate padding checks, use AES-GCM which provides confidentiality and integrity in one step. Store the ciphertext and authentication tag together in a Dynamodb attribute, and reject the request if verification fails, returning a generic error without revealing padding details.

const { DynamoDBClient, GetItemCommand } = require("@aws-sdk/client-dynamodb");
const { decrypt } = require("./crypto"); // your secure wrapper
const express = require("express");
const app = express();

app.get("/profile/:keyId", async (req, res) => {
  const client = new DynamoDBClient({ region: "us-east-1" });
  const cmd = new GetItemCommand({
    TableName: "UserProfiles",
    Key: { id: { S: req.params.keyId } },
  });
  try {
    const { Item } = await client.send(cmd);
    if (!Item || !Item.encryptedData || !Item.encryptedData.B) {
      return res.status(404).json({ error: "not_found" });
    }
    const ciphertext = Item.encryptedData.B;
    const plaintext = decrypt(ciphertext); // internally uses AES-GCM and verifies tag
    if (!plaintext) {
      // Generic failure, no distinction between padding, tag, or parsing errors
      return res.status(401).json({ error: "invalid_token" });
    }
    res.json({ profile: JSON.parse(plaintext.toString("utf8")) });
  } catch (err) {
    console.error(err);
    res.status(500).json({ error: "server_error" });
  }
});

2. Constant-time comparison when verifying integrity

If you must work with ciphertext and MAC or signature schemes, ensure comparisons are constant-time and that errors do not reveal where the failure occurred. Below is an example where a MAC is verified before any sensitive operation, and a generic error is returned on failure.

const crypto = require("crypto");

function verifyMac(ciphertext, receivedMac, secretKey) {
  const mac = crypto.createHmac("sha256", secretKey).update(ciphertext).digest("hex");
  // Use timing-safe compare; do not branch on partial matches
  return crypto.timingSafeEqual(
    Buffer.from(mac, "hex"),
    Buffer.from(receivedMac, "hex")
  );
}

app.post("/data/:itemId", async (req, res) => {
  const { ciphertext, mac } = req.body;
  const secretKey = process.env.MAC_KEY;
  if (!verifyMac(ciphertext, mac, secretKey)) {
    return res.status(401).json({ error: "invalid_data" });
  }
  // Proceed to decrypt and use data from Dynamodb safely
  res.json({ ok: true });
});

3. Secure error handling and logging

Ensure that errors from Dynamodb or decryption routines do not expose stack traces or internal paths. Use structured logging that excludes sensitive context and avoid returning different HTTP codes for cryptographic versus validation failures that could aid an attacker.

app.use((err, req, res, next) => {
  // Do not expose internal error details in production
  console.error("Request processing failed", { err: err.message, path: req.path });
  res.status(500).json({ error: "internal_error" });
});