Bleichenbacher Attack in Strapi with Dynamodb

A Bleichenbacher attack is a padding oracle attack against RSA encryption that can reveal plaintext or reveal whether a padding is valid. In Strapi with DynamoDB as the data store, this can occur when Strapi stores encrypted values (for example, a JSON Web Encryption field, a custom encrypted string column, or credentials) and an attacker can supply many modified ciphertexts to learn about validity through differences in server behavior. DynamoDB itself does not perform RSA decryption; the padding oracle arises from how Strapi processes decrypted data and returns responses.

Consider a scenario where Strapi stores user preferences encrypted with RSA-OAEP and retrieves them from DynamoDB by decrypting in application code. If Strapi returns different HTTP status codes or response bodies for invalid padding versus successful decryption, an attacker can use this as an oracle. For example, an attacker intercepts a ciphertext stored in a DynamoDB item, then sends modified ciphertexts to the Strapi endpoint. By observing whether responses indicate malformed data, validation errors, or successful operations, the attacker iteratively decrypts the ciphertext without having the RSA private key.

In DynamoDB, items are retrieved by key. If the key or an attribute is an encrypted value that is manipulated block-by-block, the service simply returns the item if the key exists or not; the oracle behavior is introduced by Strapi’s handling after the DynamoDB read. Strapi may perform validation on decrypted fields and respond with 400 for bad padding or 200 for good padding. This difference enables the Bleichenbacher attack. Additionally, if Strapi uses deterministic or partially deterministic schemes (e.g., encrypting a field that becomes part of the DynamoDB key), an attacker might perform adaptive chosen-ciphertext queries to infer structure.

Real-world impact includes exposure of sensitive configuration, tokens, or personal data stored encrypted in DynamoDB via Strapi. Attackers can recover plaintexts such as API keys, secrets, or PII. This maps to OWASP API Top 10:2023 Broken Object Level Authorization (BOLA) when combined with IDOR if decryption is tied to record access, and it touches Cryptographic Failures in the OWASP Top 10.

To detect this during a middleBrick scan, the engine tests unauthenticated endpoints that consume user-supplied encrypted or opaque values, sends modified inputs, and observes response differentials. Findings include indicators that padding validation leaks information and guidance to remove padding oracles.

Remediation centers on ensuring Strapi does not expose padding validity through responses and that cryptographic operations are performed safely. Use authenticated encryption with associated data (AEAD) such as AES-GCM instead of RSA where possible. If RSA is required, use RSAES-OAEP with constant-time padding checks and avoid returning distinct errors for padding failures. Ensure DynamoDB item structures do not inadvertently expose cryptographic state that can be used in adaptive attacks.

Below are concrete code examples for Strapi that demonstrate safe handling when storing encrypted values in DynamoDB. These examples assume you use a library like node-forge or the built-in crypto module and a DynamoDB client configured for your region.

Example 1: Encrypting a field before storing in DynamoDB with AEAD

const { randomBytes, createCipheriv, createDecipheriv } = require('crypto');

// Use AES-GCM (AEAD) instead of RSA where possible
function encryptData(plaintext, key) {
  const iv = randomBytes(12); // 96-bit IV for GCM
  const cipher = createCipheriv('aes-256-gcm', key, iv);
  let encrypted = cipher.update(plaintext, 'utf8', 'base64');
  encrypted += cipher.final('base64');
  const authTag = cipher.getAuthTag().toString('base64');
  return {
    ciphertext: encrypted,
    iv: iv.toString('base64'),
    authTag: authTag
  };
}

function decryptData(encryptedObj, key) {
  const { ciphertext, iv, authTag } = encryptedObj;
  const decipher = createDecipheriv('aes-256-gcm', key, Buffer.from(iv, 'base64'));
  decipher.setAuthTag(Buffer.from(authTag, 'base64'));
  let decrypted = decipher.update(ciphertext, 'base64', 'utf8');
  decrypted += decipher.final('utf8');
  return decrypted;
}

// In a Strapi controller, store the encrypted object in DynamoDB
const AWS = require('aws-sdk');
const dynamo = new AWS.DynamoDB.DocumentClient();

async function storeEncryptedPreference(userId, plaintextPreference, encryptionKey) {
  const encrypted = encryptData(plaintextPreference, encryptionKey);
  const params = {
    TableName: 'UserPreferences',
    Item: {
      userId: userId, // partition key
      encryptedPreference: encrypted
    }
  };
  await dynamo.put(params).promise();
  return params.Item;
}

async function getEncryptedPreference(userId, encryptionKey) {
  const params = {
    TableName: 'UserPreferences',
    Key: {
      userId: userId
    }
  };
  const result = await dynamo.get(params).promise();
  if (!result.Item || !result.Item.encryptedPreference) {
    return null;
  }
  try {
    return decryptData(result.Item.encryptedPreference, encryptionKey);
  } catch (err) {
    // Log for monitoring but return a generic error to avoid padding oracle
    console.error('Decryption failed', err);
    throw new Error('Invalid data');
  }
}

Example 2: Safe error handling to prevent oracle leaks

// Always use constant-time comparison and generic errors
const crypto = require('crypto');

function safeCompare(a, b) {
  return crypto.timingSafeEqual(Buffer.from(a), Buffer.from(b));
}

async function handleStoredValue(userId, providedToken, encryptionKey) {
  const params = {
    TableName: 'UserTokens',
    Key: {
      userId: userId
    }
  };
  const result = await dynamo.get(params).promise();
  if (!result.Item) {
    // Return a generic error to avoid distinguishing between missing item and bad token
    throw new Error('Invalid request');
  }
  try {
    const decrypted = decryptData(result.Item.tokenData, encryptionKey);
    // Use constant-time comparison if comparing secrets
    const isValid = safeCompare(providedToken, decrypted.expectedToken);
    if (!isValid) {
      throw new Error('Invalid token');
    }
    return decrypted;
  } catch (err) {
    // Do not expose internal details; generic message prevents oracle
    console.error('Processing error', err);
    throw new Error('Invalid request');
  }
}

These examples prioritize removing padding oracles by using AEAD and uniform error handling. middleBrick can validate that endpoints do not differentiate between padding failures and other errors by sending crafted ciphertexts and inspecting responses. The scan checks for indicators that could enable a Bleichenbacher-style attack in the API surface.