Bleichenbacher Attack in Hanami with Dynamodb

A Bleichenbacher attack is a cryptographic padding oracle technique that can be relevant in Hanami applications that use encryption or signature schemes (e.g., RSAES-PKCS1-v1_5) to protect or authenticate data before it is stored in DynamoDB. In this context, the vulnerability arises when error messages or timing differences reveal whether a decryption or verification operation succeeded, enabling an attacker to iteratively decrypt ciphertexts or forge valid signatures without knowing the key.

When such cryptographic operations are performed server-side in Hanami and the results are stored or looked up in DynamoDB, the combination can expose a padding oracle if responses vary based on the validity of the decrypted data. For example, an application might decrypt a token from a cookie or header using an RSA private key, then query DynamoDB using a user ID or record identifier extracted from the decrypted payload. If the application returns distinct errors (e.g., ‘invalid signature’, ‘item not found’, or timing differences) based on whether the padding is correct, the attacker can adaptively craft ciphertexts and observe responses to eventually recover plaintext.

DynamoDB itself does not introduce the oracle; the risk comes from how Hanami handles and compares cryptographic validation outcomes. Consider a Hanami action that expects a signed JWT or encrypted blob, performs decryption in Ruby, and then uses a field like user_id to load a record from DynamoDB via the AWS SDK. If the decryption fails due to bad padding, Hanami might raise an exception or return a 404/422 with a distinguishable message. These differences allow an attacker to infer whether a guessed plaintext block has correct padding, which is the core of Bleichenbacher’s adaptive chosen-ciphertext attack.

In practice, this can manifest in endpoints that accept an encrypted identifier (e.g., a reservation ID) which is decrypted and then used in a DynamoDB GetItem or Query. If the decryption step leaks validity through HTTP status codes or response content, the attacker can mount a Bleichenbacher attack to recover sensitive identifiers or session tokens. The DynamoDB storage layer becomes an oracle amplifier because successful decryption leads to a record being found, while failed decryption leads to a missing item or error, reinforcing the signal available to the attacker.

To illustrate, a Hanami controller might look like this in a vulnerable form:

class Reservation::Show < Hanami::Action
  def call(params)
    encrypted_id = params[:id]
    # Vulnerable: decryption may raise distinct errors for bad padding vs missing record
    decrypted = Crypto.rsa_private_decrypt(encrypted_id)
    record = ReservationRepository.find(decrypted[:user_id])
    if record
      response.body = { reservation: record }.to_json
    else
      response.status = 404
      response.body = { error: 'not_found' }.to_json
    end
  end
end

If Crypto.rsa_private_decrypt raises a distinct exception for bad padding versus a generic error for a missing record, an attacker can distinguish between valid and invalid padding by observing HTTP status codes or response content. This enables a Bleichenbacher attack that gradually decrypts encrypted_id without access to the private key.

Using DynamoDB in this flow compounds the issue if the application’s error handling or timing varies based on whether the decrypted key exists in the table. Even if DynamoDB access time is relatively constant, the distinction between a decryption error and a missing item creates a side channel. Therefore, securing this combination requires consistent error handling and cryptographic practices that eliminate oracle behavior.

Remediation focuses on removing padding oracle signals by ensuring that decryption failures and missing records are handled uniformly. In Hanami, this means standardizing responses and avoiding early differentiation based on padding validity. Use constant-time comparison where applicable and ensure that DynamoDB lookups do not expose differential behavior.

First, refactor the decryption step to not raise distinct exceptions for padding errors. Instead, map all decryption failures to a generic failure path that behaves identically to a missing record. For example:

class Reservation::Show < Hanami::Action
  def call(params)
    encrypted_id = params[:id]
    begin
      decrypted = Crypto.rsa_private_decrypt(encrypted_id)
    rescue Crypto::DecryptionError
      # Treat decryption failure the same as a missing record
      response.status = 404
      response.body = { error: 'not_found' }.to_json
      return
    end

    record = ReservationRepository.find(decrypted[:user_id])
    if record
      response.body = { reservation: record }.to_json
    else
      response.status = 404
      response.body = { error: 'not_found' }.to_json
    end
  end
end

Second, use a secure RSA decryption scheme with proper padding (e.g., RSA-OAEP) and avoid PKCS#1 v1.5 padding where possible, as it is more susceptible to Bleichenbacher-style attacks. If RSAES-PKCS1_v1_5 must be used, ensure decryption errors are caught and handled identically to other failures.

Third, when querying DynamoDB, avoid leaking information via timing or error messages. Use a consistent flow that always performs a lookup with the decrypted identifier and returns the same generic response on failure:

class Reservation::Show < Hanami::Action
  def call(params)
    encrypted_id = params[:id]
    begin
      decrypted = Crypto.rsa_private_decrypt(encrypted_id)
    rescue Crypto::DecryptionError
      response.status = 404
      response.body = { error: 'not_found' }.to_json
      return
    end

    # Use a repository that encapsulates DynamoDB interactions
    record = ReservationRepository.find(decrypted[:user_id])
    if record
      response.body = { reservation: record }.to_json
    else
      response.status = 404
      response.body = { error: 'not_found' }.to_json
    end
  end
end

# In a separate repository file
class ReservationRepository
  def self.find(user_id)
    dynamodb = Aws::DynamoDB::Client.new
    result = dynamodb.get_item(
      table_name: 'reservations',
      key: { 'user_id' => { s: user_id.to_s } }
    )
    result.item ? JSON.parse(result.item.to_json) : nil
  end
end

Additionally, consider using authenticated encryption with associated data (AEAD) such as AES-GCM for encrypting identifiers, which provides integrity and reduces the need for padding schemes that are vulnerable to Bleichenbacher attacks. In Hanami, integrate this via a secure crypto service object that handles both encryption and decryption with consistent error handling.

Finally, apply defense-in-depth by rate-limiting requests per client and validating inputs before cryptographic operations to reduce the attacker’s ability to automate queries. While middleBrick cannot fix these issues, its scans can help identify endpoints where unusual error patterns or timing differences might indicate a padding oracle, guiding developers toward safer implementations.