Brute Force Attack in Hanami with Hmac Signatures

A brute force attack against an API that uses Hmac Signatures in Hanami typically targets the signature verification step. In this setup, a client creates a canonical string from request components (HTTP method, path, selected headers, and body), then signs it with a shared secret to produce an Hmac-SHA256 signature sent in a header (e.g., X-Api-Signature). If the server-side verification is performed in a way that is not constant-time or leaks information about partial matches, an attacker can systematically submit many requests with slightly altered inputs, observe differences in response behavior or timing, and infer whether a guessed signature component is correct.

The vulnerability often arises when Hanami applications implement ad-hoc verification without considering side-channel risks. For example, a developer might compute the expected signature and compare it with the incoming header using a standard equality check. If the comparison returns early on the first mismatching byte, an attacker can use these timing differences to mount an online brute force or partial-replay attack. Additionally, if nonces or timestamps used to build the canonical string are not enforced strictly (e.g., replayed or accepted out of a narrow window), an attacker can reuse captured requests and iterate over secrets or parameters without triggering anti-replay controls.

Another angle specific to Hmac Signatures is weak handling of the shared secret or canonical construction. If the secret is low-entropy or derived predictably, an attacker can brute force the secret itself. If the canonical string omits important headers or includes redundant ones, the server may accept multiple variations of the same logical request, increasing the effective search space an attacker can exploit. These issues map to broader API security concerns such as weak authentication schemes and insufficient rate limiting, which are among the 12 checks middleBrick runs in parallel to detect authentication weaknesses and BOLA/IDOR risks.

In practice, a scanner like middleBrick can identify these risks by analyzing the OpenAPI/Swagger spec (including $ref resolution) and by conducting black-box tests that probe for timing inconsistencies, missing replay protection, and improper signature validation logic. The tool does not fix the implementation, but it provides prioritized findings with severity ratings and remediation guidance, helping teams understand how an attacker might chain brute force attempts with Hmac weaknesses.

To remediate brute force risks when using Hmac Signatures in Hanami, focus on constant-time comparison, strict replay/nonce controls, and robust canonicalization. Below are concrete code examples that demonstrate a secure approach.

First, use a constant-time comparison to avoid timing leaks when verifying signatures. In Ruby, ActiveSupport::SecurityUtils.secure_compare is appropriate. Here is a Hanami-compatible verification snippet:

# app/services/hmac_signature_verifier.rb
require "openssl"
require "active_support/security_utils"

class HmacSignatureVerifier
  ALGORITHM = "sha256"

  def initialize(shared_secret)
    @shared_secret = shared_secret
  end

  def valid_signature?(request_body, received_signature, canonical_string)
    expected = OpenSSL::HMAC.hexdigest(ALGORITHM, @shared_secret, canonical_string)
    ActiveSupport::SecurityUtils.secure_compare(expected, received_signature)
  end
end

Second, enforce a tight replay window and require a nonce or timestamp in the canonical string to prevent reuse. For example, include x-request-timestamp and x-request-nonce in the canonical construction and validate them server-side:

# app/validators/request_validator.rb
class RequestValidator
  REPLAY_WINDOW_SECONDS = 300

  def self.canonical_parts(request)
    [
      request.request_method,
      request.path,
      request.headers["X-Request-Timestamp"],
      request.headers["X-Request-Nonce"],
      request.body.read
    ]
  end

  def self.recent_nonce?(nonce)
    # Store seen nonces in a fast, TTL-backed store (e.g., Redis); pseudocode below
    return false if seen_nonce?(nonce)
    mark_nonce_as_seen(nonce)
    true
  end

  def self.verify_hmac(request, shared_secret)
    timestamp = request.headers["X-Request-Timestamp"]
    nonce = request.headers["X-Request-Nonce"]
    return false unless timestamp && nonce
    return false unless (Time.now.to_i - Integer(timestamp)) <= REPLAY_WINDOW_SECONDS
    return false unless recent_nonce?(nonce)

    canonical = canonical_parts(request).join("\n")
    received = request.headers["X-Api-Signature"]
    verifier = HmacSignatureVerifier.new(shared_secret)
    verifier.valid_signature?(request.body.read, received, canonical)
  end
end

Third, ensure your canonicalization includes all relevant headers and body exactly as transmitted, and reject requests with missing or malformed components. Also enforce rate limits per client key (e.g., derived from the API key used to look up the shared secret) to make brute force attempts impractical. middleBrick’s checks for Authentication, Rate Limiting, and Input Validation highlight these concerns, and its findings can guide you toward appropriate thresholds and controls.

Finally, rotate shared secrets periodically and prefer Hmac-SHA256 or stronger. Use the middleBrick CLI to validate your implementation by running middlebrick scan <url> against your endpoints and review the report to confirm that signatures are validated safely and replay protections are in place.