Injection Flaws in Sinatra with Hmac Signatures

Injection flaws in Sinatra when HMAC signatures are used for request authentication often stem from how signature verification is implemented rather than from the cryptographic primitive itself. A typical pattern accepts parameters, a timestamp, and an HMAC-SHA256 signature, then decides whether to process the request. If the application compares signatures using a simple string equality check and does not enforce constant-time comparison, an attacker can exploit timing differences to learn about the expected signature byte by byte. This is a classic example where a weak verification implementation undermines the integrity guarantees of HMAC.

Another common issue arises when user-controlled data is included in the signed payload without canonicalization. For instance, if the application signs a JSON object but the keys are serialized in varying orders, or if some parameters are omitted from the signature while still being used in business logic, an attacker can modify unsigned parameters to change behavior (such as user ID or role) without invalidating the signature. This is closely related to Insecure Direct Object References (IDOR) and BOLA when the signature does not bind the resource identifier tightly to the authenticated context. The unauthenticated attack surface of middleBrick reveals such parameter manipulation risks by testing whether unsigned or weakly bound parameters can be altered to elevate privilege or access other users’ resources.

Input validation problems compound the risk. If parameters that influence database queries or command execution are reflected into responses without strict allowlisting or escaping, an attacker who can affect the signed payload may be able to inject malicious content. Even when the HMAC is valid, the application must treat all inputs as untrusted. For example, a user_id included in the signature and later used in a SQL WHERE clause without parameterized queries can lead to SQL injection. Similarly, if the endpoint triggers external calls using values from the signed payload, Server-Side Request Forgery (SSRF) can occur. middleBrick’s checks for Input Validation and SSRF highlight these downstream effects by probing endpoints with manipulated, yet correctly signed, requests to see whether unsafe consumption leads to unintended behavior.

Consider an endpoint where the client sends resource_id, timestamp, and action, with an HMAC covering these fields. If the server uses the HMAC only to verify integrity but does not re-validate authorization for the specific resource_id against the authenticated principal, a BOLA/IDOR path is opened. An attacker can iterate over other IDs while keeping a valid HMAC for their own resources, assuming the server does not enforce ownership checks. This is why middleBrick emphasizes BOLA/IDOR testing alongside signature validation: to confirm that a valid cryptographic binding does not replace proper access control logic.

LLM/AI Security considerations also intersect here. If error messages or logs inadvertently disclose parts of the expected HMAC or system behavior, an attacker may use carefully crafted probes to infer verification logic. middleBrick’s LLM/AI Security checks, including active prompt injection tests and system prompt leakage detection, are designed to surface such information exposure risks even in API-driven services where LLM components might be involved in processing or logging requests. By running these checks in parallel with standard security scans, the tool helps identify subtle channels that could aid injection or bypass attempts.

Remediation centers on strict canonicalization, constant-time comparison, and clear separation of authorization from integrity verification. Below is a complete, realistic Sinatra example that shows a secure way to handle HMAC-SHA256 signed requests.

require 'sinatra'
require 'json'
require 'openssl'
require 'base64'
require 'securerandom'

SECRET_KEY = ENV.fetch('HMAC_SECRET') { raise 'HMAC_SECRET must be set' }

helpers do
  # Constant-time comparison to avoid timing attacks
  def secure_compare(a, b)
    return false unless a.bytesize == b.bytesize
    l = a.unpack 'C*'
    res = 0
    b.each_byte { |byte| res |= byte ^ l.shift }
    res == 0
  end

  # Build the canonical payload string for signing
  def canonical_payload(params)
    # Ensure deterministic ordering and exclude signature/timestamp handled separately if needed
    parts = []
    parts << "resource_id:#{params['resource_id']}" if params['resource_id']
    parts << "action:#{params['action']}" if params['action']
    parts << "timestamp:#{params['timestamp']}" if params['timestamp']
    parts.join('|')
  end

  # Verify HMAC signature
  def valid_signature?(params)
    return false unless params['signature']
    payload = canonical_payload(params)
    expected = OpenSSL::HMAC.hexdigest('SHA256', SECRET_KEY, payload)
    secure_compare(expected, params['signature'])
  end
end

post '/process' do
  content_type :json
  payload = JSON.parse(request.body.read)

  # Basic input validation
  unless payload['resource_id'] && payload['action'] && payload['timestamp'] && payload['signature']
    status 400
    return { error: 'missing required fields' }.to_json
  end

  # Reject requests with stale timestamps (replay protection)
  request_time = Integer(payload['timestamp'])
  now = Time.now.to_i
  if (now - request_time).abs > 300
    status 400
    return { error: 'timestamp out of window' }.to_json
  end

  unless valid_signature?(payload)
    status 401
    return { error: 'invalid signature' }.to_json
  end

  # Re-validate authorization: ensure the resource_id belongs to the requester
  # This is the BOLA/IDOR boundary that HMAC does not replace
  unless authorized_for_resource?(payload['resource_id'], current_user) # implement per app
    status 403
    return { error: 'forbidden' }.to_json
  end

  # Safe processing with parameterized queries or strict allowlists
  # Example: use Sequel or ActiveRecord with placeholders
  # DB[:events].where(resource_id: payload['resource_id'], action: allowed_action?(payload['action']))

  { status: 'ok' }.to_json
end

Key points in this remediation:

• Canonical payload: The signature covers a deterministic string built from selected fields, preventing key-order ambiguity and ensuring the server and client interpret the signed data identically.
• Constant-time comparison: secure_compare eliminates timing side channels that could leak signature information, addressing the HMAC-specific risk of adaptive chosen-message attacks.
• Replay protection: The timestamp window prevents replay attacks; this complements HMAC by adding freshness.
• Authorization separation: Even with a valid HMAC, the endpoint re-checks ownership (BOLA/IDOR) and role-based permissions. HMAC provides integrity and authentication of the payload, not authorization.
• Input validation and parameterized usage: Values used in queries or external calls are validated and never directly interpolated, mitigating SQL injection and SSRF regardless of signature validity.

When integrating with existing services, prefer environment-managed secrets, rotate keys periodically, and log verification failures without exposing sensitive data. middleBrick’s CLI can scan your Sinatra endpoints to verify that such protections are consistently applied and to detect whether signature verification is bypassed or performed on incomplete data sets.