Injection Flaws in Grape with Bearer Tokens

Grape is a Ruby API micro-framework commonly used to build RESTful JSON APIs. When Bearer Tokens are used for authentication, injection flaws arise when token-derived data is incorporated into queries, command construction, or reflection logic without proper sanitization. The combination of a flexible parameter DSL in Grape and the presence of authorization headers can unintentionally expose internal logic or data if user-controlled values are concatenated into system commands or interpreted too loosely.

Consider an endpoint that accepts an API token via the Authorization header and uses a query parameter to select a data source or filter. If the token is parsed and then used to construct a system command or a dynamic file path, an attacker who can influence the parameter may achieve command injection or path traversal. For example, a developer might write a helper that reads the token to enrich request context, then passes a user-supplied string into Open3 or system without validation. The token itself is not the secret here; it is the way derived values are handled that creates the injection surface.

Another pattern involves reflection or debugging endpoints that echo the token or metadata for troubleshooting. If these endpoints concatenate the token with unchecked input to form log messages or error strings, they can become vectors for log injection or, in rare server-side template scenarios, template injection. Even when Grape validations are used, injection can still occur if the validation layer permits certain characters that are later interpreted by a downstream processor as control characters or command separators.

SSRF is also relevant when a Bearer Token influences URL construction. An attacker might supply a URL that includes the token as a query or fragment, and if the application reuses that URL internally without strict schema and host validation, the server may make unintended internal requests. The token in this context does not cause SSRF by itself, but it can amplify impact if the request is made with higher privileges tied to the token’s scope.

Input validation checks in Grape using requires and values help reduce risk, but they must be complemented by output encoding and strict allowlisting when token-derived data flows into system-level operations. For instance, using the token to select a tenant identifier should be mapped through a whitelist rather than interpolated into SQL or shell commands. This ensures that even if an attacker influences the parameter, the execution path remains constrained.

Middleware that logs headers can also inadvertently store or transmit tokens in plaintext if combined with unsanitized message formatting. Injection here is not remote code execution but can lead to privacy violations or injection of malicious formatting into downstream systems that process logs. Proper canonicalization and context-aware escaping are necessary when token metadata is included in structured logs or forwarded to monitoring tools.

Remediation focuses on strict input handling, avoiding concatenation of user-influenced data into system commands, and ensuring token usage does not expand the attack surface. Below are concrete patterns and code examples for a Grape API.

1. Use allowlists for tenant or resource selection

Instead of using a token-derived or user-supplied string to dynamically choose a data source, map it through a predefined set of valid identifiers.

class App < Grape::API
  TENANTS = %w[alpha beta gamma].freeze

  helpers do
    def current_tenant
      # Assume token maps to a tenant; validate against allowlist
      tenant_from_token = validate_token_and_extract_tenant(request.env['HTTP_AUTHORIZATION'])
      TENANTS.include?(tenant_from_token) ? tenant_from_token : 'default'
    end

    def validate_token_and_extract_tenant(auth_header)
      return 'default' unless auth_header&.start_with?('Bearer ')
      token = auth_header.split(' ').last
      # In practice, decode or lookup tenant from token claims
      # This is a simplified example
      token_hash = { 'abc123' => 'alpha', 'def456' => 'beta' }
      token_hash.fetch(token, 'default')
    end
  end

  before { @tenant = current_tenant }

  get '/data' do
    # Safe: tenant is from an allowlist
    { tenant: @tenant, records: fetch_records_for(@tenant) }
  end
end

2. Avoid shell interpolation when using token-derived values

Never build shell commands with string interpolation that includes values influenced by the token or any user input. Use structured commands or libraries that avoid a shell entirely.

require 'open3'

class DataService
  def self.run_report(token_scope)
    # Unsafe: token_scope interpolated into shell command
    # cmd = "gpg --decrypt --recipient #{token_scope} report.gpg"

    # Safe: pass arguments as an array
    stdout, status = Open3.capture2e('gpg', '--decrypt', '--recipient', token_scope, input: File.read('report.gpg'))
    raise 'Decryption failed' unless status.success?
    stdout
  end
end

3. Validate and sanitize URLs when token influences request targets

If a Bearer Token is used in the context of making outbound requests, ensure that any URL built from user input is strictly validated to prevent SSRF. Do not trust the token alone to determine reachability.

class HttpConnector
  ALLOWED_SCHEMES = %w[https].freeze
  ALLOWED_HOSTS = %w[api.example.com internal.service.corp].freeze

  def initialize(auth_header)
    @auth_header = auth_header
  end

  def fetch(url)
    uri = URI.parse(url)
    raise 'Invalid scheme' unless ALLOWED_SCHEMES.include?(uri.scheme)
    raise 'Disallowed host' unless ALLOWED_HOSTS.include?(uri.host)

    # Use the token in the Authorization header, not in the URL
    request = Net::HTTP::Get.new(uri.request_uri)
    request['Authorization'] = @auth_header
    Net::HTTP.start(uri.hostname, uri.port, use_ssl: uri.scheme == 'https') do |http|
      http.request(request)
    end
  end
end

4. Prevent log injection via sanitization

When including token metadata in logs, ensure structured logging and avoid concatenating raw user input into message templates that could be misinterpreted by log parsers.

class AuditLogger
  def self.info(message, token_header)
    tenant = extract_tenant_safe(token_header)
    # Structured logging reduces risk of injection
    Logger.info({ event: message, tenant: tenant, timestamp: Time.now.utc }.to_json)
  end

  def self.extract_tenant_safe(auth_header)
    return 'unknown' unless auth_header&.start_with?('Bearer ')
    # Do not log the full token; extract only safe metadata
    'tenant_derived_value'
  end
end

5. Use framework validations and avoid loose parameter coercion

Grape validations should explicitly define types and avoid relying on implicit coercion that might interpret maliciously crafted values as code or commands.

params do
  requires :filter, type: String, values: { 'active' => true, 'inactive' => true }, messages: { invalid: 'must be active or inactive' }
  requires :sort, type: String, values: { 'name' => true, 'created_at' => true }
end