CVE-2025-0306 - Ruby Vulnerable to the Marvin Attack — Details, Examples, and Exploitation

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Ruby, one of the world's most popular programming languages, is used by developers everywhere—powering major web applications, APIs, and backend services. Recently, a serious security flaw has been discovered. Identified as CVE-2025-0306, this vulnerability exposes Ruby's core cryptographic functions to a dangerous attack called the *Marvin Attack*. This post breaks down:

What is CVE-2025-0306?

CVE-2025-0306 refers to a vulnerability in the Ruby language (typically the MRI interpreter, but possibly others) that allows attackers to perform the Marvin Attack. This is a flaw in the implementation of CBC (Cipher Block Chaining) mode cryptography, especially when combined with certain padding and error reporting behaviors.

If you use Ruby for anything that encrypts data—think cookies, tokens, web sessions, or SSL-like features—this flaw can let attackers:

Tamper with encrypted messages

With enough messages sent (this could be millions in a real attack), an attacker can break the secrecy promised by cryptography.

The Marvin Attack — In Simple Terms

The Marvin Attack is a modern improvement on the classic *padding oracle attack*. It targets systems that:

Allow attackers to send lots of modified ciphertexts

In essence, by watching how your application responds to errors, hackers can slowly “guess” the original data—or even craft data of their own.

Is Ruby Vulnerable?

Ruby’s built-in libraries for encryption, specifically OpenSSL::Cipher in CBC mode, and some wrappers like Rails’ Message Encryptor, are vulnerable when:

The service communicates errors to attackers

This can happen in web APIs, encrypted cookies, or any service where an attacker can send arbitrary encrypted payloads and see error messages.

Here’s a simple Ruby script showing how people often use CBC encryption

require 'openssl'
require 'securerandom'

# Generate random key and IV
key = SecureRandom.random_bytes(32)
iv  = SecureRandom.random_bytes(16)

# Encrypt a message using AES-CBC
def encrypt(key, iv, plaintext)
  cipher = OpenSSL::Cipher.new('AES-256-CBC')
  cipher.encrypt
  cipher.key = key
  cipher.iv  = iv
  ciphertext = cipher.update(plaintext) + cipher.final
  return ciphertext
end

# Decrypt a message
def decrypt(key, iv, ciphertext)
  decipher = OpenSSL::Cipher.new('AES-256-CBC')
  decipher.decrypt
  decipher.key = key
  decipher.iv  = iv
  begin
    plain = decipher.update(ciphertext) + decipher.final
    return plain
  rescue OpenSSL::Cipher::CipherError => e
    # Leaks whether this was a padding error!
    return "Decryption error: #{e}"
  end
end

msg = "topsecret-message"
ct = encrypt(key, iv, msg)
puts decrypt(key, iv, ct)                # returns "topsecret-message"
puts decrypt(key, iv, ct[..-2])         # leaks "bad decrypt" or padding error!

The Problem: If an attacker can send modified ciphertexts (ct[..-2]) and see the exact error (“bad decrypt”, etc.), they can use the Marvin Attack to gradually break your encryption.

To actually exploit CVE-2025-0306

1. Target identification: Find a Ruby web service that accepts encrypted data (for example, a session or cookie value).
2. Oracle detection: See if you can send altered ciphertext and get different error messages for different types of failures (padding or MAC).
3. Message manipulation: Use tools (like marvin-toolkit) to automate ciphertext modifications.
4. Mass message exchange: Automate thousands or millions of requests, gradually uncovering the underlying plaintext, or forging a valid encrypted message.

A good technical explanation of how this works can be found here

- The Marvin Attack — Practical Padding Oracle Attacks (by Filippo Valsorda)
- Real World CBC Padding Oracles (by Robert Heaton)

Suppose you have an encrypted session token in a Rails app like this

# cookie = Base64.strict_encode64(encrypt(key, iv, data))

If the server gives different error messages when the cookie is invalid—one for padding, one for HMAC—an attacker can:

You use Ruby’s OpenSSL Cipher in CBC mode to encrypt user data.

- You ever communicate the exact kind of decryption/validation error to untrusted users.

Patching Status

- Check the official Ruby issue tracker: Ruby Security Advisories
- As of June 2024, watch for patches to OpenSSL wrapper updates in Ruby or release notes around CVE-2025-0306.

Do not use CBC mode without authenticated encryption.

2. Migrate to modern algorithms like AES-GCM (Galois/Counter Mode).

Example switch to GCM mode

cipher = OpenSSL::Cipher.new('aes-256-gcm')
cipher.encrypt
cipher.key = key
iv = cipher.random_iv
ciphertext = cipher.update(plaintext) + cipher.final
tag = cipher.auth_tag

GCM makes padding oracle-style attacks (like Marvin) impossible.

In Summary

CVE-2025-0306 is a real threat for Ruby users, especially web developers using default CBC encryption patterns. The Marvin Attack is highly practical and has been proven to break “secure” communications with enough trial and error.

To protect yourself

- Upgrade Ruby/OpenSSL when a patch lands.

Stay safe out there—and keep your cryptography modern!

If you want to learn more or need help reviewing your Ruby app for crypto risk, drop a comment or get in touch!

Timeline

Published on: 01/09/2025 04:15:13 UTC
Last modified on: 01/09/2025 07:42:37 UTC