The vulnerability stemmed from insufficient password complexity checks. The key evidence is the patch adding :zxcvbnable to Devise modules in User.rb and requiring the devise_zxcvbn gem. Before 9.2.10, the User model only used Devise's standard :validatable which performs basic validations (length >= 8 chars, confirmation match) but doesn't enforce complexity. The test case changes from 'top-secret' to 'top-Secret12!$#' and added password strength tests confirm the absence of complexity validation was the root cause.