Shopware's default newsletter opt-in settings allow for mass sign-up abuse

Shopware is an open source e-commerce software platform. Prior to 6.6.10.3 or 6.5.8.17, the default settings for double-opt-in allow for mass unsolicited newsletter sign-ups without confirmation. Default settings are Newsletter: Double Opt-in set to active, Newsletter: Double opt-in for registered customers set to disabled, and Log-in & sign-up: Double opt-in on sign-up set to disabled. With these settings, anyone can register an account on the shop using any e-mail-address and then check the check-box in the account page to sign up for the newsletter. The recipient will receive two mails confirming registering and signing up for the newsletter, no confirmation link needed to be clicked for either. In the backend the recipient is set to “instantly active”. This vulnerability is fixed in 6.6.10.3 or 6.5.8.17.