GHSA-mfxh-vp55-7gc6 Contao: The registration module re-sends activation mails

• Packagist contao/core-bundle: >= 4.1.0, < 5.3.50 → fixed in 5.3.50 • Packagist contao/core-bundle: >= 5.4.0-RC1, < 5.7.12 → fixed in 5.7.12

`ModuleRegistration::compile()` reaches its follow-up registration branch on any POST to a page carrying the module. That branch checks neither `FORM_SUBMIT` nor the captcha result computed immediately above it, and `resendActivationMail()` leads to `OptInToken::send()`, which has no rate limit at all. Impact Anyone on the internet can make a Contao installation send unlimited mail to an address of their choosing, from the site's own sender and reputation, at one outbound message per HTTP request. That is both a nuisance for the recipient and a deliverability risk for the site operator. The same request is a reliable account oracle for "this address has a pending registration on this site", which is exactly the sort of membership fact a public site is usually expected not to disclose. Honest bound. The target must have an unconfirmed registration, that is `tl_member.disable = 1` together with an unconfirmed `reg-` opt-in token. An attacker can create that state for an arbitrary address, since registration requires no ownership proof, but on a site where `reg_activate` is off the branch is unreachable.