baserCMS is a website development framework. In versions 5.2.5 before 5.3.2 and 5.4.0 before 5.4.2, an unauthenticated visitor can supply a comma-separated address list in a public mail form's email field and cause a single auto-reply to be addressed to multiple third parties. MailMessageMailer::sendFormToUser() passes the visitor-controlled userMail value to parseAndNormalizeMailAddresses(), which splits it with explode(',') and supplies every resulting address to setTo(). Newly created forms with an email-type field are affected when the separate, optional VALID_EMAIL server-side validation is not enabled; browser type=email validation does not prevent crafted requests. The bundled contact form enables VALID_EMAIL and is not affected. The auto-reply uses the site's sender identity and includes partially attacker-controlled content, allowing abuse of the site to deliver unsolicited mail. This issue has been patched in versions 5.3.2 and 5.4.2.
https://github.com/baserproject/basercms/security/advisories/GHSA-w47c-4g8m-gjv9
https://github.com/baserproject/basercms/releases/tag/5.4.2
https://github.com/baserproject/basercms/releases/tag/5.3.2
https://github.com/baserproject/basercms/commit/99e6257c9d5e2fa2bff2351defcb9fb37f69daf1