Mailpit is an email testing tool and API for developers. Prior to version 1.28.3, Mailpit's SMTP server is vulnerable to Header Injection due to an insufficient Regular Expression used to validate "RCPT TO" and "MAIL FROM" addresses. An attacker can inject arbitrary SMTP headers (or corrupt existing ones) by including carriage return characters ("\r") in the email address. This header injection occurs because the regex intended to filter control characters fails to exclude "\r" and "\n" when used inside a character class. Version 1.28.3 fixes this issue.