phpList before 3.6.15 is vulnerable to Reflected Cross-Site Scripting (XSS) via the /lists/dl.php endpoint. An attacker can inject arbitrary JavaScript code by manipulating the id parameter, which is improperly sanitized.
https://www.phplist.org/newslist/phplist-3-6-15-release-notes/
https://github.com/phpList/phplist3/compare/v3.6.14...v3.6.15