The remote Debian host is missing a security-related update.
Michael Kaczmarczik discovered a vulnerability in the web interface template editing function of Sympa, a mailing list manager. Owner and listmasters could use this flaw to create or modify arbitrary files in the server with privileges of sympa user or owner view list config files even if edit_list.conf prohibits it.
Upgrade the sympa packages. For the stable distribution (stretch), this problem has been fixed in version 6.2.16~dfsg-3+deb9u1.