A mailman security update has been released for Debian GNU/Linux 8 LTS to address a XSS issue.

DLA 2200-1: mailman security update

Package : mailman
Version : 1:2.1.18-2+deb8u5
CVE ID : CVE-2020-12137

A vulnerability was discovered in mailman. GNU Mailman 2.x before 2.1.30
uses the .obj extension for scrubbed application/octet-stream MIME
parts. This behavior may contribute to XSS attacks against
list-archive visitors, because an HTTP reply from an archive web
server may lack a MIME type, and a web browser may perform MIME
sniffing, conclude that the MIME type should have been text/html, and
execute JavaScript code.

For Debian 8 "Jessie", this problem has been fixed in version
1:2.1.18-2+deb8u5.

We recommend that you upgrade your mailman packages.

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS