Here is what I was thinking of, a security weakness in Mailman's WebUI for subscribing, that permitted spammers to sneak arbitrary addresses onto a Mailman list's roster by generating forged POST datastreams from offsite. Description of the bugfix, which was merged starting 2.1.16rc1:

There is a new mm_cfg.py setting SUBSCRIBE_FORM_SECRET which will put a dynamically generated, hidden hash in the listinfo subscribe form and check it upon submission. Setting this will prevent automated processes (bots) from successfully POSTing web subscribes without first retrieving and parsing the form from the listinfo page. The form must also be submitted no later than FORM_LIFETIME nor no earlier than SUBSCRIBE_FORM_MIN_TIME after retrieval. Note that enabling this will break any static subscribe forms on your site. See the description in Defaults.py for more info

https://bugs.launchpad.net/mailman/+bug/1082746 https://mail.python.org/pipermail/mailman-users/2014-June/077202.html https://launchpad.net/mailman/+milestone/2.1.16rc1

SUBSCRIBE_FORM_SECRET must be set to "write your own string here" in mm_cfg.py (a site-specific hash written by the site admin.

Mailman then embeds a CSRF (cross-site request forgery) token into its subscription form. Mailman will also enforce that the form be submitted at least five seconds after it was generated.

Michael, please confirm that is set.