---
title: "Fighting Mailman Subscription Spam: The Easy Way"
categories: sysadmin
---

I recently noticed that both of the Mailman setups that I am running are being
abused for *subscription spam*: Bots would automatically attempt to subscribe
foreign email addresses to public mailing lists, resulting in a subscription
notification being sent to that address.  I am still extremely saddened by the
fact that this is a thing---whoever sends this spam has no direct benefit and no
way of selling anything (they don't control the content of the message); the
only effect is to annoy the owner of that email address, the victim.  That seems
to be enough for some. :(

Oh, and my servers' reputation goes down because people mark these emails as
spam.  So, more than enough reasons to try and stop this.

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### The Big Guns

My first reaction was to go and look for a way to add a CAPTCHA to the
subscription page.  Unfortunately, Mailman 2 itself only very recently (with
version 2.1.26) gained support for CAPTCHAs, and even that just supports
Google's reCAPTCHA.  I am not going to expose my users to Google's tracking like
that, nor am I willing to actively discriminate against people not having Google
accounts (reCAPTCHA is much more annoying if Google can't track you because you
are not logged in), so reCAPTCHA was clearly not an option.  Instead, the plan
was to look at one of the patches that add CAPTCHA support to older versions of
Mailman and implement a simple question-and-answer CAPTCHA myself.

**Update:** I previously claimed Mailman 2 does not support CAPTCHAs at all,
  which turned out to be incorrect. **/Update**

### Keep It Simple

But then, while just getting started on this and browsing the Mailman sources, I
found out about `SUBSCRIBE_FORM_SECRET`.  `SUBSCRIBE_FORM_SECRET` is a Mailman
config option that, once set to a random string, will make Mailman embed a
[CSRF token](https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet)
into the subscription form.  Mailman will also enforce that the form must be
submitted *at least* five seconds after it was generated.  Since the bots that
have found my servers so far are much less patient than that, just setting
`SUBSCRIBE_FORM_SECRET` was enough to completely get rid of the subscription
spam.

So, if you are reading this and running a Mailman installation: **Please set
`SUBSCRIBE_FORM_SECRET` and protect your setup against abuse!** Just run
`openssl rand -base64 18` to get some random string, and then add
`SUBSCRIBE_FORM_SECRET = "<random string here>"` to `/etc/mailman/mm_cfg.py`.
It's really that simple!  Just a
[four-line patch in my Ansible playbook](https://git.ralfj.de/ansible.git/commitdiff/937b170594be82e500ae726dc47de8ca9ef3dfcf)
to get this rolled out to all servers.  Note that you need to be at least on
Mailman 2.1.16 for this to work; all currently supported versions of Debian come
with a recent enough version (if you use backports on Debian 7 "Wheezy").

The more people do this, the more it will help to stop this kind of spam.  Or
rather, it'll force the spammers to upgrade their game.  I assume eventually I
*will* have to add a CAPTCHA.  Or maybe there is a simple and reliable way to
migrate to Mailman 3 before that happens---and maybe that will have more
reasonable CAPTCHA options, something beyond just reCAPTCHA.