Adding reCAPTCHA support to Mailman

The GNOME and many other infrastructures have been recently attacked by an huge amount of subscription-based spam against their Mailman istances. What the attackers were doing was simply launching a GET call against a specific REST API URL passing all the parameters it needed for a subscription request (and confirmation) to be sent out. Understanding it becomes very easy when you look at the following example taken from our apache.log:

As you can the see attackers were sending all the relevant details needed for the subscription to go forward (and specifically the full name, the email, the digest option and the password for the target list). At first we tried to either stop the spam by banning the subnets where the requests were coming from, then when it was obvious that more subnets were being used and manual intervention was needed we tried banning their User-Agents. Again no luck, the spammers were smart enough to change it every now and then making it to match an existing browser User-Agent. (with a good percentage to have a lot of false-positives)

Now you might be wondering why such an attack caused a lot of issues and pain, well, the attackers made use of addresses found around the web for their malicius subscription requests. That means we received a lot of emails from people that have never heard about the GNOME mailing lists but received around 10k subscription requests that were seemingly being sent by themselves.

It was obvious we needed to look at a backup solution and luckily someone on our support channel suggested the sysadmins recently added CAPTCHAs support to Mailman.  I’m now sharing the patch and providing a few more details on how to properly set it up on either DEB or RPM based distributions. Credits for the patch should be given to Debian Developer Tollef Fog Heen, who has been so kind to share it with us.

Before patching your installation make sure to install the python-recaptcha package (tested on Debian with Mailman 2.1.15) on DEB based distributions and python-recaptcha-client on RPM based distributions. (I personally tested it against Mailman release 2.1.15, RHEL 6)

The Patch

Additional setup

Then on the /var/lib/mailman/templates/en/listinfo.html template (right below <mm-digest-question-end>)  add:

Make also sure to generate a public and private key at and add the following paramaters on your file:


Loading reCAPTCHAs images from a trusted HTTPS source can be done by changing the following line:


EPEL 6 related details

A few additional details should be provided in case you are setting this up against a RHEL 6 host: (or any other machine using the EPEL 6 package python-recaptcha-client-1.0.5-3.1.el6)

Importing the recaptcha.client module will fail for some strange reason, importing it correctly can be done this way:

and then fix the imports also making sure sys.path.append(“/usr/share/pyshared”) is not there:

That’s not all, the package still won’t work as expected given the API_SSL_SERVER, API_SERVER and VERIFY_SERVER variables on are outdated (filed as bug #1093855), substitute them with the following ones:

And then on line 76:

That should be all! Enjoy!

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