DKIM signatures stripped from -owner messages with anonymous lists

[Robert Mathews]

If a list is set to be an anonymous list, and a message is sent to the -owner address, Mailman strips any existing DKIM header.

This means that if someone from a DMARC-restricted address (e.g. yahoo.com) is sending a message that would get forwarded to an owner at a DMARC-checking ISP (e.g. yahoo.com), the message is rejected: it fails the DMARC check due to non-matching SPF and missing DKIM headers. If the DKIM header was left intact, it should work, since Mailman didn't modify the body for an -owner message.

It makes privacy sense to always strip DKIM headers on messages that will be posted to an anonymous list. And it can work out okay because DMARC munging mitigation can be applied afterwards.

But it doesn't seem to make sense to do the same for -owner messages on anonymous lists. Mailman doesn't apply other anonymous list modifications, like hiding the "From:" header, for -owner messages, as far as I can tell.

This happens, by the way, due to Defaults.py:

# This is the pipeline which messages sent to the -owner address go through
OWNER_PIPELINE = [
    'SpamDetect',
    'Replybot',
    'CleanseDKIM',
    'OwnerRecips',
    'ToOutgoing',
    ]

Is 'CleanseDKIM' really helpful in this -owner flow? Removing it would solve this problem. Alternately, perhaps CleanseDKIM could be taught to exempt -owner addresses on anonymous lists.

[Mark Sapiro (msapiro)]

Thanks for your report. I agree that CleanseDKIM should not be in OWNER_PIPELINE and I have removed it for the next release.

In the mean time, you can always put

OWNER_PIPELINE.remove('CleanseDKIM')

in mm_cfg.py.