Mitigating DMARC damage to third party mail
DMARC is an anti-phishing technology. It has recently been repurposed by large mail providers for public webmail domains.
DMARC alignment requires that the From: line address in a mail message match either a DKIM signature d= domain, or an SPF-validated bounce address domain. Depending on a domain's published policy, unaligned mail may be quarantined, i.e., relegated to the spam folder, or rejected with a 5xx SMTP code by recipient systems. Mailing lists and other software that send legitimate but unaligned mail often find that mail rejected. Problems include recipients not getting the mail they want, and in some cases the normal list bounce processing removes the recipients from lists.
- 1 Sending side workarounds
- 1.1 Exclude domains that require alignment
- 1.2 Turn off all message modifications
- 1.3 Replace address with a generic one
- 1.4 Add fixed string such as .invalid to addresses
- 1.5 Rewrite addresses to forwarding addresses
- 1.6 Message wrapping
- 1.7 Mandatory digests
- 1.8 Ignore DMARC bounces
- 1.9 Third Party Authorization
- 2 Recipient workarounds
- 3 Cooperative solutions
- 4 Authorization approaches
Sending side workarounds
Techniques which can be applied unilaterally by any operator that sends unaligned mail.
Exclude domains that require alignment
Check the DMARC policy on incoming mail, and refuse mail from domains with quarantine or reject polcies.
- Users in affected domains have to get new addresses or leave.
- Operation otherwise unaffected.
Turn off all message modifications
Disable subject tags, message footers, and anything else that might break a DKIM signature.
- Still doesn't work for domains that validate mail with SPF.
- Doesn't help mail-an-article.
- Users can't recognize list mail by subject tags or anything else.
- Sorting by subject tags doesn't work.
- Often impractical when footers have required content.
Replace address with a generic one
Lists rewrite the From: line to use the list's address rather than the author's, either for all mail, or for mail from domains with policies.
Address may be moved into address comment, e.g.
From: foo list for Marissa mmeyer-at-yahoo.com <firstname.lastname@example.org>
Author's address may be placed in a Reply-To: header, give or take the usual issues about lists and Reply-To.
- Reply to author unavailable without Reply-To:
- Address book techniques based on recognizing From: address don't work or work differently.
- Author not visible in MUA message list.
- Mail sorting rules don't work the same.
- Encourages phishing by training users that address in the comment is equivalent to address.
- Breaks S/MIME signatures
Add fixed string such as .invalid to addresses
Add a suffix to From: line addresses with policies.
From: Marissa <email@example.com>
- Sorting rules might break depending on match details
- Reply to author requires usenet-style demunging
- Encourages phishing by training users that munged address is equivalent to address.
- Breaks S/MIME signatures
- Many mail systems reject mail if the From: address domain doesn't resolve.
Rewrite addresses to forwarding addresses
Rewrite addresses to a valid address that will forward to the original address, either using a wildcard domain suffix or a pseudorandom address.
From: Marissa <firstname.lastname@example.org>
From: Marissa <email@example.com>
- List function largely unchanged
- Requires list operator to run a forwarder, which must be managed to avoid relaying spam
- Forwarding addresses likely to end up in user address books, which means either the forwarder has to work forever or the address book entries will suffer bit rot. (Too bad no SMTP software supports 251 and 551 reply codes any more.)
- Encourages phishing by training users that modified address is equivalent to address.
- Breaks S/MIME signatures
Wrap each message in an outer message, similar to single-message digests.
- Looks like digests to users unless they have (so far nonexistent) MUA unwrapping extensions
- May encourage phishing depending on how MUA displays message.
Require all users, or users on recipient systems that enforce DMARC, to get list mail as digests
- All the usual reasons people don't like digests, mail is delayed until the digest is sent, replying to individual messages is harder, replying to the whole digest loses threads, etc.
Ignore DMARC bounces
Check for DMARC codes in bounce messages and disregard them for list management.
- Recipients lose mail
- Bounces may affect list reputation and delivery
Third Party Authorization
TPA is intended to replace RFC 6541 third-party signatures
TPA-Label allows either a community or domain managed list of informally federated domains using a single and small DNS transaction. Deployment only requires the trusted domain and the system evaluating DMARC alignment to implement TPA-Label processing. Few third-party services will be required to make any changes.
TPA-Label helps direct which message element provides alignment and can signal when a domain is excluded from the federation for policy reasons. The exclusion circumvents repeated processing during review of DMARC feedback against outbound logs which is only done by the Trusted Domain (email provider).
See the Internet Draft.
Techniques that can be applied unilaterally by receipient systems and users.
The recipient system or perhaps each individual recipient maintains a list of IPs or DKIM signatures for mailing lists and other non-hostile forwarders. Probably wouldn't be absolute whitelist, just skip DMARC alignment check, due to spammers compromising users' accounts and sending spam to addresses in their address books, including lists to which they are subscribed.
- Lets forwarders work the way they always have.
- A lot of work for recipient systems or users.
- Unsophisticated users could mistakenly whitelist malicious senders.
Techniques which need mutual cooperation in order to work.
Third party maintains DMARC-bypass whitelist of IPs and/or DKIM signatures of senders who send legitimate unaligned mail. Could probably seed with forwarders known to large mail systems. Probably wouldn't be absolute whitelist, just skip DMARC alignment check.
- Someone has to do it.
- Eligible senders need to find out about it and apply, if not seeded.
- Could allow more spam or phish depending on how careful forwarders are.
Per sender whitelist
Senders publish list of DKIM signatures allowed to send for them, like in RFC 6541.
- A lot of work for each sender
- Large senders unlikely to know all of the third parties that their users use.
Remailers include a header reporting on the authentication of the incoming message. AR header is intended to detect spear-phishing if the remailer doesn't.
- Since the header is trivially forgable, recipients have to know who to trust. (See whitelist above.)
- List operation unaffected if it works.
- Doesn't help mail-an-article.
Original sender includes a header with a signed token that identifies the relay. See this conditional DKIM signature draft for a possible implementation.
- A variety of bad things if the relay is compromised or tries to play games.
- Senders will probably only want to include them in mail to trustworthy relays. (See whitelist above.)
Ways to get a DKIM author signature or something similar on the message.
Authenticated Received Chain (ARC)
ARC defines a chain of DKIM-like signatures applied by forwarders so that a receiving host can estimate whether a message was authenticated when it was sent. Currently under development by an IETF working group, with drafts of the protocol spec and a usage guide.
- No changes to user-visible parts of messages. If it works, mailing lists and other forwarders can continue working as they always have,
- Proposed by large mail systems so they are likely to implement it.
- ARC headers are easy to forge, so recipients have to track forwarder reputation before believing ARC headers.
- Forwarders depend on recipients to implement it.
Signing key delegation
The author domain delegates a signing key to the list operator.
- Requires that mail providers trust the list operators.
- Scales poorly since each list operator needs to get a key from each sending domain.
Use API or SUBMIT to send mail through the author domain's server
- Requires a a password at each author domain
- Web submission (mail and article) could use API and oauth to avoid disclosing password
- Otherwise need users' password for SUBMIT
- Author domain likely to force its own bounce address
- Author domain likely to limit volume
As above, but send one copy back to the list to get signed, then remail that copy
- As above but avoids bounce address and volume issues
- Only works if all recipients get the same message, not customized per-recipient