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Posts Tagged ‘Reporting and Conformance’

DMARC: The State of Domain-based Email Authentication – Part 2

Monday, September 11th, 2017

Building a safer email ecosystem with DMARC

In our previous post, we described two techniques for authenticating an email sender:

  • Sender Policy Framework (SPF), IETF RFC 7208, which verifies if the sending MTA is indeed authorized to send mail on behalf of a domain; and
  • DomainKeys Identified Mail (DKIM), IETF RFC 6376, where a domain shows “ownership” of a mail it sends by signing portions of it so that critical aspects cannot be forged by intermediaries.

Like most technologies, these are just individual weapons in the arsenal for fighting phishing and spam. Weapons, like all tools, need to be properly used if they are to be effective. Unfortunately, as we described in the earlier post, both SPF and DKIM are deployed in a manner that reduces their usefulness. With SPF, the validation policy set by the sender is often chosen in a manner that leaves handling authentication failures at the discretion of the recipient. DKIM, on the other hand, does not even have an explicit policy directive set by the sender. Moreover, in a heterogeneous mail environment, some perfectly legitimate MTAs might not be capable of signing messages.

Building a safer email system with DMARC

Thus, receivers in actual deployments tend to “soft fail” any SPF and/or DKIM validation failures as there are reasonable situations when legitimate mail can fail such checks. A common example is forwarded mail (which fails SPF), or mail sent via a mailing list (which fails DKIM). Mail providers consider it better to deliver most mail (even if some are fake or spammy) rather than risk dropping legitimate mail. Thus, neither of these techniques individually or combined provide clear guidance to receivers, and the resulting actions can be inconsistent.

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Stopping Forged Email 3: DMARC to the Rescue

Monday, March 2nd, 2015

We have recently looked at how hackers and spammers can send forged email and then seen how these forged messages can be almost identical to legitimate messages from the purported senders.  In fact, we learned that generally all you can trust in an inbound email message is the internet IP address of the server talking to your inbound email server — as this cannot realistically be forged in any way that would still enable you to receive the message.

In our previous two posts in this series, we examined how SPF and DKIM can be used to help limit forged email messages based on validating if a message was sent by an approved server by looking at the IP address delivering the email message to you and based on digitally signing messages.  We found that while SPF and DKIM can work, they has many significant limitations that cause them to fall or be insufficient to stop forgeries in many cases.

However, SPF and DKIM address the forgery problem in very different and, in many respects, very complementary ways. For this reason, many organizations use both technologies.

If you are using both technologies and you have a good amount of control over where your domain’s messages are coming from, then you can step up your game by using DMARC — Domain-based Message Authentication, Reporting and Conformance. 

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