DMARC Escape

Services that depend on email notification experience customer attrition when notifications become phishing targets. leveraged either SPF or DKIM records to request message Reject or Quarantine handling when From header field domains do not align with either of these records. From header fields also having the role of Sender occurs with transactional notifications. However when the From header field does not play this role or does not share the same domain as the Sender header field, a different identity may need to supplant the role of Author to ensure delivery whenever DMARC policy requests Reject or Quarantine handling. section 10.5 offers feeble advice by stating although mediator transformations may conform with standards, this may not avoid message Reject or Quarantine handling nor does it allow provisions for alignment with verified Sender header fields likely preferable to diverted placement of valid and legitimate messages. Efforts at handling messages based on the Author role rather than the Sender is at the heart of the problem. Only the Sender role can establish a trustworthy message source and ensure domain alignment with SPF or DKIM records, not the Author. If DMARC had a provision for Sender header field alignment, this would enable effective exclusion of known bad or inclusion of known good domains by MTAs or MUAs. Admittedly, this mode of operation would not benefit those only seeking to protect email notifications from being phished, but would accommodate normal email exchanges. Such a provision demands ensuring the identity of the Sender permitting acceptance is shown to recipients. A feature allowing and displaying Sender alignment in MUAs is commercially available for domains handling normal user email. Such a provision would also allow DMARC to be more compliant with and less likely to disrupt messages undergoing mediator transformations or originating from verified, but different domains. Large Email Service Providers (ESPs) are able to manage their service in a manner not tolerated with smaller providers. Many large ESPs ignore abuse reports indicating compromised or abused user accounts or even message security being exploited. They are in a class best described as Too Big to Fail. DMARC permits these ESPs to export their managerial role onto receivers who must now cope with disrupted services. Both SPF and DKIM favor these ESPs by being referenced from a domain not actually managing the sending of messages. These authorization mechanisms leave others to suffer the repercussions of unbeknownst access. SPF authorizes outbound IP addresses used by domains to send mail. Until DMARC, SPF largely only squelched Delivery Status Notifications (DSNs) emitted from spoofed sources and DKIM never required domain alignment with From header fields. Now DMARC attempts to exclude messages where either DKIM or SPF records do not align with the From header field domains. DMARC lacks any fallback strategy when DMARC domains allow users to interact with mediators unable to retain compliance with DMARC. DMARC refuses to adopt assertions to indicate a policy that allows alignment with Sender header fields on the basis few recipients see these header fields and insist this leads to increased phishing. In reaction, some advocate use of . This proposes a new DKIM signature in the hope ESPs disrupting standards compliant messages with restrictive DMARC policy will instead use this mechanism to delegate their signing to third-party domains listed within the signature's header field. This new DKIM signature may require some selected header fields be retained unaltered while allowing the entire message body to change. A strategy that creates interesting situations when the header fields selected by this new DKIM signature does not coincide with those signed by the third-party DKIM signature designated to forward altered messages. DKIM is unable to ensure where a signed message fragment originates and is unable to constrain overall message volume or an associated number of recipients. While permitting third-parties to sign a different domain's From header field, the retained header fields represent those that DMARC advocates have claimed will not prevent phishing. Especially retention of never seen Message-IDs or Date header fields where any asserted expiry must still allow reasonable intervals for delivery with a window open to potentially subscribed malefactors. Even the selection of message destinations to receive this new DKIM delegation signature represents similar vetting required of that has an advantage of being able to mitigate detected sources of abuse. An effort to deploy TPA-Label can be greatly reduced for the customers of large ESPs by having DMARC reference a consolidated and centrally managed TPA-Label zone. ESPs unwilling to accept DMARC alignment with Sender header fields as a fallback scheme seem equally unlikely to include a new DKIM signature that dynamically delegates a proxy signer for their domain. Especially since a mailing-list is unlikely to reject messages a DMARC domain may consider egregious. A scheme that would allow any such message to be replicated without limit within whatever expiry time adopted. Even the mailing-list may not see all header fields a malefactor might employ in their campaign. Users naturally expect to have an ability to use email services gainfully employed for decades. Instead, due to some ESPs, users encounter the service's messages either being rejected or quarantined and ironically are finding the identity of the Author becoming more difficult to ascertain as these services abandon the role of the From header field as a practical means to ensure delivery. DMARC's use of DKIM or SPF means malefactors only need an ability to exploit either scheme. An unfortunate progression that is making email less reliable and the identity of the Author less certain. As DMARC becomes more broadly deployed, how will improved SMTP security via opportunistic DANE TLS be introduced which should soon offer a secure global host identity scheme for email? DNSSEC/DANE overcomes security weaknesses found in both routing and message exchange. While some claim DNSSEC/DANE is not practical they also misrepresent weaker methods based on IP address authorization or signed message fragments as representing domain authentication. IP address based authorization or potentially malformed message fragments can not safely verify the binding of a domain with that of a message. DMARC even offers malefactors feedback that can enhance the exploitation effectiveness or leak relationship information that can be used to facilitate deceptions. Misleading use of the term "authentication" which conflicts with and occurs with and .

A domain referenced by SPF or a domain confirmed in a DKIM signature has not posed a problem since seldom was acceptance based on From header field Domain Alignment with a domain used by these two methods. However, when acceptance is based on From header field alignment in the case of which may use either SPF or DKIM related domains, this may disrupt many Third-Party Services where the expected reaction to this problem has the effect of deprecating the use of the From header field retaining the role of Author. The disruption becomes egregious when messages from the domain's own users are rejected based on an erroneous level of the domain's asserted alignment practices. At the strictest alignment level, erroneous assertions not only disrupt messages from their users, it also affects subscriptions or services for other users of affected third-party services. SPF normally provides a form of authorization by listing IP addresses of authorized outbound servers. In many cases, these servers represent a shared resource used by perhaps thousands of domains. SPF is unable to verify an IP address represents the actions of a claimed domain which does not meet the definition of "$ authentication" in . DKIM intended to establish increased levels of trust based upon verified DKIM signatures controlling acceptance and what a user sees within the From header field. But DKIM failed to include in its header stack processing a scheme to actively guard against pre-pended header fields. This would ensure acceptance is not based on verified DKIM signatures that fail to prevent header field spoofing. Even now, this weakness allows malefactors to exploit DKIM signature acceptance established by Large ESPs to spoof ANY other domain, even when prohibited within the Signer's network. DKIM signatures are verified by a process that MUST examine the entire header field stack and yet needs some prior unreported and unknown message structure verification. Inclusion of this undefined process has proven problematic in preventing header spoofing. It took several years for one of the largest service providers to notice this oversight long after arguments were made about this risk. Ignoring essential header field stack validation that MUST occur represents an oversight in the DKIM deployment specifications that at one time had been partially addressed by the earlier DMARC specifications. It seems even this validation was removed by what might be described as a misguided insistence such processing is to remain the responsibility of the transport. Section 3.3 clearly indicates messages SHOULD NOT be rejected based on perceived defects in message structure. Section 7.1 also warns against preventing spoofing within the SMTP transport and suggests much safer PGP or S/MIME, both of which benefit by deployment of DANE. DMARC was developed to curtail phishing attempts leading to user attrition with high volume transactional services. Unfortunately, DMARC is being (ab)used to lessen phishing attempts related to general user accounts where there seems little interest at finding a solution for the problems this creates.

For domains handling normal user email, a special DMARC policy assertion "p" request policy that is suitable for public email users which recognizes alignment with Sender header fields when present. This makes an assumption users employ Mail User Agents that display the identity contained in the Sender header field when used as a basis for acceptance. A new "IM-From" header field re-establishes the Author role for RFC5322.From domains affected by DMARC. This header permits re-locating the Author role to a new header to establish the integrity of third-party services. Establishing a new header prevents confusion caused by unknown alternatives, such as Reply-To, or Original-From, or indirectly through the use of Original Authentication Results header (OAR). Munging Reply-To or From header fields removes information essential for establishing side discussions rather than having all conversations on the list. Not all conversations are suitable for the entire forum. A different domain is specifically excluded from actions caused by non-alignment when authorized by the DMARC domain using . DMARC could make an assertion of "sam=tpa; and tpa=third-party-authority.example.com;" when the DMARC domain offers the Specific Advisory Methods "sam=" tag indicating the third-party advisory methods supported. The "tpa=" tag can also indicate the domain location where third-party-authorization hashes have been consolidated with an assumed prefix of "_smtp._tpa.<tpa-domain>". A few large domains have had a high percentage of user accounts compromised. These events gave malefactors access to prior private exchanges and contact lists. Even after accounts were reclaimed, malefactors continue sending convincing spoofed messages from other sources. To mitigate harm, some domains have asserted DMARC Alignment policies similar to those used by domains that only emit transactional messaging where a prior DMARC recommendation of restricted use was normally heeded. In addition, some domains also recommended "reject" rather than "quarantine" as a misalignment response. In conjunction with misleading DMARC alignment assertions, rejection becomes a highly disruptive choice. Currently, the least disruptive adjustment made by receivers faced with Third-Party services used by a RFC5322.From domain is to override their policy of "reject" with "quarantine" to allow delivery of the message causing users to search through their "quarantine" folder for otherwise lost messages. Alternatively, the From header field may replace the Author role with that of the Sender by asserting a policy of "p" intended to assert the strongest protection suitable for a domain supporting email being used by the general public which allows alignment to occur with Sender header fields. Some have suggested the From header field contents be retained in the Reply-To header. This document offers an alternative to the use of X-Original-From header field and that it be given the name IM-From header field that has additional semantics not available with the normal From header field. Use of IM-From header field claims the role of Author that has been lost due to DMARC. It is unfair to place a large burden on receivers and expect them to remain cooperative. Prior to making alignment assertions likely to disrupt services handling legitimate messages, it is possible for RFC5322.From domains to make assertions which allow compliance with normal email handling. When RFC5322.From domains proactively guard against disrupting legitimate messages, receivers are more likely to cooperate with their recommendations. When the asserted policies prove disruptive over time, DMARC should offer receivers reasonable overrides.

Deterrents based upon reputation and/or path based scoring strategies that utilize a variety of originating header fields has proved ineffective. These header fields often remain invisible to recipients, and contain domains exploited for periods measured in hours to avoid any Whack-A-Mole like response. Even long term reputations have issues due to an intermix of messages from compromised accounts. Content filtering is unable to keep up with the polymorphic abuse. Few recipients will inspect the stack of message header fields, or be able to draw useful conclusions from a profusion of unfriendly information. As a result, many recipients deal with abuse by sorting messages into groups based on assumed sources found in a few originating header fields. DMARC represents an open registry that offers domain specific guidance for DKIM/SPF alignment sending practices to determine whether messages should be delivered, quarantined, or refused. However, appropriate actions become unclear whenever Third-Party Services are involved. Although DMARC warns of a potential for disruption, the specific handling requested by DMARC is very limited. DMARC expects receivers to devise their own special handling to mitigate disruptions that DMARC assertions might cause for legitimate messaging. This is unfortunate, since the necessary feedback is given to the DMARC asserting domain and not to the cooperating receivers. When a Third Party domain does not employ DKIM or SPF or does not include Authentication-Results header fields or perhaps (OAR) or its "X-" version could allow authorizations to be exploited. For Third Party domains not applying DMARC but capture the OAR, past compliance with DMARC based on the OAR can be made a requirement for authorization. While conceivably Domain Alignment might just rely on the content of the Original-Authentication-Results header, whether to trust this, or any other message content can not be based on the mere acceptance of the message alone. Whether false content even effects message acceptance would be difficult to determine. Only the DMARC asserting domain is able to make this type of determination based on their knowledge of outbound messages and corrections needed based on DMARC feedback. im-from = "IM-From:" (mailbox-list / address-list) CRLF address = mailbox / group group = display-name ":" [group-list] ";" [CFWS] mailbox = name-addr / addr-spec addr-spec = local-part "@" domain ["/" resourcepart] name-addr = [display-name] angle-addr angle-addr = [CFWS] "<" addr-spec ">" [CFWS] group = display-name ":" [group-list] ";" [CFWS] display-name = phrase mailbox-list = (mailbox *("," mailbox)) address-list = (address *("," address)) group-list = mailbox-list / CFWS mailbox-list = (mailbox *("," mailbox)) Use of the IM-From header group display name can be used to replace the use of list tags embedded in the Subject header field. Since this header is ignored by DMARC, it can also retain the identity of the Author. This field also permits the use of the resourcepart extension to support XMPP endpoints as defined in and which is to be ignored otherwise.

DMARC policy assertions are transitory so exclusion of users within a DMARC domain is never assured. Unless all valid Third-Party Domains have been authorized or allowed a suitable From header field alternative, personally identifiable information will be exchanged within the DMARC feedback. This feedback can unintentionally expose private exchanges made on behalf of the RFC5322.From domain's users. To the greatest extent possible, this feedback information should not be shared with other domains not offering the information. This feedback can even identify mailing-list subscribers that never sent any message to the list, or invoices made on behalf of an accountant's client.

This draft extends Domain Alignment validation practices that depend on DKIM or SPF . Most related security matters are discussed in those specifications. Additional considerations are also included in . Some receivers mistakenly bypass validation of the header fields because a signature from a Trusted Domain had been confirmed as perhaps suggested in . Validation of the header stack MUST NOT be omitted unless the message is not accepted for other reasons. Services that depend only upon path authorizations might permit the RFC5322.From domain to be spoofed and obtain acceptance. During such events, the RFC5322.From domain might need to retract its authorization from the service. For this reason, path related validation based on IP addresses should only be used as a carefully monitored interim solution.