Individual D. Otis Internet-Draft D. Rand Intended status: Informational Trend Micro Expires: November 13, 2013 May 12, 2013 DKIM is Harmful as Specified draft-otis-dkim-harmful-00 Abstract Currently, email lacks conventions ensuring SMTP clients can be identified by an authenticated domain. Unfortunately many hope to use DKIM as an alternative, but it is independent of intended recipients and domains accountable for sending the message. This means DKIM is poorly suited for establishing abuse assessments for unsolicited messaging of commercial email otherwise known as SPAM, nor was this initially DKIM's intent. DKIM lacks message context essential to ensure fair assessment and to ensure this assessment is not poisoned. DKIM was instead intended to establish increased levels of trust based upon valid DKIM signatures controlling acceptance and what a user sees within the FROM header field. But DKIM failed to guard against pre-pended header fields where any acceptance based on valid DKIM signatures is sure to exclude header field spoofing, especially that of the FROM. This weakness allows malefactors to exploit DKIM signature acceptance established by high-volume DKIM domains to spoof ANY other domain, even when prohibited within the Signer's network. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any Otis & Rand Expires November 13, 2013 [Page 1] Internet-Draft DKIM-HARMFUL May 2013 time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 13, 2013. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Otis & Rand Expires November 13, 2013 [Page 2] Internet-Draft DKIM-HARMFUL May 2013 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Maintaining Trust . . . . . . . . . . . . . . . . . . . . . . 4 3. Responding to Defects and Exploitation . . . . . . . . . . . . 5 4. SMTP Can't . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. DKIM Vulnerability . . . . . . . . . . . . . . . . . . . . . . 6 6. Barriers to an Authenticated Domain . . . . . . . . . . . . . 7 7. Domains as a Basis for Managing Traffic . . . . . . . . . . . 8 8. XMPP Shows the Way Forward . . . . . . . . . . . . . . . . . . 8 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 10. Security Considerations . . . . . . . . . . . . . . . . . . . 9 11. References - Informative . . . . . . . . . . . . . . . . . . . 9 Appendix A. DKIM Examples . . . . . . . . . . . . . . . . . . . . 10 Appendix B. Stats . . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 Otis & Rand Expires November 13, 2013 [Page 3] Internet-Draft DKIM-HARMFUL May 2013 1. Introduction Currently, IPv4 address reputation provides the primary basis for defending SMTP open services. Use of IP addresses in this role completely breaks down when dealing with IPv6 [RFC2460]. There are currently 18,209,237,415,985,168 /64 equivalent IPv6 prefixes routed. [v6-BGP-Rpts]. In comparison, for IPv4 there are 2,614,711,792 IP addresses routed. While IPv4 is reaching its maximum, IPv6 has about 0.1% of the available /64 prefix routed and this continues to grow rapidly. Unlike IPv4, there is no practical means to scan reverse DNS namespace within IPv6 since each /64 prefix may contain any number of PTR records ranging up to 184,000,000,000,000,000,000. A technique commonly employed to automate IPv4 address categorization of suitable hosts is to check whether reverse PTR records appear to represent valid hostnames. Those that represent 4 decimal numbers are often considered unacceptable, for example. Our processing of reverse DNS namespace in cooperation with network providers now excludes about 38%, or about 1,000,000,000 IPv4 addresses. Comparing IPv6 /64 prefixes with the remainder of routable IPv4 addresses shows there are 11.3 million times more IPv6 /64 prefixes needing categorization. In addition, there is no practical means to facilitate this effort. Some also suggest there will not be a significant increase in the number of servers running over IPv6 and since their overall number should be comparable, email should still be dealing with a similar number of IP addresses. Unlike IPv4, IPv6 does not constrain the number of IP addresses assigned to a network interface. This feature allows each connection from a server to originate from a different IP address over the entire life of its operation while also having an ability to effortlessly change /64 prefixes. The potential increase allowed by IPv6 may prove explosive. Many now hope DKIM will be able to offer a domain identity to provide a basis for acceptance to replace that of the IP address used by SMTP clients. With a lack of uptake by commercial reputation services, there are proposals within the IETF aimed at establishing DKIM as a basis for reputation schemes in the Repute WG. When such an ill- considered effort is combined with DKIM's inability to detect invalid prefixed header fields, the results can prove highly harmful. 2. Maintaining Trust Not every subsystem or protocol layer should be expected to repeat previous security checks, however critical checks should not be assumed, especially those that involve a trivial amount of effort. Otis & Rand Expires November 13, 2013 [Page 4] Internet-Draft DKIM-HARMFUL May 2013 With high levels of abuse resulting from email's open nature, delegating checks in a structured manner better conserves essential resources. However, email's highly distributed store and forward protocol could not function if rigid message structures were enforced by the transport. New authentication or presentation requirements may involve small structural adjustments. For example, internationalization introduced a format negotiation not assured to survive beyond the next hop. 3. Responding to Defects and Exploitation With the advent of aviation, the world marveled at the skill and intellect taking us to ever greater heights. With aviation, faults threatening security, that when found, demanded our attention and diligence to effect repair. As with aviation, the success of email has risen to great heights. Email has become an integral component in general commerce and the maintenance of security such as reporting system failures, break-in attempts, and facilitating account access recovery. Reporting or predicting failure should not be viewed as exhibiting a lack of respect for achieved accomplishments. Noting and repairing faults only signify the importance of email's prominent role. As with most security related protocols, responding to noted defects is fairly common. Not responding to discovered defects in a security related protocol would be shocking. 4. SMTP Can't SMTP [RFC5321] recommends against rejecting messages based upon perceived defects in the message structure. This liberal acceptance permits evolutionary changes in message specifications starting at [RFC0822] that was based on [RFC0733] replaced by [RFC2822] and again by [RFC5322], [RFC6152], [RFC6532], and [RFC6854]; the second to last paragraph in section 3 of [RFC5321] provides a definitive statement messages should not be rejected due to perceived defects in the [RFC0822] message structure. The initial reference to [RFC0822] in this paragraph offers two foot notes with the second referencing the latest version of [RFC0822] which is [RFC5322] which itself has recently been updated. The impact of initially removing text specifically indicating which header fields are not to repeat is unknown. This information was implied within the then-new ABNF notation. Clarifying text for this requirement did not return until the [RFC0822] revision 19 years later which also indicates this specification's success at providing a foundation that allowed email to flourish. Otis & Rand Expires November 13, 2013 [Page 5] Internet-Draft DKIM-HARMFUL May 2013 There are many SMTP servers that have been in operation for decades with years passing between security patches. Such an accomplishment is most remarkable considering the volume of traffic being handled, often from highly malicious sources. This amazing stability and scalability with high levels of security would not have been possible if SMTP had been expected to validate message formats. Expecting SMTP to validate message formats to protect against vulnerabilities pertaining to protocols such as DKIM does not scale. The general use of DKIM permits signature checks subsequent to acceptance where only the status of signatures determines internal placement. As such, it becomes critical to ensure a valid signature is never declared having malformed header field stacks. To accomplish this, the DKIM specification must change. 5. DKIM Vulnerability DKIM permits a vulnerability by not checking the message header field stack for invalid repeats when signing or verifying a signature. The DKIM signature process must walk both down and then up the header field stack while selecting the header fields to be included in the hash process of the signature. The DKIM process will even ignore prefixed FROM header fields which is the only header field always included. The WG concluded that "listing non-existent header fields as signed" hack added in non-normative language together with opinions that checking for invalid repeated header fields is not to be considered DKIM's problem. See section 8.15 of [RFC6376] where this issue was expressed as not an attack against the trust DKIM intends to convey, and thus not a concern for DKIM. Nevertheless, improperly formed messages may display only the first of multiple header fields that, as a result of erroneous assumptions of there being no invalid repeated header fields, the prefixed header fields are likely to be displayed in lieu of those signed while not impacting DKIM's signature validity. DKIM incorrectly assumed the header field stack's starting condition, which DKIM itself is best able to determine, and is an option in the OpenDKIM implementation. This is likely to astonish most recipients that DKIM failed to make a robust effort to maintain the trust it is attempting to convey. Three members of the WG authored proposed changes aimed specifically at addressing this issue [DKIM-MH-Attack]. At the time, some expressed concerns about whether this might set back DKIM's standardization process. As such, DKIM Signers may sign malformed messages (e.g., violate [RFC5322]) and be in compliance with DKIM specifications. In addition, receivers may verify these Otis & Rand Expires November 13, 2013 [Page 6] Internet-Draft DKIM-HARMFUL May 2013 messages as having valid signatures despite multiple instances of a header field only permitted to occur once and also be in compliance with DKIM specifications. See addendum for examples. Use of DKIM on such messages exposes a vulnerability in the evaluation process. Rather than ensuring essential checks are made prior to producing a result, a wasteful hack was later suggested where extra non-existent header fields could be included in the list of signed header fields. Any pre-pended header field added after signing would thereby change resulting hashes and invalidate the signature. Not all domains are attempting to achieve the same level of trust and may be more sensitive to incurring incremental storage requirements. Some domains may even inadvertently sign invalid repeated header fields because this check had not been required in the DKIM process. These same DKIM domains are also likely to establish themselves as being Too Big To Block. These TBTB domains can then be used to spoof other domains that may have otherwise established a high level of trust by implementing the hack where, due to this defect in DKIM, can still do nothing in their defense from the perspective of now deceived recipients. This vulnerability in DKIM represents an exploit allowing serious attacks caused by erroneous assumptions made in DKIM's signature process. There is also a header field, which because of its label, may potentially mislead recipients into believing it contains valid "Authentication-Results" [RFC5451]. Common phrases such as "Authentication-Results", "pass", and "fail", rather than use of result codes belies introductory claims this header is not intended for direct human consumption. 6. Barriers to an Authenticated Domain Some advocate use of DKIM as a means to obtain domain references based on the increased prevalence of this protocol. DKIM is independent of the domain actually sending the message and the recipient by design. Unfortunately, DKIM also does not attempt to protect against likely abuses that are also beyond the control of the signing domain in which DKIM signature validity conveys no assurance pre-fixed header fields have not changed what recipients see. As such, DKIM signing domains can not be held accountable for incidents of abuse appearing to violate subscription policies or that spoof other domains. Because of DKIM's vulnerability to header field spoofing, it would never be safe to express positive reputations either. Any such assurance could be exploited by malefactors to deceive those trusting DKIM results. In short, a DKIM signed domain as currently defined, Otis & Rand Expires November 13, 2013 [Page 7] Internet-Draft DKIM-HARMFUL May 2013 can never be safely used in any context, other than the most rigid exclusion of any unsigned content which is well beyond any existing implementation. It can never be used for email reputation as currently defined. 7. Domains as a Basis for Managing Traffic A manageable basis for assessments can leverage a smaller number of related domains, compared to IPv6 or even IPv4 addresses. Although technically the domain name space can be larger than the massively large IPv6 address space, in practice it is not. One hundred thousand domains control 90% of Internet traffic out of approximately 100 million domains active each month. The top 150 domains control 50% of the traffic, and the top 2,500 domains control 75%. This level of domain consolidation permits effective fast-path white- listing. Improvements achieved using domains to consolidate the threat landscape can easily justify added cryptographic authentication burdens. Even APL resource records [RFC3123] can authenticate EHLO using a single DNS transaction, but this would not allow IPv6 email to be more easily managed, which cryptographic technology can offer. 8. XMPP Shows the Way Forward In addition to SMTP [RFC5321] using StartTLS [RFC3207] XMPP [RFC6122] uses StartTLS [RFC6120] over a different port with many of the features used by web servers such as [RFC2560] as one means to increase scalability. It seems plausible that by defining SMTP access over a different port is where a new authentication and international requirements can be resolved together. Of course, port 25 can be used, where it might require StartTLS in the case of IPv6 connections. Many administrators overlook a serious problem made much worse by chatty protocols that impose processing delays. Examining server logs will not reveal any problem either, because the limited resource being consumed is the number of outstanding connections TCP is able to support. Reaching this limit will prevent new connections from being instantiated but this is not logged as an event. Over time administrators may hear complaints email is not being delivered or just see an ever growing percentage of spam. 9. IANA Considerations This document requires no IANA consideration. Otis & Rand Expires November 13, 2013 [Page 8] Internet-Draft DKIM-HARMFUL May 2013 10. Security Considerations This draft intends to describe serious security concerns raised with use of DKIM exacerbated with IPv6 email. The contained recommendations are expected to reduce the security concerns. To ensure security, the DKIM specification must change. 11. References - Informative [DKIM-MH-Attack] http://trac.tools.ietf.org/wg/dkim/trac/ticket/24, "Multiple-header-attack alternative proposal", April 2011. [RFC0733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson, "Standard for the format of ARPA network text messages", RFC 733, November 1977. [RFC0822] Crocker, D., "Standard for the format of ARPA Internet text messages", STD 11, RFC 822, August 1982. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999. [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [RFC3123] Koch, P., "A DNS RR Type for Lists of Address Prefixes (APL RR)", RFC 3123, June 2001. [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, February 2002. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006. [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Otis & Rand Expires November 13, 2013 [Page 9] Internet-Draft DKIM-HARMFUL May 2013 Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, September 2007. [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension for Authentication", RFC 4954, July 2007. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [RFC5451] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 5451, April 2009. [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, March 2011. [RFC6122] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Address Format", RFC 6122, March 2011. [RFC6152] Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP Service Extension for 8-bit MIME Transport", STD 71, RFC 6152, March 2011. [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, September 2011. [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized Email Headers", RFC 6532, February 2012. [RFC6854] Leiba, B., "Update to Internet Message Format to Allow Group Syntax in the "From:" and "Sender:" Header Fields", RFC 6854, March 2013. [v6-BGP-Rpts] http://bgp.potaroo.net/v6/as6447/, "BGP Routing Table Analysis Reports/IPv6/AS6447 views", May 2013. Appendix A. DKIM Examples From Random User Tue Mar 12 12:07:37 2013 X-Apparently-To: just4spamdlr@yahoo.com via 72.30.237.8; Tue, 12 Mar 2013 12:08:37 -0700 Return-Path: Received-SPF: neutral (192.83.249.65 is neither permitted nor denied by domain of gmail.com) A3RleHQvcGxhaW4DAzACA3RleHQvaHRtbAMDMQ-- Otis & Rand Expires November 13, 2013 [Page 10] Internet-Draft DKIM-HARMFUL May 2013 X-YMailISG: Po8J_9cWLDuz5QIo_tChc7OagZYPBIscsK7APx8FMj835hEX clyJxoQr6Ojy40ccEugqmkym_ayJu65fKm.KJY73k6aprxb9s7Bj6P32lpml 6yGzxWFYdNXCwcxHtFGdhKe3v7Tjh8x051jkxjIqfuS0vo8J5rZOr.Z__6vD 4wiGFDUwFHNUWAwuz_pwp7pZ5HCivuuuyszYVvH0eIFsrQ9crR.rrk_3EQU2 Xkv_fInlGDFR8fafFPMOgQ7QOrHhy0zQUbptDEFGdh1QVOyLwIpjwEC7264k 4MqxUH7zz_M5JOQzj6dJslH0.iz5y9Sgp6y6kTUHAVP2f_t1hMeRvf3F7WJ6 1yY2rZJALIME1CtiNKQJoDctzgGFRnh_5mo415MvUcEIH7qqS5RFgWtXEQpd JIpyYlECDXVUcuASoLmzbuGSiCEVLq7f4EiBTAsaMwXJ07OgXBR.QYDw3VfA Z0AcfnFrUVHNLZtLaFukQKzdk9c6SpHFHSuCAsvLPuZeRy4Ij5ndXd7viyCS IkAHsnhG_u3.nZr3zUDFOrqw8sEKphobj6ZJ8KEXtuhr_tx.94abE1JRJYi5 fukj2h8y9s.K10ZxoTClaw41_DD8fxESbyfyTRPytiEXUdK1WEjgS3rAZ0TA WPJPDr063xLYk20UY0V.N5J15lBCtqZcde_9pdXwxVySyXo1KEQOaH3TNRBZ AKMFuCC7NF56aklkiUgk2EWm8iYoHsFez5_HtOz1zmc1dv4mNFOPTaNrXF2X qjFiwfdUipupIlAEc6pIdv0_le.xvz1jnaewEOyxo4dKd2XLVvybLfsLY16U FzLS9MJJ1wC0Cmf3G2SbOmT4ZiAvPjyv8QnHzbSDDDy3hqg8F0uEE03sJ5dm on5FxOHZZ1wCH7DL1QAXpZYxYWKV.h3q69dKQMl6HbnmfT_WZQY4X8uKXqkZ o34v.YmvJxHSRCSmhFpug1EstpJ4gHVitl_eJzT_n6xYQwhNAuMZ9uRjN2xE 1Lf7NpgzRf9bFvOpJAlyLoK5Xvxbx711cMgEUfGIha_JtL1P7hyfncRszHDv txgUYzcsVvRyAyVvwDAM.TEBsFhAtqqwOibqo2l5xCBj2yXRbKJ0EOC1JDMs HA-- X-Originating-IP: [192.83.249.65] Authentication-Results: mta1225.mail.bf1.yahoo.com from=gmail.com; domainkeys=neutral (no sig); from=gmail.com; dkim=pass (ok) Received: from 127.0.0.1 (EHLO rdaver.bungi.com) (192.83.249.65) by mta1225.mail.bf1.yahoo.com with SMTP; Tue, 12 Mar 2013 12:08:36 -0700 Received: by rdaver.bungi.com via smail with stdio id for Just4spamdlr@yahoo.com; Tue, 12 Mar 2013 12:08:33 -0700 (PDT) (Smail-3.2.0.94 1997-Apr-22 #591 built 2011-Feb-5) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:x-received:date:message-id:subject:from:to :content-type; bh=PS9xMxYwwTGwWXbCd8bjBBm2rwb79wVOSDLhmp+k4b4=; b=qnYVUccLSAi2DGJdUgDDIP9A3uPk3PaxgqhYLBn6xU382MsCi/ICFgKAoFPuwM7BvL AuSuqL6P54cIJ3Pn36h2xmXy+ucNr5r5OqIY63rtvj6Apjr4uW1PzG47J7BGEiP9iwDZ PLTzl9ZLpZXvZZpTCJOXUQP2HF8q6aivCblYZIQcCdVRCftG+A4z0+dEyTHbxoAMx9U3 GFISRRHcZ7k7GAyYmLrSr3fUTjvpa1YWoNK+IcSALC2tKVSW5FP1IQAT07f1e8+bOgHh JleaQIw8b1Vjlzhs4hFKLdedmjQqjDJXVP/K3J+t/ggfYn4H547fu6Pb5syKZIiuPf1e yJqA== MIME-Version: 1.0 X-Received: by 10.220.221.143 with SMTP id ic15mr6773333vcb.32.1363115257152; Tue, 12 Mar 2013 12:07:37 -0700 (PDT) Received: by 10.52.70.169 with HTTP; Tue, 12 Mar 2013 12:07:37 -0700 (PDT) Date: Tue, 12 Mar 2013 09:07:37 -1000 Message-ID: Otis & Rand Expires November 13, 2013 [Page 11] Internet-Draft DKIM-HARMFUL May 2013 Subject: An example signed message From: Random User To: just4spamdlr@yahoo.com Content-Type: multipart/alternative; boundary=14dae9cdc33bb0ff5204d7bf00ff Content-Length: 280 reporting valid signature From Fake User Tue Mar 12 12:07:37 2013 X-Apparently-To: just4spamdlr@yahoo.com via 72.30.237.8; Tue, 12 Mar 2013 12:09:01 -0700 Return-Path: Received-SPF: neutral (192.83.249.65 is neither permitted nor denied by domain of gmail.com) A3RleHQvcGxhaW4DAzACA3RleHQvaHRtbAMDMQ-- X-YMailISG: gFqc.ysWLDtqkdjDpSCH39uGWhgFfnsGdWobzNb5os6sP0We _L38eAdX.VKZWQ2F75gFwoipcPyj4g0uKMm_vSayLjrnps9lBxMGLvtTE8kT XYxIw6vZb4aFZ_jEcpoRntvJDkZQl4XSGWGakfmJ5G2blTWZ_i1BVkBvj0Sv jEymvhoIXZTb_l8C0Jh69ot3MgrNBvjhrBmhCK3sziUtDPpKQPJb_lxCnYKN O0SiArQ_TUXrCRFRNsyEiJxzVfSgJWIdsCV5BN3cp..NZ17X8fguB.YxNQjt qjVcGMd4IjQioY.a4f1luQxuiCN1yWvYqiLpP6eOCQhMrHt9XOdk32HAXNuJ GBraVtjrySTl9Db7PpRC46wlMs3iIUHl3z0d4o6293sMA5qFmnbczGoLRGFs RUVlBJuRoJCSYZh5LOwbj0RPQNX2Nmw.LHwF7SY3XcZWFUjvUQQ2sdx63m_J Mgy7JHAwBTVH6ytULsbXvu38a5GIYHccfNnDKVjtsrIg9qBDpVASHrRkncL0 MFLy5FHLb_XBW1TPztCFtlRViKr_HFxMob6aZIte6T57AMqlV2YAHwVNObwx WE8ZWTkKNWbXqJYytd3vyuyAHfuseBFP_Jfmj0zVtg52EXpIlDiTANEOTamP zeu23QbeRWJd_Gpz9bbGw_OorPdcV.WJOQ29DHpiYAQRgWjJNLjkd8dI.vuM vs1Fr7LOiE3wRpSU5AW_hrR4anvGrnwSPOQaFmpNE0pl8n.Vomrp.5NU8cgU QYI1UCSPoE_HK5Som2HMPYZFQv0pJSu1NeitXlRM3DHkIMvW4aVYqrHSNVjl gGCFFx77c25QW.XAGtySBYWcTzcUlHP4fMa7Wli4u06C4N3pDPiQoXKOC10U koXUMKFYmedaZYvEeQRPO3_8xHwKyZ.QInDsnQRwPFWYKvcWCJu4c5zxDMG4 h1AsyT3CM80nZXk8.ZGhzfTgo810Xjn_OJVgUfkG1z3..ReN990deaWJY8F5 _j6lRWLZZRzCMwOGpJ6I.jgaN5mNk38Kj6.NYLFCpMTEIt28jIRHD85cfpa3 iOL3drg1TIKQWrEhS9u3H29niQ_hjHbk7ys6uSJvowilRwO8eB2s.Wz0 X-Originating-IP: [192.83.249.65] Authentication-Results: mta1266.mail.bf1.yahoo.com from=gmail.com; domainkeys=neutral (no sig); from=gmail.com; dkim=pass (ok) Received: from 127.0.0.1 (EHLO rdaver.bungi.com) (192.83.249.65) by mta1266.mail.bf1.yahoo.com with SMTP; Tue, 12 Mar 2013 12:09:00 -0700 Received: by rdaver.bungi.com via smail with stdio id for Just4spamdlr@yahoo.com; Tue, 12 Mar 2013 12:09:00 -0700 (PDT) (Smail-3.2.0.94 1997-Apr-22 #591 built 2011-Feb-5) From: Fake User DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:x-received:date:message-id:subject:from:to Otis & Rand Expires November 13, 2013 [Page 12] Internet-Draft DKIM-HARMFUL May 2013 :content-type; bh=PS9xMxYwwTGwWXbCd8bjBBm2rwb79wVOSDLhmp+k4b4=; b=qnYVUccLSAi2DGJdUgDDIP9A3uPk3PaxgqhYLBn6xU382MsCi/ICFgKAoFPuwM7BvL AuSuqL6P54cIJ3Pn36h2xmXy+ucNr5r5OqIY63rtvj6Apjr4uW1PzG47J7BGEiP9iwDZ PLTzl9ZLpZXvZZpTCJOXUQP2HF8q6aivCblYZIQcCdVRCftG+A4z0+dEyTHbxoAMx9U3 GFISRRHcZ7k7GAyYmLrSr3fUTjvpa1YWoNK+IcSALC2tKVSW5FP1IQAT07f1e8+bOgHh JleaQIw8b1Vjlzhs4hFKLdedmjQqjDJXVP/K3J+t/ggfYn4H547fu6Pb5syKZIiuPf1e yJqA== MIME-Version: 1.0 X-Received: by 10.220.221.143 with SMTP id ic15mr6773333vcb.32.1363115257152; Tue, 12 Mar 2013 12:07:37 -0700 (PDT) Received: by 10.52.70.169 with HTTP; Tue, 12 Mar 2013 12:07:37 -0700 (PDT) Date: Tue, 12 Mar 2013 09:07:37 -1000 Message-ID: Subject: An example signed message From: Random User To: just4spamdlr@yahoo.com Content-Type: multipart/alternative; boundary=14dae9cdc33bb0ff5204d7bf00ff Content-Length: 280 spoofed DKIM with valid signature Appendix B. Stats Total spams: 9438 DKIM pass: 688 (about 25% relayed from large ESPs) DKIM fail: 189 DKIM pass w/multiple from: 28 (about 2% on average) Unsigned: 8561 Looking at a few minutes of spam. Authors' Addresses Douglas Otis Trend Micro 10101 N. De Anza Blvd Cupertino, CA 95014 USA Phone: +1.408.257-1500 Email: doug_otis@trendmicro.com Otis & Rand Expires November 13, 2013 [Page 13] Internet-Draft DKIM-HARMFUL May 2013 Dave Rand Trend Micro 10101 N. De Anza Blvd Cupertino, CA 95014 USA Phone: +1.408.257-1500 Email: dave_rand@trendmicro.com Otis & Rand Expires November 13, 2013 [Page 14]