| Individual | D. Otis |
| Internet-Draft | D. Rand |
| Intended status: Informational | Trend Micro |
| Expires: November 10, 2013 | May 09, 2013 |
IPv6 Email Authentication
draft-otis-ipv6-email-authent-01
IPv6 facilitates network routing over an incredibly vast address space, but abuse mitigation resolving to underlying structures of routes or blocks of prefixes would be highly disruptive due to collateral effects. Use of domains minimizes unintended coincidental blocking while offering the consolidation necessary to facilitate connection management. Currently, email lacks conventions ensuring SMTP clients can be identified by an authenticated domain.
DKIM is independent of intended recipients and domains accountable for having sent email. SPF normally requires several text based responses imposing high overhead to query various locations. These locations can be constructed by email-address local-part macros which can flood caches or leverage DNS name compression to further increase network DDoS amplifications when receivers' attempt to verify message sources which may then only offer authorization, not authentication of accountable domains. Most email abuse, including what might be imposed by SPF, is prevented with comprehensive mapping of the address space, but such mapping is impractical with IPv6.
An effective authenticated domain name alternative is needed to provide a basis for assessing and reacting to abusive behavior. For most high scale protocols, this involves cryptography.
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].
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/.
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This Internet-Draft will expire on November 10, 2013.
Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
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There are currently 18,209,211,661,910,032 /64 equivalent IPv6 [RFC2460] 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 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 exclude about 38%, or about 1,000,000,000 IPv4 addresses. Comparing IPv6 /64 prefixes with the remainder of rout-able IPv4 addresses shows there are 11.3 million times more IPv6 /64 prefixes needing categorization that also happen to lack a practical means for facilitating this effort.
Management of either a list of names or a list of numbers involves logging the associated connections for subsequent review. Often limits are imposed over some period to constrain overall log growth when establishing behavior. When based on currently routed IPv6 /64 prefixes, storing just a single bit per rout-able prefix requires 6 million billion bytes or 5,650 Terra-bytes of storage just to track simple use. Storage requirements for simple address use is likely to grow another seven fold as more providers adopt IPv6. Categorization of this use will require additional bits to retain counts and indexes further multiplying storage needs.
Some suggest there will not be a significant increase in the number of servers running over IPv6 and since the overall number should be comparable, email should 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 life of its operation while also having an ability to effortlessly change /64 prefixes. The increase may prove explosive.
Extending IP address reputation to IPv6 would also indicate which mailboxes are used to detect abuse and afford malefactors a means to wash mailing lists to sidestep enforcement of subscription policies. Detection of unsolicited commercial email is a fundamental element in email reputation. Traditional address reputation techniques with IPv6 will not retain effective detection. Even if detection were practical, no scheme can distribute highly sparse IP address information quickly enough to counter an IPv6 source's agility at evading IP address based mitigation.
Checking reverse DNS entries per SMTP connection requires queries against the entire IP address. For IPv6, reverse DNS checks by SMTP servers may inadvertently induce DDoS effects against DNS while also consuming the server's connection resources waiting for reverse DNS responses. Prior IPv4 mapping of reverse DNS avoided a sizable waste of resources while defending open services. However, prior mapping of reverse DNS is not practical with IPv6 making it impossible to defend against DDoS effects or excessive resource loss while checking the reverse namespace.
Alternatively, costs related to blocking port 25 by Customer-Premises Equipment should be negligible. Network providers should offer different services to isolate residential from commercial use, in a manner analogous to IPv4 reverse PTR records designating dynamic and static assignments. Residential systems sending email to SMTP's public port 25, for the most part, represent compromised systems. Blocking this category of traffic would be much safer than expecting recipients to check the reverse namespace. Windows, OS X, and iOS employ the IPv6 privacy extensions [RFC4941] where any problematic IP address is likely to be reassigned different addresses regularly and remain valid beyond the reassignment interval.
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. 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.
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.
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.
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.
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 the "listing non-existent header fields as signed" hack added in non-normative language together with opinions that checking for invalid repeated header fields is to be considered SMTP's problem. 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 conditions, which itself is best able to determine. This is likely to astonish most recipients that DKIM failed to make a robust effort to maintain the trust it is attempting to convey. As such, DKIM Signers may sign malformed messages (e.g., violate [RFC5322]). In addition, receivers will verify these messages as having valid signatures despite multiple instances of a header field only permitted to occur once. See addendum for examples.
Use of DKIM on such messages exposes a vulnerability in the evaluation process. Rather 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.
Many advocate either SPF or DKIM as a means to obtain domain references based on the increased prevalence of these protocols. 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.
Access to outbound SMTP servers may or may not restrict the Mail Parameter use. A strategy used to control abuse might rely upon rate limits and denial of access subsequent to abuse reports. Reasons for publishing SPF records might be to mitigate backscatter based on records that offer Non-Delivery-Notifications (NDNs) authorization by the Mail From parameter where the loss of some of these notifications is considered acceptable. This mechanism does not offer a positive basis for identification. Its intent is to constrain the number of invalid NDNs being returned when someone spoofs their domain. Evidence of this intent might be denoted by SPF record's ending or providers overriding them with "?all".
SPF failure rates of even a few percent are too high for it to offer a solid basis for either acceptance or rejection. There are some policy strategies for specific domains that attempt to combine SPF and DKIM where when either of their related domains manage to pass, only then is the message to be placed in the in-box.
Domains that reference SPF resource records should not be considered authenticated because the SPF process authorized the SMTP client. Often an SMTP client is shared by many domains. As such, it would be incorrect to assume SPFv1 records that authorize a provider by way of the Mail Parameters is only permitted by authenticated accounts from that domain.
As a result, no domain information in SPF or DKIM, under current circumstances, can be used as a domain reference for either acceptance or reputation.
When IPv6 becomes more commonly used, some will attempt white-listing IP addresses as a practical albeit non-scalable method to deal with the highly increased address range confronted when dealing with IPv6. There are currently no practical solutions able to scale to the challenging range of IP addresses that might be controlled by malefactors.
Unlike most DNS resources that segregate IPv4 and IPv6 datasets, SPF consolidates both IPv4 and IPv6 addresses into a common list comprised of a chain of DNS transactions. In addition to explicit chaining, SPF expects receivers to process the first SPF resource record which may contain 10 SPF mechanisms. Each mechanism may then require an additional 10 DNS transactions to resolve the mechanism's status. According to [spf-all.com] out of 112,311,175 domains, 24,073,182 (21.4 %) publish SPF records where only 12,768 (0.053%) employ macros. Even the 0.053% figure overstates use since many represent questionable server farm deployments. These are questionable since many major providers do not process SPF records that contain macros. Non-implementation of macros is very good from a security and reliability perspective, but problematic for email's integrity when used.
Ask a group of email providers about SPF, and you'll likely hear SPF is used to identify IP addresses used by a domain to send email. With this information, they can check these addresses against popular reputation services to determine where there are problems. Silent discard or placement into spam folders reduces delivery status integrity and makes supporting unreliable delivery difficult. When SPF was first introduced, AOL claimed they used SPF to create IP address white-lists based upon domains with whom they collaborate. White-listing would be less effective if macros were used in SPF records to change results.
SPF also includes macros run by recipients that can expand the local-part of the Mail From parameter or the IP address of the SMTP client into labels used in subsequent DNS queries. Expressing an IPv6 address using the SPF macro in the reverse order nibble form can comprise a query containing more than 32 labels. If local-part macros are used to locate 10 MX records, the combined targets for these MX records can comprise a set of 100 separate hostnames. Local-part macros can also be used to chain SPF record sets making forensic review difficult. The latest [I-D.ietf-spfbis-4408bis] offers a "(do not use)" statement for the PTR mechanism and the {%p} macro. Because macros pose a serious interchange risk when relied upon, all macros should be listed as "(do not use)". In the case of the PTR mechanism, the {%p} and {%l} macros should include the statement "(do not use and do not implement)."
The use of the PTR mechanism may place a burden on the DNS servers offered by the sender's network provider. The use of {%l} macro can potentially place a sizeable burden on ANY network at little cost to malefactors. The change in this version of SPF that limits the number of NX Domain responses will constrain attacks to being against any wildcard resource. Unfortunately, synthetic domains have become a popular alternative to the use of Web cookies.
It should also be pointed out use of macros may not function when [RFC6531] permits international email addresses which includes both local-part and domain portions of the email address. Tests and comments by large email providers show an understandable reluctance to process SPF macros where often they simply do not implement the macro portion of SPF.
The SPF review documented in [RFC6686] failed to provide a breakdown on macro use. SPF's macros can prove hazardous, especially with IPv6. Rather than permitting receivers to decide their acceptance methods, an SPF resource record based on the Mail From parameter can induce 100 DNS transactions using labels constructed from random strings found in the message local-part. While recent changes made in the current version of SPF recommends against its use, to process the reverse namespace may place an extreme burden on DNS, its cache, and the SMTP server's connection resources. Too bad unrelated DNS providers where not afforded similar consideration.
[I-D.ietf-spfbis-4408bis] purports to be documenting current use. It dropped the SPF resource record (99) due to sparse use and recommends against reverse PTR use. If a similar threshold were applied, all the SPF macro expansion aspects of this protocol would be removed as well. Several large providers have offered their assurance that their SPF libraries had these macros removed. Any effort hoping to define current use MUST carefully reconsider the inclusion of this ill-considered macro feature. Otherwise, it can not be suggested SPF-bis simply documents current use.
Danger imposed by the use of the local-part macro is inherent in the query required to support both an IPv6 expansion, in conjunction with the expansion of local-part macros induced by possibly cached SPF records. The local-part, along with a range of IP addresses made readily possible by IPv6, can be manipulated to induce a flurry of large DNS transactions directed toward any otherwise uninvolved domain, all directed by cached DNS records that contain active content.
It is never a good idea to allow free attacks enabled through foreign scripts. The SPF local-part macro would allow a cached DNS record to be repeatedly exploited by a spam campaign with an attacker consuming little of their resources beyond what they already use in the campaign. In this case, their recipients change the domains queried on behalf of the attackers based upon the same SPF resource record. High levels of amplification still apply if SPF specifications are not changed and resulting implementation are then assumed part of some greater security requirement and are no longer deemed experimental. The SPF macro DOS overview demonstrated in 2006 [otis-spf-dos-exploit] did not consider IPv6 use which offers higher gain, where the gain achieved sending a single 1KB message to a single recipient offered a 1:156 reflected gain over IPv4 that can never be filtered or blocked by using BCP38 [RFC2827]. When made against a wildcard record, even higher gains can be achieved.
Logically, local-part macros would not safely provide positive results without the query being combined with an IP address list of SMTP clients in some form. A list structure would need to be repeated for each user. Rcpt To using schemes like BATV provides an expedient way where the purported sender determines whether a NDN can be returned without the risk associated with active content placed within DNS resource records. Several parsers ignore local-part macro expansion since they rarely play any role in providing positive results.
The [I-D.ietf-spfbis-4408bis] Addendum E indicates a possible use of SPF macros such as exists:{%l}._spf.{%d} or exists:{%i}_spf.{%d} can be used in "specialized" DNS servers able to understand encrypted local-parts or react to inordinate query rates which suggests use of resource records having extremely short TTLs. Such an approach can add a sizeable burden on recipients and targeted DNS servers. Such a scheme represents a poor alternative to providing authenticated domain identifiers. The general intent and purpose of SPF is to offer Non-Delivery Notification authorization where no macros are required and by most practical measure are not being used.
The Purported Responsible Address (PRA) alternative attempted to overcome the relatively high path registration failure rates normally experienced with SPF. Neither scheme offered authenticated domain identifiers and left recipients prone to being spoofed by relying on authorizations. DKIM was intended to help solve these related spoofing issues, but failed as currently specified.
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.
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 port 465 is where a new authentication and international requirements can be resolved together.
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.
This document requires no IANA consideration.
This draft intends to describe serious security concerns raised with use of IPv6 email. The contained recommendations are expected to reduce the security concerns.
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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 <m1UFUZI-00KeXRC@rdaver.bungi.com>
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 <fake.user@gmail.com>
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: <CA+VnpPKv0s-p2nKkAkNHS4V2SxZehw_6S9QF5p1p2ji+FMof=Q@mail.gmail.com>
Subject: An example signed message
From: Random User <random.j.user.994@gmail.com>
To: just4spamdlr@yahoo.com
Content-Type: multipart/alternative; boundary=14dae9cdc33bb0ff5204d7bf00ff
Content-Length: 280
spoofed DKIM with valid signature
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.