Internet Engineering Task Force (IETF) D. Margolis Request for Comments: 8461 M. Risher Category: Standards Track Google, Inc. ISSN: 2070-1721 B. Ramakrishnan Oath, Inc. A. Brotman Comcast, Inc. J. Jones Microsoft, Inc. September 2018
SMTP MTA Strict Transport Security (MTA-STS)
Abstract
SMTP MTA Strict Transport Security (MTA-STS) is a mechanism enabling mail service providers (SPs) to declare their ability to receive Transport Layer Security (TLS) secure SMTP connections and to specify whether sending SMTP servers should refuse to deliver to MX hosts that do not offer TLS with a trusted server certificate.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8461.
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Copyright Notice
Copyright (c) 2018 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 (https://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.
The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and hosts to negotiate the use of a TLS channel for encrypted mail transmission.
While this opportunistic encryption protocol by itself provides a high barrier against passive man-in-the-middle traffic interception, any attacker who can delete parts of the SMTP session (such as the "250 STARTTLS" response) or who can redirect the entire SMTP session (perhaps by overwriting the resolved MX record of the delivery domain) can perform downgrade or interception attacks.
This document defines a mechanism for recipient domains to publish policies, via a combination of DNS and HTTPS, specifying:
o whether MTAs sending mail to this domain can expect PKIX- authenticated TLS support
o what a conforming client should do with messages when TLS cannot be successfully negotiated
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
We also define the following terms for further use in this document:
o MTA-STS Policy: A commitment by the Policy Domain to support TLS authenticated with PKIX [RFC5280] for the specified MX hosts.
o Policy Domain: The domain for which an MTA-STS Policy is defined. This is the next-hop domain; when sending mail to "alice@example.com", this would ordinarily be "example.com", but this may be overridden by explicit routing rules (as described in Section 3.4, "Policy Selection for Smart Hosts and Subdomains").
o Policy Host: The HTTPS host that serves the MTA-STS Policy for a Policy Domain. Rules for constructing the hostname are described in Section 3.2, "MTA-STS Policies".
o Sender or Sending MTA: The SMTP MTA sending an email message.
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o ABNF: Augmented Backus-Naur Form, a syntax for formally specifying syntax, defined in [RFC5234] and [RFC7405].
The DNS-Based Authentication of a Named Entities (DANE) TLSA record [RFC7672] is similar, in that DANE is also designed to upgrade unauthenticated encryption or plaintext transmission into authenticated, downgrade-resistant encrypted transmission. DANE requires DNSSEC [RFC4033] for authentication; the mechanism described here instead relies on certification authorities (CAs) and does not require DNSSEC, at a cost of risking malicious downgrades. For a thorough discussion of this trade-off, see Section 10, "Security Considerations".
In addition, MTA-STS provides an optional testing-only mode, enabling soft deployments to detect policy failures; partial deployments can be achieved in DANE by deploying TLSA records only for some of a domain's MXes, but such a mechanism is not possible for the per- domain policies used by MTA-STS.
The primary motivation of MTA-STS is to provide a mechanism for domains to ensure transport security even when deploying DNSSEC is undesirable or impractical. However, MTA-STS is designed not to interfere with DANE deployments when the two overlap; in particular, senders who implement MTA-STS validation MUST NOT allow MTA-STS Policy validation to override a failing DANE validation.
MTA-STS policies are distributed via HTTPS from a "well-known" [RFC5785] path served within the Policy Domain, and their presence and current version are indicated by a TXT record at the Policy Domain. These TXT records additionally contain a policy "id" field, allowing Sending MTAs to check that a cached policy is still current without performing an HTTPS request.
To discover if a recipient domain implements MTA-STS, a sender need only resolve a single TXT record. To see if an updated policy is available for a domain for which the sender has a previously cached policy, the sender need only check the TXT record's version "id" against the cached value.
The MTA-STS TXT record is a TXT record with the name "_mta-sts" at the Policy Domain. For the domain "example.com", this record would be "_mta-sts.example.com". MTA-STS TXT records MUST be US-ASCII, semicolon-separated key/value pairs containing the following fields:
o "v" (plaintext, required): Currently, only "STSv1" is supported.
o "id" (plaintext, required): A short string used to track policy updates. This string MUST uniquely identify a given instance of a policy, such that senders can determine when the policy has been updated by comparing to the "id" of a previously seen policy. There is no implied ordering of "id" fields between revisions.
An example TXT record is as below:
_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"
The formal definition of the "_mta-sts" TXT record, defined using ABNF [RFC7405], is as follows:
The TXT record MUST begin with the sts-version field; the order of other fields is not significant. If multiple TXT records for "_mta-sts" are returned by the resolver, records that do not begin with "v=STSv1;" are discarded. If the number of resulting records is not one, or if the resulting record is syntactically invalid, senders MUST assume the recipient domain does not have an available MTA-STS
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Policy and skip the remaining steps of policy discovery. (Note that the absence of a usable TXT record is not by itself sufficient to remove a sender's previously cached policy for the Policy Domain, as discussed in Section 5.1, "Policy Application Control Flow".) If the resulting TXT record contains multiple strings, then the record MUST be treated as if those strings are concatenated without adding spaces.
The "_mta-sts" record MAY return a CNAME that points (directly or via other CNAMEs) to a TXT record, in which case senders MUST follow the CNAME pointers. This can be used for policy delegation, as described in Section 8.2.
The policy itself is a set of key/value pairs (similar to header fields in [RFC5322]) served via the HTTPS GET method from the fixed "well-known" [RFC5785] path of ".well-known/mta-sts.txt" served by the Policy Host. The Policy Host DNS name is constructed by prepending "mta-sts" to the Policy Domain.
When fetching a policy, senders SHOULD validate that the media type is "text/plain" to guard against cases where web servers allow untrusted users to host non-text content (typically, HTML or images) at a user-defined path. All parameters other than charset=utf-8 or charset=us-ascii are ignored. Additional "Content-Type" parameters are also ignored.
This resource contains the following CRLF-separated key/value pairs:
o "version": Currently, only "STSv1" is supported.
o "mode": One of "enforce", "testing", or "none", indicating the expected behavior of a Sending MTA in the case of a policy validation failure. See Section 5, "Policy Application", for more details about the three modes.
o "max_age": Max lifetime of the policy (plaintext non-negative integer seconds, maximum value of 31557600). Well-behaved clients SHOULD cache a policy for up to this value from the last policy fetch time. To mitigate the risks of attacks at policy refresh time, it is expected that this value typically be in the range of weeks or greater.
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o "mx": Allowed MX patterns. One or more patterns matching allowed MX hosts for the Policy Domain. As an example,
mx: mail.example.com <CRLF> mx: *.example.net
indicates that mail for this domain might be handled by MX "mail.example.com" or any MX at "example.net". Valid patterns can be either fully specified names ("example.com") or suffixes prefixed by a wildcard ("*.example.net"). If a policy specifies more than one MX, each MX MUST have its own "mx:" key, and each MX key/value pair MUST be on its own line in the policy file. In the case of Internationalized Domain Names [RFC5891], the "mx" value MUST specify the Punycode-encoded A-label [RFC3492] to match against, and not the Unicode-encoded U-label. The full semantics of certificate validation (including the use of wildcard patterns) are described in Section 4.1, "MX Host Validation".
sts-policy-field = sts-policy-version / ; required once sts-policy-mode / ; required once sts-policy-max-age / ; required once sts-policy-mx / ; required at least once, except when ; mode is "none" sts-policy-extension ; other fields
Parsers MUST accept TXT records and policy files that are syntactically valid (i.e., valid key/value pairs separated by semicolons for TXT records), possibly containing additional key/value pairs not specified in this document, in which case unknown fields SHALL be ignored. If any non-repeated field -- i.e., all fields excepting "mx" -- is duplicated, all entries except for the first SHALL be ignored.
Policy bodies are, as described above, retrieved by Sending MTAs via HTTPS [RFC2818]. During the TLS handshake initiated to fetch a new or updated policy from the Policy Host, the Policy Host HTTPS server MUST present an X.509 certificate that is valid for the "mta-sts" DNS-ID [RFC6125] (e.g., "mta-sts.example.com") as described below, chain to a root CA that is trusted by the Sending MTA, and be non- expired. It is expected that Sending MTAs use a set of trusted CAs similar to those in widely deployed web browsers and operating systems. See [RFC5280] for more details about certificate verification.
The certificate is valid for the Policy Host (i.e., "mta-sts" prepended to the Policy Domain) with respect to the rules described in [RFC6125], with the following application-specific considerations:
o Matching is performed only against the DNS-ID identifiers.
o DNS domain names in server certificates MAY contain the wildcard character '*' as the complete left-most label within the identifier.
The certificate MAY be checked for revocation via the Online Certificate Status Protocol (OCSP) [RFC6960], certificate revocation lists (CRLs), or some other mechanism.
Policies fetched via HTTPS are only valid if the HTTP response code is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP caching (as specified in [RFC7234]) MUST NOT be used.
Senders may wish to rate-limit the frequency of attempts to fetch the HTTPS endpoint even if a valid TXT record for the recipient domain exists. In the case where the HTTPS GET fails, implementers SHOULD limit further attempts to a period of five minutes or longer per version ID, to avoid overwhelming resource-constrained recipients with cascading failures.
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Senders MAY impose a timeout on the HTTPS GET and/or a limit on the maximum size of the response body to avoid long delays or resource exhaustion during attempted policy updates. A suggested timeout is one minute, and a suggested maximum policy size is 64 kilobytes; Policy Hosts SHOULD respond to requests with a complete policy body within that timeout and size limit.
If a valid TXT record is found but no policy can be fetched via HTTPS (for any reason), and there is no valid (non-expired) previously cached policy, senders MUST continue with delivery as though the domain has not implemented MTA-STS.
Conversely, if no "live" policy can be discovered via DNS or fetched via HTTPS, but a valid (non-expired) policy exists in the sender's cache, the sender MUST apply that cached policy.
Finally, to mitigate the risk of persistent interference with policy refresh, as discussed in-depth in Section 10, MTAs SHOULD proactively refresh cached policies before they expire; a suggested refresh frequency is once per day. To enable administrators to discover problems with policy refresh, MTAs SHOULD alert administrators (through the use of logs or similar) when such attempts fail, unless the cached policy mode is "none".
3.4. Policy Selection for Smart Hosts and Subdomains
When sending mail via a "smart host" -- an administratively configured intermediate SMTP relay, which is different from the message recipient's server as determined from DNS -- compliant senders MUST treat the smart host domain as the Policy Domain for the purposes of policy discovery and application. This specification does not provide a means of associating policies with email addresses that employ Address Literals [RFC5321].
When sending mail to a mailbox at a subdomain, compliant senders MUST NOT attempt to fetch a policy from the parent zone. Thus, for mail sent to "user@mail.example.com", the policy can be fetched only from "mail.example.com", not "example.com".
When sending to an MX at a domain for which the sender has a valid and non-expired MTA-STS Policy, a Sending MTA honoring MTA-STS MUST check whether:
1. At least one of the policy's "mx" patterns matches the selected MX host, as described in Section 4.1, "MX Host Validation".
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2. The recipient mail server supports STARTTLS and offers a PKIX- based TLS certificate, during TLS handshake, which is valid for that host, as described in Section 4.2, "Recipient MTA Certificate Validation".
When these conditions are not met, a policy is said to fail to validate. This section does not dictate the behavior of Sending MTAs when the above conditions are not met; see Section 5, "Policy Application", for a description of Sending MTA behavior when policy validation fails.
A receiving candidate MX host is valid according to an applied MTA- STS Policy if the MX record name matches one or more of the "mx" fields in the applied policy. Matching is identical to the rules given in [RFC6125], with the restriction that the wildcard character '*' may only be used to match the entire left-most label in the presented identifier. Thus, the mx pattern "*.example.com" matches "mail.example.com" but not "example.com" or "foo.bar.example.com".
The certificate presented by the receiving MTA MUST not be expired and MUST chain to a root CA that is trusted by the Sending MTA. The certificate MUST have a subject alternative name (SAN) [RFC5280] with a DNS-ID [RFC6125] matching the hostname, per the rules given in [RFC6125]. The MX's certificate MAY also be checked for revocation via OCSP [RFC6960], CRLs [RFC6818], or some other mechanism.
When sending to an MX at a domain for which the sender has a valid, non-expired MTA-STS Policy, a Sending MTA honoring MTA-STS applies the result of a policy validation failure in one of two ways, depending on the value of the policy "mode" field:
1. "enforce": In this mode, Sending MTAs MUST NOT deliver the message to hosts that fail MX matching or certificate validation or that do not support STARTTLS.
2. "testing": In this mode, Sending MTAs that also implement the TLSRPT (TLS Reporting) specification [RFC8460] send a report indicating policy application failures (as long as TLSRPT is also implemented by the recipient domain); in any case, messages may be delivered as though there were no MTA-STS validation failure.
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3. "none": In this mode, Sending MTAs should treat the Policy Domain as though it does not have any active policy; see Section 8.3, "Removing MTA-STS", for use of this mode value.
When a message fails to deliver due to an "enforce" policy, a compliant MTA MUST NOT permanently fail to deliver messages before checking, via DNS, for the presence of an updated policy at the Policy Domain. (In all cases, MTAs SHOULD treat such failures as transient errors and retry delivery later.) This allows implementing domains to update long-lived policies on the fly.
An example control flow for a compliant sender consists of the following steps:
1. Check for a cached policy whose time-since-fetch has not exceeded its "max_age". If none exists, attempt to fetch a new policy (perhaps asynchronously, so as not to block message delivery). Optionally, Sending MTAs may unconditionally check for a new policy at this step.
2. For each candidate MX, in order of MX priority, attempt to deliver the message. If a policy is present with an "enforce" mode, when attempting to deliver to each candidate MX, ensure STARTTLS support and host identity validity as described in Section 4, "Policy Validation". If a candidate fails validation, continue to the next candidate (if there is one).
3. A message delivery attempt MUST NOT be permanently failed until the sender has first checked for the presence of a new policy (as indicated by the "id" field in the "_mta-sts" TXT record). If a new policy is not found, existing rules for the case of temporary message delivery failures apply (as discussed in [RFC5321], Section 4.5.4.1).
MTA-STS is intended to be used along with TLSRPT [RFC8460] in order to ensure that implementing domains can detect cases of both benign and malicious failures and to ensure that failures that indicate an active attack are discoverable. As such, senders that also implement TLSRPT SHOULD treat the following events as reportable failures:
o HTTPS policy fetch failures when a valid TXT record is present.
o Policy fetch failures of any kind when a valid policy exists in the policy cache, except if that policy's mode is "none".
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o Delivery attempts in which a contacted MX does not support STARTTLS or does not present a certificate that validates according to the applied policy, except if that policy's mode is "none".
To ensure that the server sends the right certificate chain, the SMTP client MUST have support for the TLS Server Name Indication (SNI) extension [RFC6066]. When connecting to an HTTP server to retrieve the MTA-STS Policy, the SNI extension MUST contain the name of the Policy Host (e.g., "mta-sts.example.com"). When connecting to an SMTP server, the SNI extension MUST contain the MX hostname.
HTTP servers used to deliver MTA-STS policies MAY rely on SNI to determine which certificate chain to present to the client. HTTP servers MUST respond with a certificate chain that matches the policy hostname or abort the TLS handshake if unable to do so. Clients that do not send SNI information may not see the expected certificate chain.
SMTP servers MAY rely on SNI to determine which certificate chain to present to the client. However, servers that have one identity and a single matching certificate do not require SNI support. Servers MUST NOT enforce the use of SNI by clients, as the client may be using unauthenticated opportunistic TLS and may not expect any particular certificate from the server. If the client sends no SNI extension or sends an SNI extension for an unsupported server name, the server MUST simply send a fallback certificate chain of its choice. The reason for not enforcing strict matching of the requested SNI hostname is that MTA-STS TLS clients may be typically willing to accept multiple server names but can only send one name in the SNI extension. The server's fallback certificate may match a different name that is acceptable to the client, e.g., the original next-hop domain.
MTAs supporting MTA-STS MUST have support for TLS 1.2 [RFC5246] or TLS 1.3 [RFC8446] or higher. The general TLS usage guidance in [RFC7525] SHOULD be followed.
Updating the policy requires that the owner make changes in two places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and at the corresponding HTTPS endpoint. As a result, recipients should expect that a policy will continue to be used by senders until both the HTTPS and TXT endpoints are updated and the TXT record's TTL has passed.
In other words, a sender who is unable to successfully deliver a message while applying a cache of the recipient's now-outdated policy may be unable to discover that a new policy exists until the DNS TTL has passed. Recipients SHOULD therefore ensure that old policies continue to work for message delivery during this period of time, or risk message delays.
Recipients SHOULD also update the HTTPS policy body before updating the TXT record; this ordering avoids the risk that senders, seeing a new TXT record, mistakenly cache the old policy from HTTPS.
Domain owners commonly delegate SMTP hosting to a different organization, such as an ISP or a web host. In such a case, they may wish to also delegate the MTA-STS Policy to the same organization, which can be accomplished with two changes.
First, the Policy Domain must point the "_mta-sts" record, via CNAME, to the "_mta-sts" record maintained by the provider. This allows the provider to control update signaling.
Second, the Policy Domain must point the "well-known" policy location to the provider. This can be done either by setting the "mta-sts" record to an IP address or CNAME specified by the provider and by giving the provider a TLS certificate that is valid for that host or by setting up a "reverse proxy" (also known as a "gateway") server for the Policy Domain's Policy Host, configured to serve proxied responses from the Policy Host of the provider.
For example, given a user domain "user.example" hosted by a mail provider "provider.example", the following configuration would allow policy delegation:
DNS:
_mta-sts.user.example. IN CNAME _mta-sts.provider.example.
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Policy:
> GET /.well-known/mta-sts.txt Host: mta-sts.user.example < HTTP/1.1 200 OK # Response proxies content from # https://mta-sts.provider.example
Note that in all such cases, the policy endpoint ("https://mta-sts.user.example/.well-known/mta-sts.txt" in this example) must still present a certificate valid for the Policy Host ("mta-sts.user.example"), and not for that host at the provider's domain ("mta-sts.provider.example").
Note that while Sending MTAs MUST NOT use HTTP caching when fetching policies via HTTPS, such caching may nonetheless be useful to a reverse proxy configured as described in this section. An HTTPS policy endpoint expecting to be proxied for multiple hosted domains -- as with a large mail hosting provider or similar -- may wish to indicate an HTTP Cache-Control "max-age" response directive (as specified in [RFC7234]) of 60 seconds as a reasonable value to save reverse proxies an unnecessarily high-rate of proxied policy fetching.
In order to facilitate clean opt-out of MTA-STS by implementing Policy Domains, and to distinguish clearly between failures that indicate attacks and those that indicate such opt-outs, MTA-STS implements the "none" mode, which allows validated policies to indicate authoritatively that the Policy Domain wishes to no longer implement MTA-STS and may, in the future, remove the MTA-STS TXT and policy endpoints entirely.
A suggested workflow to implement such an opt out is as follows:
1. Publish a new policy with "mode" equal to "none" and a small "max_age" (e.g., one day).
2. Publish a new TXT record to trigger fetching of the new policy.
3. When all previously served policies have expired -- normally this is the time the previously published policy was last served plus that policy's "max_age", but note that policies older than the previously published policy may have been served with a greater "max_age" than the previously published policy, allowing overlapping policy caches -- safely remove the TXT record and HTTPS endpoint.
Implementers of send-time MTA-STS validation in mail transfer agents should take note of the risks of modifying the logic of traversing MX candidate lists. Because an MTA-STS Policy can be used to prefilter invalid MX candidates from the MX candidate list, it is tempting to implement a "two-pass" model, where MX candidates are first filtered for possible validity according to the MTA-STS Policy, and then the remaining candidates are attempted in order as without an MTA-STS Policy. This may lead to incorrect implementations, such as message loops; instead, it is recommended that implementers traverse the MX candidate list as usual, and treat invalid candidates as though they were unreachable (i.e., as though there were some transient error when trying to deliver to that candidate).
One consequence of validating MX hosts in order of ordinary candidate traversal is that in the event a higher-priority MX is MTA-STS valid and a lower-priority MX is not, senders may never encounter the lower-priority MX, leading to a risk that policy misconfigurations that apply only to "backup" MXes may only be discovered in the case of primary MX failure.
IANA has created a new registry titled "MTA-STS TXT Record Fields". The initial entries in the registry are:
+------------+--------------------+-------------------------+ | Field Name | Description | Reference | +------------+--------------------+-------------------------+ | v | Record version | Section 3.1 of RFC 8461 | | id | Policy instance ID | Section 3.1 of RFC 8461 | +------------+--------------------+-------------------------+
New fields are added to this registry using IANA's "Expert Review" policy [RFC8126].
SMTP MTA-STS attempts to protect against an active attacker trying to intercept or tamper with mail between hosts that support STARTTLS. There are two classes of attacks considered:
o Foiling TLS negotiation (for example, by deleting the "250 STARTTLS" response from a server or altering TLS session negotiation). This would result in the SMTP session occurring over plaintext, despite both parties supporting TLS.
o Impersonating the destination mail server, whereby the sender might deliver the message to an impostor, who could then monitor and/or modify messages despite opportunistic TLS. This impersonation could be accomplished by spoofing the DNS MX record for the recipient domain or by redirecting client connections intended for the legitimate recipient server (for example, by altering BGP routing tables).
MTA-STS can thwart such attacks only if the sender is able to previously obtain and cache a policy for the recipient domain, and only if the attacker is unable to obtain a valid certificate that complies with that policy. Below, we consider specific attacks on this model.
SMTP MTA-STS relies on certificate validation via PKIX-based TLS identity checking [RFC6125]. Attackers who are able to obtain a valid certificate for the targeted recipient mail service (e.g., by compromising a CA) are thus able to circumvent STS authentication.
Since MTA-STS uses DNS TXT records for policy discovery, an attacker who is able to block DNS responses can suppress the discovery of an MTA-STS Policy, making the Policy Domain appear not to have an MTA- STS Policy. The sender policy cache is designed to resist this attack by decreasing the frequency of policy discovery and thus reducing the window of vulnerability; it is nonetheless a risk that attackers who can predict or induce policy discovery -- for example, by inducing a sending domain to send mail to a never-before-contacted recipient while carrying out a man-in-the-middle attack -- may be able to foil policy discovery and effectively downgrade the security of the message delivery.
Since this attack depends upon intercepting initial policy discovery, implementers SHOULD prefer policy "max_age" values to be as long as is practical.
Because this attack is also possible upon refresh of a cached policy, implementers SHOULD NOT wait until a cached policy has expired before checking for an update; if senders attempt to refresh the cache regularly (for example, by fetching the current live policy in a background task that runs daily or weekly, regardless of the state of the "_mta-sts" TXT record, and updating their cache's "max age" accordingly), an attacker would have to foil policy discovery consistently over the lifetime of a cached policy to prevent a successful refresh.
Additionally, MTAs SHOULD alert administrators to repeated policy refresh failures long before cached policies expire (through warning logs or similar applicable mechanisms), allowing administrators to detect such a persistent attack on policy refresh. (However, they should not implement such alerts if the cached policy has a "none" mode, to allow clean MTA-STS removal, as described in Section 8.3.)
Resistance to downgrade attacks of this nature -- due to the ability to authoritatively determine "lack of a record" even for non- participating recipients -- is a feature of DANE, due to its use of DNSSEC for policy discovery.
We additionally consider the Denial-of-Service risk posed by an attacker who can modify the DNS records for a recipient domain. Absent MTA-STS, such an attacker can cause a Sending MTA to cache invalid MX records, but only for however long the sending resolver caches those records. With MTA-STS, the attacker can additionally advertise a new, long "max_age" MTA-STS Policy with "mx" constraints
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that validate the malicious MX record, causing senders to cache the policy and refuse to deliver messages once the victim has resecured the MX records.
This attack is mitigated in part by the ability of a victim domain to (at any time) publish a new policy updating the cached, malicious policy, though this does require the victim domain to both obtain a valid CA-signed certificate and to understand and properly configure MTA-STS.
Similarly, we consider the possibility of domains that deliberately allow untrusted users to serve untrusted content on user-specified subdomains. In some cases (e.g., the service "tumblr.com"), this takes the form of providing HTTPS hosting of user-registered subdomains; in other cases (e.g. dynamic DNS providers), this takes the form of allowing untrusted users to register custom DNS records at the provider's domain.
In these cases, there is a risk that untrusted users would be able to serve custom content at the "mta-sts" host, including serving an illegitimate MTA-STS Policy. We believe this attack is rendered more difficult by the need for the attacker to also serve the "_mta-sts" TXT record on the same domain -- something not, to our knowledge, widely provided to untrusted users. This attack is additionally mitigated by the aforementioned ability for a victim domain to update an invalid policy at any future date.
Even if an attacker cannot modify a served policy, the potential exists for configurations that allow attackers on the same domain to receive mail for that domain. For example, an easy configuration option when authoring an MTA-STS Policy for "example.com" is to set the "mx" equal to "*.example.com"; in this case, recipient domains must consider the risk that any user possessing a valid hostname and CA-signed certificate (for example, "dhcp-123.example.com") will, from the perspective of MTA-STS Policy validation, be a valid MX host for that domain.
A number of risks apply to the PKI system that is used for certificate authentication, both of the "mta-sts" HTTPS host's certificate and the SMTP servers' certificates. These risks are broadly applicable within the Web PKI ecosystem and are not specific to MTA-STS; nonetheless, they deserve some consideration in this context.
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Broadly speaking, attackers may compromise the system by obtaining certificates under fraudulent circumstances (i.e., by impersonating the legitimate owner of the victim domain), by compromising a CA or Delegate Authority's private keys, by obtaining a legitimate certificate issued to the victim domain, and similar.
One approach commonly employed by web browsers to help mitigate against some of these attacks is to allow for revocation of compromised or fraudulent certificates via OCSP [RFC6960] or CRLs [RFC6818]. Such mechanisms themselves represent trade-offs and are not universally implemented; we nonetheless recommend implementers of MTA-STS to implement revocation mechanisms that are most applicable to their implementations.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/info/rfc5280>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 6960, DOI 10.17487/RFC6960, June 2013, <https://www.rfc-editor.org/info/rfc6960>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10.17487/RFC7672, October 2015, <https://www.rfc-editor.org/info/rfc7672>.
The owner of "example.com" wishes to begin using MTA-STS with a policy that will solicit reports from senders without affecting how the messages are processed, in order to verify the identity of MXes that handle mail for "example.com", confirm that TLS is correctly used, and ensure that certificates presented by the recipient MX validate.
MTA-STS Policy indicator TXT RR:
_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"
Below is pseudocode demonstrating the logic of a compliant Sending MTA.
While this pseudocode implementation suggests synchronous policy retrieval in the delivery path, that may be undesirable in a working implementation, and we expect some implementers to instead prefer a background fetch that does not block delivery when no cached policy is present.
func isEnforce(policy) { // Return true if the policy mode is "enforce". }
func isNonExpired(policy) { // Return true if the policy is not expired. }
func tryStartTls(connection) { // Attempt to open an SMTP STARTTLS connection with the MX. }
func certMatches(connection, host) {
Margolis, et al. Standards Track [Page 25]
RFC 8461 MTA-STS September 2018
// Assume a handy function to return if the server // certificate presented in "connection" is valid for "host". }
func policyMatches(candidate, policy) { for mx in policy.mx { // Literal match. if mx == candidate { return true } // Wildcard matches only the leftmost label. // Wildcards must always be followed by a '.'. if mx[0] == '*' { parts = SplitN(candidate, '.', 2) // Split on the first '.'. if len(parts) > 1 && parts[1] == mx[2:] { return true } } } return false }
func tryDeliverMail(connection, message) { // Attempt to deliver "message" via "connection". }
func tryGetNewPolicy(domain) { // Check for an MTA-STS TXT record for "domain" in DNS, and return // the indicated policy. }
func cachePolicy(domain, policy) { // Store "policy" as the cached policy for "domain". }
func tryGetCachedPolicy(domain) { // Return a cached policy for "domain". }
func reportError(error) { // Report an error via TLSRPT. }
func tryMxAccordingTo(message, mx, policy) { connection := connect(mx) if !connection { return false // Can't connect to the MX, so it's not an MTA-STS // error.