--- 1/draft-ietf-uta-smtp-tlsrpt-18.txt 2018-05-02 10:13:26.016000590 -0700 +++ 2/draft-ietf-uta-smtp-tlsrpt-19.txt 2018-05-02 10:13:26.080002118 -0700 @@ -1,55 +1,55 @@ Using TLS in Applications D. Margolis Internet-Draft Google, Inc Intended status: Standards Track A. Brotman -Expires: October 6, 2018 Comcast, Inc +Expires: November 3, 2018 Comcast, Inc B. Ramakrishnan Yahoo!, Inc J. Jones Microsoft, Inc M. Risher Google, Inc - April 4, 2018 + May 2, 2018 SMTP TLS Reporting - draft-ietf-uta-smtp-tlsrpt-18 + draft-ietf-uta-smtp-tlsrpt-19 Abstract A number of protocols exist for establishing encrypted channels between SMTP Mail Transfer Agents, including STARTTLS, DANE TLSA, and MTA-STS. These protocols can fail due to misconfiguration or active attack, leading to undelivered messages or delivery over unencrypted or unauthenticated channels. This document describes a reporting mechanism and format by which sending systems can share statistics and specific information about potential failures with recipient domains. Recipient domains can then use this information to both - detect potential attackers and diagnose unintentional + detect potential attacks and diagnose unintentional misconfigurations. 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 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 October 6, 2018. + This Internet-Draft will expire on November 3, 2018. 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents @@ -83,45 +83,47 @@ 5. Report Delivery . . . . . . . . . . . . . . . . . . . . . . . 14 5.1. Report Filename . . . . . . . . . . . . . . . . . . . . . 14 5.2. Compression . . . . . . . . . . . . . . . . . . . . . . . 15 5.3. Email Transport . . . . . . . . . . . . . . . . . . . . . 15 5.3.1. Example Report . . . . . . . . . . . . . . . . . . . 16 5.4. HTTPS Transport . . . . . . . . . . . . . . . . . . . . . 17 5.5. Delivery Retry . . . . . . . . . . . . . . . . . . . . . 18 5.6. Metadata Variances . . . . . . . . . . . . . . . . . . . 18 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 6.1. Message headers . . . . . . . . . . . . . . . . . . . . . 18 - 6.2. Report Type . . . . . . . . . . . . . . . . . . . . . . . 19 - 6.3. application/tlsrpt+json Media Type . . . . . . . . . . . 19 - 6.4. application/tlsrpt+gzip Media Type . . . . . . . . . . . 20 - 6.5. STARTTLS Validation Result Types . . . . . . . . . . . . 21 + 6.2. Report Type . . . . . . . . . . . . . . . . . . . . . . . 18 + 6.3. +gzip Media Type Suffix . . . . . . . . . . . . . . . . . 19 + 6.4. application/tlsrpt+json Media Type . . . . . . . . . . . 20 + 6.5. application/tlsrpt+gzip Media Type . . . . . . . . . . . 21 + 6.6. STARTTLS Validation Result Types . . . . . . . . . . . . 22 7. Security Considerations . . . . . . . . . . . . . . . . . . . 22 - 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 - 8.1. Normative References . . . . . . . . . . . . . . . . . . 23 - 8.2. Informative References . . . . . . . . . . . . . . . . . 25 - Appendix A. Example Reporting Policy . . . . . . . . . . . . . . 25 - A.1. Report using MAILTO . . . . . . . . . . . . . . . . . . . 26 - A.2. Report using HTTPS . . . . . . . . . . . . . . . . . . . 26 - Appendix B. Example JSON Report . . . . . . . . . . . . . . . . 26 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 + 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 + 8.1. Normative References . . . . . . . . . . . . . . . . . . 24 + 8.2. Informative References . . . . . . . . . . . . . . . . . 26 + Appendix A. Example Reporting Policy . . . . . . . . . . . . . . 27 + A.1. Report using MAILTO . . . . . . . . . . . . . . . . . . . 27 + A.2. Report using HTTPS . . . . . . . . . . . . . . . . . . . 27 + Appendix B. Example JSON Report . . . . . . . . . . . . . . . . 27 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 1. Introduction The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and hosts to establish secure SMTP sessions over TLS. The protocol design uses an approach that has come to be known as "Opportunistic - Security" (OS) [RFC7435], which maintains interoperability with - clients that do not support STARTTLS but means that any attacker who - can delete parts of the SMTP session (such as the "250 STARTTLS" - response) or redirect the entire SMTP session (perhaps by overwriting - the resolved MX record of the delivery domain) can perform a - downgrade or interception attack. + Security" (OS) [RFC7435]. This method maintains interoperability + with clients that do not support STARTTLS, but means that any + attacker could potentially eavesdrop on a session. An attacker could + perform a downgrade or interception attack by deleting parts of the + SMTP session (such as the "250 STARTTLS" response) or redirect the + entire SMTP session (perhaps by overwriting the resolved MX record of + the delivery domain). Because such "downgrade attacks" are not necessarily apparent to the receiving MTA, this document defines a mechanism for sending domains to report on failures at multiple stages of the MTA-to-MTA conversation. Recipient domains may also use the mechanisms defined by MTA-STS [I-D.ietf-uta-mta-sts] or DANE [RFC6698] to publish additional encryption and authentication requirements; this document defines a mechanism for sending domains that are compatible with MTA-STS or @@ -135,42 +137,49 @@ also serve as a heartbeat that systems are successfully negotiating TLS during sessions as expected. This document is intended as a companion to the specification for SMTP MTA Strict Transport Security [I-D.ietf-uta-mta-sts], as well as adds reporting abilities for those implementing DANE [RFC7672]. 1.1. Terminology 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]. + "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 definition of the expected TLS availability, - behavior, and desired actions for a given domain when a sending - MTA encounters problems in negotiating a secure channel. MTA-STS - is defined in [I-D.ietf-uta-mta-sts]. + o MTA-STS Policy: A mechanism by which administrators can specify + the expected TLS availability, presented identity, and desired + actions for a given email recipient domain. MTA-STS is defined in + [I-D.ietf-uta-mta-sts]. - o DANE Policy: A mechanism by which administrators can supply a - record that can be used to validate the certificate presented by - an MTA. DANE is defined in [RFC6698] and [RFC7672]. + o DANE Policy: A mechanism by which administrators can specify + constraints to be used to validate certificates presented by an + MTA. DANE is defined in [RFC6698] and [RFC7672]. o TLSRPT Policy: A policy specifying the endpoint to which sending MTAs should deliver reports. o Policy Domain: The domain against which an MTA-STS or DANE Policy - is defined. + is defined. This should be the same as the recipient envelope + domain [RFC5321], such as if the message were going to + "alice@example.com', the policy domain would be "example.com". o Sending MTA: The MTA initiating the relay of an email message. + o Aggregate Report URI (rua): A comma-separated list of locations + where the report is to be submitted. + 2. Related Technologies o This document is intended as a companion to the specification for SMTP MTA Strict Transport Security [I-D.ietf-uta-mta-sts]. o SMTP-TLSRPT defines a mechanism for sending domains that are compatible with MTA-STS or DANE to share success and failure statistics with recipient domains. DANE is defined in [RFC6698] and MTA-STS is defined in [I-D.ietf-uta-mta-sts]. @@ -178,43 +187,49 @@ A domain publishes a record to its DNS indicating that it wishes to receive reports. These SMTP TLSRPT policies are distributed via DNS from the Policy Domain's zone, as TXT records (similar to DMARC policies) under the name "_smtp._tls". For example, for the Policy Domain "example.com", the recipient's TLSRPT policy can be retrieved from "_smtp._tls.example.com". Policies consist of the following directives: - o "v": This value MUST be equal to "TLSRPTv1". + o "v": This document defines version 1 of TLSRPT, for which this + value MUST be equal to "TLSRPTv1". Other versions may be defined + in later documents. o "rua": A URI specifying the endpoint to which aggregate information about policy validation results should be sent (see Section 4, "Reporting Schema", for more information). Two URI schemes are supported: "mailto" and "https". As with DMARC [RFC7489], the policy domain can specify a comma-separated list of URIs. o In the case of "https", reports should be submitted via POST ([RFC7231]) to the specified URI. Report submitters MAY ignore certificate validation errors when submitting reports via https. o In the case of "mailto", reports should be submitted to the specified email address ([RFC6068]). When sending failure reports via SMTP, sending MTAs MUST deliver reports despite any TLS- related failures and SHOULD NOT include this SMTP session in the - next report. This may mean that the reports are delivered in the - clear. Additionally, reports sent via SMTP MUST contain a valid - DKIM [RFC6376] signature by the reporting domain. Reports lacking - such a signature MUST be ignored by the recipient. DKIM - signatures must not use the "l=" attribute to limit the body - length used in the signature. + next report. When sending failure reports via HTTPS, sending MTAs + MAY deliver reports despite any TLS-related faliures. This may + mean that the reports are delivered in the clear. Reports sent + via SMTP MUST contain a valid DKIM [RFC6376] signature by the + reporting domain. Reports lacking such a signature MUST be + ignored by the recipient. DKIM signatures must not use the "l=" + attribute to limit the body length used in the signature. The + DKIM TXT record must contain the appropriate service type + declaration, "s=tlsrpt", and if not present the receiving system + SHOULD ignore reports signed using this record. The formal definition of the "_smtp._tls" TXT record, defined using [RFC5234] & [RFC7405], is as follows: tlsrpt-record = tlsrpt-version 1*(field-delim tlsrpt-field) [field-delim] field-delim = *WSP ";" *WSP tlsrpt-field = tlsrpt-rua / ; Note that the @@ -238,32 +253,30 @@ DIGIT / "_" / "-" / ".") tlsrpt-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding "=", ";", SP, and control ; chars If multiple TXT records for "_smtp._tls" are returned by the resolver, records which do not begin with "v=TLSRPTv1;" are discarded. If the number of resulting records is not one, senders MUST assume the recipient domain does not implement TLSRPT. If the - resulting TXT record contains multiple strings, then the record MUST - be treated as if those strings are concatenated together without - adding spaces. + resulting TXT record contains multiple strings (as described in + Section 3.1.3 of [RFC4408]), then the record MUST be treated as if + those strings are concatenated together without adding spaces. The record supports the abillity to declare more than one rua, and if there exists more than one, the reporter MAY attempt to deliver to each of the supported rua destinations. A receiver MAY opt to only attempt delivery to one of the endpoints, however the report SHOULD NOT be considered successfully delivered until one of the endpoints - accepts delivery of the report. If the reporter does not support one - of the report mechanisms, then it SHOULD NOT attempt delivery to - those rua destinations. + accepts delivery of the report. Parsers MUST accept TXT records which are syntactically valid (i.e. valid key-value pairs separated by semi-colons) and implementing a superset of this specification, in which case unknown fields SHALL be ignored. 3.1. Example Reporting Policy 3.1.1. Report using MAILTO @@ -308,23 +321,23 @@ o Aggregate counts, comprising result type, sending MTA IP, receiving MTA hostname, session count, and an optional additional information field containing a URI for recipients to review further information on a failure type. Note that the failure types are non-exclusive; an aggregate report may contain overlapping "counts" of failure types when a single send attempt encountered multiple errors. Reporters may report multiple applied policies (for example, an MTA-STS policy and a DANE TLSA - record for the same domain and MX); even in the case where only a - single policy was applied, the "policies" field of the report body - MUST be an array and not a singular value. + record for the same domain and MX). Because of this, even in the + case where only a single policy was applied, the "policies" field of + the report body MUST be an array and not a singular value. In the case of multiple failure types, the "failure-details" array would contain multiple entries. Each entry would have its own set of infomation pertaining to that failure type. 4.1. Report Time-frame The report SHOULD cover a full day, from 0000-2400 UTC. This should allow for easier correlation of failure events. To avoid a Denial of Service against the system processing the reports, the reports should @@ -517,65 +530,71 @@ STARTTLS connection. o "receiving-mx-helo": (optional) The HELO or EHLO string from the banner announced during the reported session. o "receiving-ip": The destination IP address that was using when creating the outbound session. It is provided as a string representation of an IPv4 (see below) or IPv6 ([RFC5952]) address in dot-decimal or colon-hexadecimal notation. - o "total-successful-session-count": The aggregate number (integer) - of successfully negotiated TLS-enabled connections to the - receiving site. + o "total-successful-session-count": The aggregate count (integer, + encoded as a JSON number) of successfully negotiated TLS-enabled + connections to the receiving site. - o "total-failure-session-count": The aggregate number (integer) of - failures to negotiate a TLS-enabled connection to the receiving - site. + o "total-failure-session-count": The aggregate count (integer, + encoded as a JSON number) of failures to negotiate a TLS-enabled + connection to the receiving site. o "failed-session-count": The number of (attempted) sessions that - match the relevant "result-type" for this section. + match the relevant "result-type" for this section (integer, + encoded as a JSON number). o "additional-info-uri": An optional URI [RFC3986] pointing to additional information around the relevant "result-type". For example, this URI might host the complete certificate chain presented during an attempted STARTTLS session. o "failure-reason-code": A text field to include a TLS-related error code or error message. - For report purposes, an IPv4 Address is defined as: IPv4address = - dec-octet "." dec-octet "." dec-octet "." dec-octet - dec-octet = DIGIT ; 0-9 / %x31-39 DIGIT ; 10-99 / "1" 2DIGIT ; - 100-199 / "2" %x30-34 DIGIT ; 200-249 / "25" %x30-35 ; 250-255 + For report purposes, an IPv4 Address is defined via the following + ABNF: + + IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet + dec-octet = DIGIT ; 0-9 + / %x31-39 DIGIT ; 10-99 + / "1" 2DIGIT ; 100-199 + / "2" %x30-34 DIGIT ; 200-249 + / "25" %x30-35 ; 250-255 4.5. Policy Samples Part of the report body includes the policy that is applied when attemping relay to the destination. - For DANE TLSA policies, a JSON array of strings each representing the - RDATA of a single TLSA resource record as a space-separated list of - its four TLSA fields; the fields are in presentation format (defined - in RFC6698 Section 2.2) with no internal spaces or grouping - parentheses: + For DANE TLSA policies, this is a JSON array of strings each + representing the RDATA of a single TLSA resource record as a space- + separated list of its four TLSA fields; the fields are in + presentation format (defined in [RFC6698] Section 2.2) with no + internal spaces or grouping parentheses: [ "3 0 1 1F850A337E6DB9C609C522D136A475638CC43E1ED424F8EEC8513D747D1D085D", "3 0 1 12350A337E6DB9C6123522D136A475638CC43E1ED424F8EEC8513D747D1D1234" ] - For the MTA-STS policy, an array of JSON strings that represents the - policy that is declared by the receiving site, including any errors - that may be present. Note that where there are multiple "mx" values, - they must be listed as separate "mx" elements in the policy array, - rather as a single nested "mx" sub-array. + For MTA-STS policies, this is an array of JSON strings that + represents the policy that is declared by the receiving site, + including any errors that may be present. Note that where there are + multiple "mx" values, they must be listed as separate "mx" elements + in the policy array, rather as a single nested "mx" sub-array. [ "version: STSv1", "mode: report", "mx: mx1.example.com", "mx: mx2.example.com", "mx: mx.backup-example.com", "max_age: 12345678" ] 5. Report Delivery Reports can be delivered either as an email message via SMTP or via HTTP POST. 5.1. Report Filename @@ -612,25 +631,26 @@ "unique-id" allows an optional unique ID generated by the Sending MTA to distinguish among multiple reports generated simultaneously by different sources within the same Policy Domain. For example, this is a possible filename for a compressed report to the Policy Domain "example.net" from the Sending MTA "mail.sndr.example.com": "mail.sndr.example.com!example.net!1470013207!1470186007!001.json.gz" 5.2. Compression - The report SHOULD be subjected to GZIP compression for both email and - HTTPS transport. Declining to apply compression can cause the report - to be too large for a receiver to process (a commonly observed - receiver limit is ten megabytes); compressing the file increases the - chances of acceptance of the report at some compute cost. + The report SHOULD be subjected to GZIP [RFC1952] compression for both + email and HTTPS transport. Declining to apply compression can cause + the report to be too large for a receiver to process (a commonly + observed receiver limit is ten megabytes); compressing the file + increases the chances of acceptance of the report at some compute + cost. 5.3. Email Transport The report MAY be delivered by email. To make the reports machine- parsable for the receivers, we define a top-level media type "multipart/report" with a new parameter "report-type="tlsrpt"". Inside it, there are two parts: The first part is human readable, typically "text/plain", and the second part is machine readable with a new media type defined called "application/tlsrpt+json". If compressed, the report should use the media type "application/ @@ -723,32 +743,27 @@ Note that, when sending failure reports via SMTP, sending MTAs MUST NOT honor MTA-STS or DANE TLSA failures. 5.4. HTTPS Transport The report MAY be delivered by POST to HTTPS. If compressed, the report SHOULD use the media type "application/tlsrpt+gzip", and "application/tlsrpt+json" otherwise (see section Section 6, "IANA Considerations"). - A reporting entity SHOULD expect a "successful" response from the - accepting HTTPS server, typically a 200 or 201 HTTP code [RFC7231]. - Other codes could indicate a delivery failure, and may be retried as - per local policy. The receiving system is not expected to process + The receiving system MUST return a "successful" response from its + HTTPS server, typically a 200 or 201 HTTP code [RFC7321]. Other + codes could indicate a delivery failure, and may be retried as per + local sender policy. The receiving system is not expected to process reports at receipt time, and MAY store them for processing at a later time. - Alternately, if a receiving system offers "Accept-Encoding" value of - "gzip", the sending system MAY use "Content-Encoding: gzip" as an - HTTP header as appropriate. This can be used in place of delivering - a compressed file as the payload. - 5.5. Delivery Retry In the event of a delivery failure, regardless of the delivery method, a sender SHOULD attempt redelivery for up to 24hrs after the initial attempt. As previously stated the reports are optional, so while it is ideal to attempt redelivery, it is not required. If multiple retries are attempted, ideally they SHOULD be done with exponential backoff. 5.6. Metadata Variances @@ -780,21 +795,70 @@ Specification document(s): this one 6.2. Report Type This document registers a new parameter "report-type="tlsrpt"" under "multipart/report" top-level media type for use with [RFC6522]. The media type suitable for use as a report-type is defined in the following section. -6.3. application/tlsrpt+json Media Type +6.3. +gzip Media Type Suffix + + This document registers a new media type suffix "+gzip". The GZIP + format is a public domain, cross-platform, interoperable file storage + and transfer format, specified in [RFC1952]; it supports compression + and is used as the underlying representation by a variety of file + formats. The media type "application/gzip" has been registered for + such files. The suffix "+gzip" MAY be used with any media type whose + representation follows that established for "application/gzip". The + media type structured syntax suffix registration form follows: + + Type name: GZIP file storage and transfer format + + +suffix: +gzip + + References: [RFC1952][RFC6713] + + Encoding considerations: GZIP is a binary encoding. + + Fragment identifier considerations: The syntax and semantics of + fragment identifiers specified for +gzip SHOULD be as specified for + "application/gzip". (At publication of this document, there is no + fragment identification syntax defined for "application/gzip".) The + syntax and semantics for fragment identifiers for a specific "xxx/ + yyy+gzip" SHOULD be processed as follows: + + For cases defined in +gzip, where the fragment identifier + resolves per the +gzip rules, then process as specified in + +gzip. + + For cases defined in +gzip, where the fragment identifier does + not resolve per the +gzip rules, then process as specified in + "xxx/yyy+gzip". + + For cases not defined in +gzip, then process as specified in + "xxx/yyy+gzip". + + Interoperability considerations: n/a + + Security considerations: GZIP format doesn't provide encryption. See + also security considerations of [RFC6713]. Each individual media + type registered with a +gzip suffix can have additional security + considerations + + Contact: art@ietf.org + + Author/Change controller: Internet Engineering Task Force + (mailto:iesg@ietf.org). + +6.4. application/tlsrpt+json Media Type This document registers multiple media types, beginning with Table 1 below. +-------------+----------------+-------------+-------------------+ | Type | Subtype | File extn | Specification | +-------------+----------------+-------------+-------------------+ | application | tlsrpt+json | .json | Section 5.3 | +-------------+----------------+-------------+-------------------+ Table 1: SMTP TLS Reporting Media Type @@ -805,51 +869,51 @@ Required parameters: n/a Optional parameters: n/a Encoding considerations: Encoding considerations are identical to those specified for the "application/json" media type. See [RFC7493]. Security considerations: Security considerations relating to SMTP TLS - Reporting are discussed in Section 7. + Reporting are discussed in Section 7. Security considerations + related to zlib compression are discussed in [RFC6713]. Interoperability considerations: This document specifies format of conforming messages and the interpretation thereof. Published specification: Section 5.3 of this document. Applications that use this media type: Mail User Agents (MUA) and Mail Transfer Agents. Additional information: - Magic number(s): n/a + Magic number(s): The first two bytes are 0x1f, 0x8b. File extension(s): ".json" Macintosh file type code(s): n/a Person & email address to contact for further information: See Authors' Addresses section. Intended usage: COMMON - Restrictions on usage: n/a Author: See Authors' Addresses section. Change controller: Internet Engineering Task Force (mailto:iesg@ietf.org). -6.4. application/tlsrpt+gzip Media Type +6.5. application/tlsrpt+gzip Media Type +-------------+----------------+-------------+-------------------+ | Type | Subtype | File extn | Specification | +-------------+----------------+-------------+-------------------+ | application | tlsrpt+gzip | .gz | Section 5.3 | +-------------+----------------+-------------+-------------------+ Table 2: SMTP TLS Reporting Media Type Type name: application @@ -885,21 +949,21 @@ Intended usage: COMMON Restrictions on usage: n/a Author: See Authors' Addresses section. Change controller: Internet Engineering Task Force (mailto:iesg@ietf.org). -6.5. STARTTLS Validation Result Types +6.6. STARTTLS Validation Result Types This document creates a new registry, "STARTTLS Validation Result Types". The initial entries in the registry are: +-------------------------------+ | Result Type | +-------------------------------+ | "starttls-not-supported" | | "certificate-host-mismatch" | | "certificate-expired" | @@ -935,20 +999,35 @@ Implementers are advised to take precautions against evaluating the contents of the report. o Report snooping: An attacker could create a bogus TLSRPT record to receive statistics about a domain the attacker does not own. Since an attacker able to poison DNS is already able to receive counts of SMTP connections (and, absent DANE or MTA-STS policies, actual SMTP message payloads), this does not present a significant new vulnerability. + o Ignoring HTTPS validation when submitting reports: When reporting + benign misconfigurations, it is likely that a misconfigured SMTP + server may also mean a misconfigured HTTPS server; as a result, + reporters who required HTTPS validity on the reporting endpoint + may fail to alert administrators about such misconfigurations. + Conversely, in the event of an actual attack, an attacker who + wished to create a gap in reporting and could intercept HTTPS + reports could, just as easily, simply thwart the resolution of the + TLSRPT TXT record or establishment of the TCP session to the HTTPS + endpoint. Furthermore, such a man-in-the-middle attacker could + discover most or all of the metadata exposed in a report merely + through passive observation. As a result, we consider the risks + of failure to deliver reports on misconfigurations to outweigh + those of attackers intercepting reports. + o Reports as DDoS: TLSRPT allows specifying destinations for the reports that are outside the authority of the Policy Domain, which allows domains to delegate processing of reports to a partner organization. However, an attacker who controls the Policy Domain DNS could also use this mechanism to direct the reports to an unwitting victim, flooding that victim with excessive reports. DMARC [RFC7489] defines a solution for verifying delegation to avoid such attacks; the need for this is greater with DMARC, however, because DMARC allows an attacker to trigger reports to a target from an innocent third party by sending that third party @@ -960,52 +1039,62 @@ Finally, because TLSRPT is intended to help administrators discover man-in-the-middle attacks against transport-layer encryption, including attacks designed to thwart negotiation of encrypted connections (by downgrading opportunistic encryption or, in the case of MTA-STS, preventing discovery of a new MTA-STS policy), we must also consider the risk that an adversary who can induce such a downgrade attack can also prevent discovery of the TLSRPT TXT record (and thus prevent discovery of the successful downgrade attack). Administrators are thus encouraged to deploy TLSRPT TXT records with - a large TTL (reducing the window for successful attacks against DNS - resolution of the record) or to deploy DNSSEC on the deploying zone. + a large TTL (reducing the window for successful application of + transient attacks against DNS resolution of the record) or to deploy + DNSSEC on the deploying zone. 8. References 8.1. Normative References [I-D.ietf-uta-mta-sts] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., and J. Jones, "SMTP MTA Strict Transport Security (MTA- - STS)", draft-ietf-uta-mta-sts-14 (work in progress), - January 2018. + STS)", draft-ietf-uta-mta-sts-15 (work in progress), April + 2018. + + [RFC1952] Deutsch, P., "GZIP file format specification version 4.3", + RFC 1952, DOI 10.17487/RFC1952, May 1996, + . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, . [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, . [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, . + [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) + for Authorizing Use of Domains in E-Mail, Version 1", + RFC 4408, DOI 10.17487/RFC4408, April 2006, + . + [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [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, . @@ -1047,54 +1136,68 @@ [RFC6522] Kucherawy, M., Ed., "The Multipart/Report Media Type for the Reporting of Mail System Administrative Messages", STD 73, RFC 6522, DOI 10.17487/RFC6522, January 2012, . [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 2012, . + [RFC6713] Levine, J., "The 'application/zlib' and 'application/gzip' + Media Types", RFC 6713, DOI 10.17487/RFC6713, August 2012, + . + [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014, . [RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493, DOI 10.17487/RFC7493, March 2015, . [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, . + [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC + 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, + May 2017, . + 8.2. Informative References [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, February 2002, . [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, . [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004, . + [RFC7321] McGrew, D. and P. Hoffman, "Cryptographic Algorithm + Implementation Requirements and Usage Guidance for + Encapsulating Security Payload (ESP) and Authentication + Header (AH)", RFC 7321, DOI 10.17487/RFC7321, August 2014, + . + [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, December 2014, . [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 2015, . [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based Message Authentication, Reporting, and Conformance @@ -1137,35 +1241,35 @@ "mx: .mail.company-y.example","max_age: 86400"], "policy-domain": "company-y.example", "mx-host": ".mail.company-y.example" }, "summary": { "total-successful-session-count": 5326, "total-failure-session-count": 303 }, "failure-details": [{ "result-type": "certificate-expired", - "sending-mta-ip": "98.136.216.25", + "sending-mta-ip": "2001:db8:abcd:0012::1", "receiving-mx-hostname": "mx1.mail.company-y.example", "failed-session-count": 100 }, { "result-type": "starttls-not-supported", - "sending-mta-ip": "98.22.33.99", + "sending-mta-ip": "2001:db8:abcd:0013::1", "receiving-mx-hostname": "mx2.mail.company-y.example", - "receiving-ip": "192.168.14.72", + "receiving-ip": "203.0.113.56", "failed-session-count": 200, "additional-information": "https://reports.company-x.example/ report_info ? id = 5065427 c - 23 d3# StarttlsNotSupported " }, { "result-type": "validation-failure", - "sending-mta-ip": "47.97.15.2", - "receiving-ip": "10.72.84.12", + "sending-mta-ip": "198.51.100.62", + "receiving-ip": "203.0.113.58", "receiving-mx-hostname": "mx-backup.mail.company-y.example", "failed-session-count": 3, "failure-error-code": "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED" }] }] } Authors' Addresses Daniel Margolis