Skip to main content

Encrypted Client Hello Deployment Considerations

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Andrew Campling , Paul A. Vixie , David Wright , Arnaud Taddei , Simon Edwards
Last updated 2023-07-10 (Latest revision 2023-03-26)
RFC stream (None)
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date (None)
Responsible AD (None)
Send notices to (None)
opsec                                                     A. J. Campling
Internet-Draft                                    419 Consulting Limited
Intended status: Informational                                  P. Vixie
Expires: 12 January 2024                                        Red Barn
                                                               D. Wright
                                                UK Safer Internet Centre
                                                               A. Taddei
                                                              S. Edwards
                                                            11 July 2023

            Encrypted Client Hello Deployment Considerations


   (Editorial note: to be updated as the text in the main body of the
   document is finalised) This document is intended to inform the
   community about the impact of the deployment of the proposed
   Encrypted Client Hello (ECH) standard that encrypts Server Name
   Indication (SNI) and other data.  Data encapsulated by ECH (ie data
   included in the encrypted ClientHelloInner) is of legitimate interest
   to on-path security actors including those providing inline malware
   detection, parental controls, content filtering to prevent access to
   malware and other risky traffic, mandatory security controls etc.

   The document includes observations on current use cases for SNI data
   in a variety of contexts.  It highlights how the use of that data is
   important to the operators of both public and private networks and
   shows how the loss of access to SNI data will cause difficulties in
   the provision of a range of services to end-users, including the
   potential weakening of cybersecurity defences.  Some mitigations are
   identified that may be useful for inclusion by those considering the
   adoption of support for ECH in their software.

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

   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 12 January 2024.

Copyright Notice

   Copyright (c) 2023 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 (
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Background
     1.2.  Scope, objectives and limits of this document
   2.  General considerations about the encryption of the Client Hello
     2.1.  About encrypting the Server Name Indication (SNI)
     2.2.  Why are middleboxes using the SNI?
     2.3.  The case of Proxies
     2.4.  Why relying on the SNI and not the DNS
     2.5.  About the unreliability of the SNI
       2.5.1.  With TLS 1.2
       2.5.2.  With TLS 1.3
       2.5.3.  With TLS 1.3 with ECH extension
   3.  The Education Sector
     3.1.  Context
     3.2.  Why Content Filtering Matters to Schools
     3.3.  Mitigations
     3.4.  Implications
   4.  Impact of ECH in private network contexts (Enterprises or other
     4.1.  Context
       4.1.1.  The main requirements
       4.1.2.  A degrading threat landscape
     4.2.  Additional considerations
     4.3.  Implications
       4.3.1.  Examples of regulatory implications
       4.3.2.  Impact of ECH deployment on Network Security Operations
       4.3.3.  Specific implications for SMBs
   5.  Public Network Service Providers
     5.1.  Context
     5.2.  Mitigations
       5.2.1.  Current approaches and procedures
       5.2.2.  The blocking use case
     5.3.  Implications
   6.  General issues
     6.1.  Threat Detection
     6.2.  Endpoint security limits
     6.3.  Network management
     6.4.  Future operational deployment issues due to the
           introduction of the Client Facing servers themselves
     6.5.  Migration issues
   7.  Potential further development of this work
     7.1.  Potential development of this document.
     7.2.  Potential development outside of the scope of this document
   8.  Conclusion
   9.  Security Considerations
   10. IANA Considerations
   11. Acknowledgment
   12. Appendix A - Initial data illustrating SNI unreliability
     12.1.  Data collections
     12.2.  Consumer Network Traffic
     12.3.  Corporate Customer Traffic
     12.4.  Take away observations
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Authors' Addresses

1.  Introduction

1.1.  Background

   In order to establish its handshake, the TLS protocol needs to start
   with a first handshake message called the Client Hello.  As this
   handshake message is in clear text, it exposes metadata, e.g. the
   Server Name Indication (SNI) which allow middleboxes on path to make
   policy decisions, in particular but not only for security reasons.
   As part of a wider initiative to achieve end-to-end encryption, a
   proposed extension to TLS 1.3 called Encrypted Client Hello (ECH)
   [I-D.draft-ietf-tls-esni] is attempting to encrypt all the remaining
   metadata in the clear.

   There are use cases where encryption of the SNI data may be a useful
   precaution to reduce the risk of pervasive monitoring that offers
   some benefits (e.g Enterprises offering services for their own
   customers will appreciate that their customers' privacy be better
   protected).  However ECH presents challenges for other use cases
   (e.g.  Enterprises needing network security controls for compliance

   The Internet was envisaged as a network of networks, each able to
   determine what data to transmit and receive from their peers.
   Developments like ECH mark a fundamental change in the architecture
   of the Internet, allowing opaque paths to be established from
   endpoints to commercial services, some potentially without the
   knowledge or permission of the device owners.  This change should not
   be undertaken lightly given both the architectural impact on the
   Internet and potentially adverse security implications for end users.
   Given these implications, it certainly should not be undertaken
   without either the knowledge of or consultation with end users, as
   outlined in [RFC8890].

   Whilst it is reasonable to counter that VPNs also establish opaque
   paths, a primary difference is that the use of a VPN is a deliberate
   act by the user, rather than a choice made by client software,
   potentially without either the knowledge and/or consent of the end-
   user or device owner.

   [RFC7258] discusses the critical need to protect users' privacy when
   developing IETF specifications and also recognises that making
   networks unmanageable to mitigate pervasive monitoring is not an
   acceptable outcome.

   [RFC8404] discusses current security and network operations as well
   as management practices that may be impacted by the shift to
   increased use of encryption to help guide protocol development in
   support of manageable and secure networks.  As [RFC8404] notes, "the
   implications for enterprises that own the data on their networks or
   that have explicit agreements that permit the monitoring of user
   traffic are very different from those for service providers who may
   be accessing content in a way that violates privacy considerations".

1.2.  Scope, objectives and limits of this document

   This document considers the implications of ECH for private, edge and
   public networks using the examples of education establishments,
   enterprises and public operators.  It addresses the limitations of
   [RFC8744] by providing more information about the issues posed by the
   introduction of ECH due to the loss of visibility of SNI data on
   private networks building on the report from a roundtable discussion

   The objective of this document is to detail some operational impacts
   of ECH.  It will focus specifically on the impact of encrypting the
   SNI data by ECH, but it should be noted that other elements in the
   client hello may also be relevant for some on-path security methods.

   The data encapsulated by ECH is of legitimate interest to on-path
   security actors including those providing inline malware detection,
   firewalls, parental controls, content filtering to prevent access to
   malware and other risky traffic, mandatory security controls (e.g.
   Data Loss Prevention) etc.  Beyond network security, there are
   various operational impacts of different types e.g. network
   management, content filtering, etc.

   Whilst this document identifies operational issues:

   *  it does not consider solutions nor question the development of the
      ECH proposal itself

   *  it doesn't attempt to be exhaustive,

   *  it will start by focusing on one category of middleboxes: proxies.

2.  General considerations about the encryption of the Client Hello

2.1.  About encrypting the Server Name Indication (SNI)

   [RFC8744] describes the general problem of encrypting the Server Name
   Identification (SNI) TLS extension.  The document includes a brief
   description of what it characterises as "unanticipated" usage of SNI
   information (section 2.1) as well as a brief (two paragraph)
   assessment of alternative options in the event that the SNI data is
   encrypted (section 2.3).

   The text in [RFC8744] suggests that most of the unanticipated SNI
   usage "could also be implemented by monitoring DNS traffic or
   controlling DNS usage", although it does then acknowledge the
   difficulties posed by encrypted DNS protocols.  It asserts, with
   limited evidence, that "most of 'the unanticipated usage' functions
   can, however, be realized by other means", although without
   considering or quantifying the affordability, operational complexity,
   technical capability of affected parties or privacy implications that
   might be involved.  It is unclear from the document whether any
   stakeholders that may be impacted by the encryption of SNI data have
   been consulted; it certainly does not appear to be the case that any
   such consultation has taken place.

   The characterisation of "unanticipated usage" of SNI data could be
   taken to imply that such usage was not approved and therefore
   inappropriate in some manner.  The reality is that the development of
   the Internet has many examples of permissionless innovation and so
   this "unanticipated usage" of SNI data should not be dismissed as
   lacking in either importance or validity.

2.2.  Why are middleboxes using the SNI?

   For middleboxes to be able to perform their job they need to identify
   the destination of the requested communication.  Before TLS1.3 a
   middlebox could rely on, at least, 3 metadata sources: The
   certificate, the DNS name and the SNI.  A middlebox may have used
   some or all of these metadata to determine the destination in the
   best possible way.  Yet, as part of the current initiative to
   complete end-to-end encryption, the certificate was encrypted into
   TLS1.3, then DoH/DoT/DoQ are encrypting the DNS flow to its resolver
   making it harder for middleboxes to use this information.  Even if
   the DNS data can be accessed, it can be misleading in some situations
   (does it point to the real destination, or just the site hosting
   server name, or a proxy?) and the SNI was invented to overcome some
   of the limitations of the DNS data by providing additional
   information.  However, the SNI in itself may be unreliable which is
   why middleboxes start by non-trusting it until they have validated
   the information that it provides.

2.3.  The case of Proxies

   A proxy server is a server application that acts as an intermediary
   between a client requesting a resource and the server providing that
   resource.  Instead of connecting directly, the client directs the
   request to the proxy server which evaluates the request before
   performing the required network activity.  Proxies are used for
   various purposes including load balancing, privacy and security.

   Proxies can be used explicity or transparently.

   In explicit proxy model, proxies are accessed by configuring a user's
   application or network settings, with traffic diverted to the proxy
   rather than the target destination.

   With "transparent" proxying, the proxy intercepts packets directed to
   the destination, making it seem as though the request is handled by
   the target destination itself.

   A key advantage of transparent proxies is that they work without
   requiring the configuration of user devices or software.  They are
   commonly used by organisations to provide content filtering for
   devices that they don't own that are connected to their networks.
   For example, some education environments use transparent proxies to
   implement support for “bring your own device” (BYOD) without needing
   to load software on third- party devices.

   Transparent proxies use SNI data to understand whether a user is
   accessing inappropriate content without the need to inspect data
   beyond the SNI field.  Because of this, encryption of the SNI field,
   as is the case with ECH, will disrupt the use of transparent proxies,
   requiring far more intrusive data inspection to be undertaken

2.4.  Why relying on the SNI and not the DNS

   As per the Introduction section of [RFC8744] "More and more services
   are colocated on multiplexed servers," the SNI was introduced to
   allow "direct connections to the appropriate service implementation".

   By design, a proxy cannot rely only on the DNS to ensure establishing
   the connection, the SNI is simply required by design.

2.5.  About the unreliability of the SNI

   SNI is by design not reliable therefore prompting the question for
   why and how middelboxes are using it in the first place.  Before
   anything, in Security, in general, unreliability is a useful source
   of information in itself.

   Referring to [RFC6066], TLS extenisons, including SNI, are designed
   to be backward compatible.  This means that if the server doesn't
   recognize the SNI value, the TLS handshake should continue anyway.

   In other terms, SNI value can 1) be empty or 2) have an alternative
   name which is different from the real name of the destination server
   without impacting the establishment of the TLS session.  This can
   also be easily exploited by bad actors indeed to bypass security
   middle boxes.  E.g A malware could just be coded to provide an SNI
   value that is mapped to finance or healthcare categories to bypass
   inspection (it is not that easy, but there is a way to do it).

2.5.1.  With TLS 1.2

   User client generates “ClientHello” with SNI in plain text to the
   destination server.  If accepted, the server responds to the user
   client request with the “ServerHello” message containing the intended
   server certificate alongside other encryption-related information in
   the plain text mode as well.  Middleboxes can then see/inspect 1/ the
   SNI in the “ClientHello” and 2/ the server certificate details in the
   “ServerHello” message.  Middleboxes can perform selective inspection
   based on the destination service details (service requested by the
   user client) extracted from the server certificate.  I would like to
   note that depending on the middlebox design, it is more about a
   correlation of different source of information to confirm the right
   destination service.  But still only information within the server
   certificate is reliable to perform accurate web categorization and
   then do selective inspection of any kind of web/content filtering.

2.5.2.  With TLS 1.3

   TLS 1.3 improves significantly over the TLS 1.2 in terms of security
   and privacy aspects.  More specifically, in terms of privacy, it
   overcomes the plain text server certificate exchange issue by masking
   the server’s host identity through the encrypted server certificate.
   So as middleboxes inspection capabilities are designed based on the
   server certificate, all vendors worked on how to adapt their
   capabilities to support TLS 1.3 (e.g ServerHello encryption but not

   So how do providers support TLS 1.3 inspection then?

   The most common technique is first to get the SNI from the
   ClientHello (which is still shared in plain text format.) and then
   replays / establish a new full TLS session initiated from the proxy
   server itself to the destination server to retrieve the server
   certificate details before determining the web category; Once
   identified, selective inspection can be performed on the real TLS
   session initiated by the user client.

   As the SNI is not reliable, proxies accept the SNI asis but do
   without trusting it, then they perform checks at various level to
   verify this SNI and they step by step enrich the evaluation,
   therefore bringing more possibilites to interpret which policy to
   apply.  This could end up with the proxy deciding to block the
   connection, or the proxy to let the connection happened with a
   verified or corrected SNI.

   See Appendix for some initial data gathering on the reliability of
   the SNI.

2.5.3.  With TLS 1.3 with ECH extension

   The entire legacy ClientHello message (Inner ClientHello) is
   encrypted, encapsulated and sent as part of the new ClientHello
   wrapper message (Outer ClientHello); So middleboxes cannot identity
   the destination service anymore and cannot replay the TLS session to
   the destination server because it has no clue about it.  (May be DNS
   knows but not all the time, see other issue here created by myself).

   So TLS ECH has an impact on enterprise security and compliance
   (including selective inspection) not because of SNI (which is not
   reliable) encryption but because there is no other information about
   the destination server is available and that can be used to fetch and
   retrieve the server certificate (Which is indeed reliable) to perform
   web categorization.

   We don’t care about SNI and its reliability, we need just to know the
   destination service to get the destination server certificate

3.  The Education Sector

3.1.  Context

   Focusing specifically on the education sector, the primary issue
   caused by ECH is that it is likely to circumvent the safeguards
   applied to protect children through content filtering, whether in the
   school or home environments, adding to adverse impacts already
   introduced through the use of encrypted DNS protocols such as DNS
   over HTTPS [RFC8484].

   Content filtering that leverages SNI information is used by education
   establishments to protect children from exposure to malicious, adult,
   extremist and other content that is deemed either age-inappropriate
   or unsuitable for other reasons.  Any bypassing of content filtering
   by client software on devices will be problematic and may compromise
   duties placed on education establishments.  For example: schools in
   England and Wales have obligations to provide "appropriate filtering
   systems" [KCSE]; schools in the US use Internet filters and implement
   other measures to protect children from harmful online content as a
   condition for the receipt of certain federal funding, especially
   E-rate funds [CIPA].

3.2.  Why Content Filtering Matters to Schools

   The impact that ineffective content filtering can have on an
   educational institutions should not be underestimated.  For example,
   a coroner in the UK in 2021 ruled that a school's failure to prevent
   a pupil from accessing harmful material online on its equipment
   contributed to her taking her own life [Coroner].  In this particular
   instance, the filtering software installed at the school was either
   faulty or incorrectly configured but the case highlights the harmful
   risks posed if the filtering is bypassed by client software using

3.3.  Mitigations

   Whilst it may be possible for schools to overcome some of the issues
   ECH raises by adopting similar controls to those used by enterprises,
   it should be noted that most schools have a very different budget for
   IT compared to enterprises and usually have very limited technical
   support capabilities.  Therefore, even where technical solutions
   exist that may allow them to continue to meet their compliance
   obligations, affordability and operational expertise will present
   them with significant difficulties.

   Absent funding and technical expertise, schools will need to consider
   the best way forward that allows them to remain compliant.  If client
   software does not allow ECH to be disabled, any such software that
   implements support for ECH may need to be removed from school devices
   and replaced, assuming that suitable alternatives are available.
   This will have a negative impact on budgets and may be operationally
   challenging if institutions have made a significant investment in the
   deployment and use of particular applications and technologies.

   There are instances where policies in education establishments allow
   for the use of equipment not owned by the institution, including
   personal devices and the devices of contractors and site visitors.
   These devices are unlikely to be configured to use the institution's
   proxy but can nevertheless connect to the school network using a
   transparent proxy (see below).  Transparent proxies used for
   filtering will typically use SNI data to understand whether a user is
   accessing inappropriate data, so encrypting the SNI field will
   disrupt the use of these transparent proxies.

3.4.  Implications

   In the event that transparent proxies are no longer effective,
   institutions will either have to require more invasive software to be
   installed on third party devices before they can be used along with
   ensuring they have the capability to comprehend and adequately manage
   these technologies or will have to prevent those devices from
   operating.  Neither option is desirable.

4.  Impact of ECH in private network contexts (Enterprises or other

4.1.  Context

4.1.1.  The main requirements

   Enterprises and Organizations need to protect themselves for a vast
   number of reasons, mainly:

   *  Reduce their Risks.  And in particular as part of any Cyber
      Resilience strategy.

   *  Protect their Reputation.  The term Reputation includes many
      aspects way beyond the traditional enterprises and organization
      assets (data, etc.).

   *  Comply to a growing diverse set of Policies, Regulations,
      Certifications, Labeling and Guidelines.  These requirements are
      growing in both scope and complexity as they are added to by
      various national and regional bodies around the world.

4.1.2.  A degrading threat landscape

   In addition, the general threat landscape which was already very
   large (see [I-D.draft-mcfadden-smart-threat-changes]), has
   significantly increased in three ways:

   *  COVID crisis generally accelerated the overall attack landscape.
      Indeed as the crisis forced many enterprises and organizations to
      accelerate their digital transformation, it increased the
      opportunity for cyber criminals and nation states to launch more
      attacks, leverage innovations to their advantage, better select
      their targets, increase their efficiency and increase their
      rewards, in particular with Ransomware based attacks.

   *  The Supply Chain is under stress as per the [SOLARWIND] attack

   *  Nation State attacks are continuing to evolve, for example as
      noted to those linked to the current Ukraine crisis.

   Attacks are now damaging enterprises and other organizations with
   ransomware being the number 1 issue by a considerable margin.  The
   attacks are increasing in severity, to the extent that this is now
   being measured at macroscopic level in some countries:

   *  EUR1B loss of revenue for French organizations from January to
      August 2022 [LOSSINREVENUE]

   *  Loss in capitalisation between 1-5% [LOSSINCAP]

   *  Degradation by credit notation agencies [LOSSINCREDITSCORE]

   Another implication from the COVID crisis is the acceleration of BYOD
   with the current reliance on remote working.  This has created two
   side effects for remote employees, contractors and third parties that
   need to connect to one or more enterprise networks on a temporary

   *  need to use a VPN access to the corporate network, which brings
      all the benefits (e.g. protected access to corporate network) and
      risks that VPNs may open (e.g. lateral movement when the end point
      is compromised),

   *  need to access a cloud proxy which requires an agent to be
      installed on the device to steer the traffic to the right place.

4.2.  Additional considerations

   In such circumstances, requiring software or custom configurations to
   be installed on those devices may be problematic (see

   This is why network security solutions are required and this is why
   the use of ECH to prevent access to the SNI data makes it impossible
   for blue teams to defend (see the next sections for details).

   Finally there is a global shortage of cybersecurity personnel.  Any
   expansion of technical requirements, for example to mitigate the
   operational challenges through the introduction of ECH, will
   exacerbate the problem.

   All the above conditions are weighing on capabilities to defend,

   *  Directly: a lack of visibility on a key meta data like the SNI
      will cause significant issues to enterprises and organizations

   *  Indirectly: should ECH happen and should alternative be provided,
      managing migrations to any alternative not requiring access to the
      SNI, in these conditions, is undesirable from a timing, resources,
      capacities and risks perspectives.

4.3.  Implications

4.3.1.  Examples of regulatory implications

   Regulators are accelerating their lawfare capabilities at accelerated
   pace and new legislations is impacting on the actions of enterprises
   with increased precision.  The EU GDPR had ripple effects such as
   requiring Financial Institutions to use selective decrypt in order to
   implement Data Loss Prevention.  The recent indication that US
   regulators are in the process of levying fines of $200m each on a
   number of institutions because they were unable to track all
   communications by their employees using WhatsApp or Signal ,
   [Bloomberg], creates new auditability constraints.  It is with
   growing concern that an ECH enabled ecosystem may clash with future
   regulatory requirements.

4.3.2.  Impact of ECH deployment on Network Security Operations

   Enterprises approach to endpoint control varies significantly
   depending on size, use cases and a vast number of other factors.

   For example, large enterprises generally exert control over their
   endpoints, yet to the limits of some use cases they may need to
   implement, e.g.  BYOD.  The latter was accelerated, as per above, due
   to COVID forcing more flexibility in the extended work force
   (employees, contractors, etc.).

   On the contrary, small and some medium businesses may not be in the
   position to control their endpoints to the same extent (see specific
   implications for SMBs section below).

   As some Browser makers made the use of ECH optional, this gives a
   first approach for enterprises to disable ECH for their employees.

   However this doesn't provide an holistic solution.  Indeed
   enterprises will need to consider a number of issues:

   *  Browsers which do not offer an option to disable ECH

   *  Browsers that will make ECH non optional in the future

   *  Non-browsers applications which are designed to use libraries
      enforcing ECH, without any option to disable it

   *  All the range of BYOD use cases where enterprises do not control
      the endpoint

   *  Adversaries leveraging ECH e.g. to hide their command and control
      communications, e.g. in Ransomware cases.

   Whilst, disabling ECH wherever possible provides one approach to
   mitigate ECH deployment issues, as per above, other mitigations
   approaches need to be offered to enterprises.

   (Editor's note: we need to describe how to strip the RRs to force a
   global disabling of ECH, yet mindful it might not be sufficient if an
   adversary finds a way to not use the enterprise DNS resolver)  Reminders on Network Security

   Network Security is a set of security capabilities which is
   articulated as part of a defense strategy, e.g.  Defense In Depth
   [NIST-DID], Zero Trust, SASE/SSE, etc. and can trigger and enable
   other security capabilities such as sandboxing, Data Loss Prevention,
   Cloud Access Service Broker (CASB), etc.  One constituency is a Web
   Proxy, combining both a TLS proxy and an application level (HTTP)

   In the same way that [I-D.draft-ietf-opsec-ns-impact] showed the
   impact of TLS1.3 on operational security, a loss of visibility of the
   SNI as indicator of compromise (see
   [I-D.draft-ietf-opsec-indicators-of-compromise]) has two main
   implications  Implications from loss of Meta Data

   The loss of visibility of the SNI, at TLS level, will prevent
   transparent proxies from applying corporate policies to manage risk
   and compliancy.  Typical examples:

   *  categories of compromised sites cannot be applied anymore,
      exposing employees and their organisations to potential
      cybersecurity risks; alternative approaches to block access to
      theses sites need to be found

   *  corporate lists of excluded sites for compliance or policy reasons
      need alternatives ways to be blocked.  Implications from loss of Selective Decrypt

   TLS proxies also have the ability to selectively intercept, avoiding
   any visibility into or modification of the original application
   protocol payload - but such selective intercept relies heavily on
   knowledge of the origin content server hostname, which can be
   extracted in plaintext from the TLS ClientHello SNI (server name)

   This capability allows the application proxy, in particular an HTTPS
   proxy to engage efficiently specific security controls, e.g.  Data
   Loss Prevention, Sandboxing, etc.

   The loss of SNI visibility will make it more difficult for corporate
   user flows to be intercepted, with it becoming impossible for BYOD
   use cases.

   This will create inefficiencies, will require more resources and will
   increase security risks.  It will also be counter productive for
   privacy as it may require the proxy to decrypt the whole TLS

4.3.3.  Specific implications for SMBs

   Small and Medium Business (SMBs) form a particularly vulnerable
   subset of enterprises and organizations and span from Small Office
   Home Office (SOHO, sometimes a one person business) to Medium
   Business with strong variations depending on the country (a 50
   employee company is considered the upper range of SMB business in
   developing countries while it is up to 25,000 in some developed

   Similarly to the above education use case and irrespective of
   definitions, many SMBs have very limited in-house capabilities to
   defend themselves, with security often outsourced to Managed Security
   Service Providers (typically network operators, mid range and small
   service providers).

5.  Public Network Service Providers

5.1.  Context

   In Public Networks the national, regional and international
   legislator has to balance between freedom of access to the
   information on the one hand, and safety of the Internet and the
   protection of other fundamental rights on the other hand.

   There are 2 main approaches:

   *  First, there are countries which do not have any specific
      legislation on the issue of blocking, filtering and takedown of
      illegal Internet content: there is no legislative or other
      regulatory system put in place by the state with a view to
      defining the conditions and the procedures to be respected by
      those who engage in the blocking, filtering or takedown of online
      material.  In the absence of a specific or targeted legal
      framework, several countries rely on an existing "general" legal
      framework that is not specific to the Internet to conduct what is,
      generally speaking, limited blocking or takedown of unlawful
      online material.  Here the approach has been differentiated in
      relying on self regulation from the private sector or limited
      political or legislative intervention to specific areas.

   *  The other approach has been to set up a legal framework
      specifically aimed at the regulation of the Internet and other
      digital media, including the blocking, filtering and removal of
      Internet content.  Such legislation typically provides for the
      legal grounds on which blocking or removal may be warranted, the
      administrative or judicial authority which has competence to take
      appropriate action and the procedures to be followed.

5.2.  Mitigations

5.2.1.  Current approaches and procedures

   In relation to specific areas where the public interest has to be
   protected more strongly, such as child abuse crimes, terrorism,
   criminality and national security, many states have a framework for
   the urgent removal of Internet content regarding the above materials
   without the need of a court order.  In such circumstances,
   administrative authorities, police authorities or public prosecutors
   are given specific powers to order Internet access providers to block
   access without advance judicial authority.  It is common to see such
   orders requiring action on the part of the Internet access provider
   within 24 hours, and without any notice being given to the content
   provider or host themselves.

   Particularly in relation to material concerning child abuse and other
   serious crimes, many countries adopt a “list” system, whereby a
   central list of blocked URLs or domain names are maintained and
   updated by the relevant administrative authority.  This is notified
   to the relevant Internet access providers, who are required to ensure
   that blocking is enforced.  Additionally in some states the
   authorities can request the removal of content that infringes
   intellectual property, privacy or defamation rights.  In this case
   the removal need to be requested by a court order.

   Generally speaking, the grounds relied on broadly correspond to the
   interests protected under Article 10(2) of the European Convention of
   Human Rights (ECHR), namely: the protection of national security,
   territorial integrity or public safety, the prevention of disorder or
   crime, the protection of health or morals, the protection of the
   reputation or rights of others, and the prevention of the disclosure
   of information received in confidence.  From the methodology we have
   to distinguish between blocking or takedown of content.

   *  The blocking, filtering or prevention of access to Internet
      content are generally technical measures intended to restrict
      access to information or resources typically hosted in another
      jurisdiction.  Such action is normally taken by the Internet
      access provider through hardware or software products that block
      specific targeted content from being received or displayed on the
      devices of customers of the Internet access provider.

   *  Takedown or removal of Internet content, on the other hand, will
      instead broadly refer to demands or measures aimed at the website
      operator (or "host") to remove or delete the offending website
      content or sub content.

   In these considerations we will refer to blocking only.

5.2.2.  The blocking use case

   This can be achieved through a number of techniques, including the
   blocking of the Domain Name System (DNS), the analysis of the SNI
   field or the Uniform Resource Locator (URL).  Given the increasing
   adoption of encryption techniques often a mixture of the above
   techniques is needed.

   In particular for the most serious crimes such as child abuse or
   national security many countries adopt a “list” methodology, where a
   central list of blocked Domains or URLs is maintained by the
   authorities and updated on a regular basis (daily or even hourly) and
   shared with Public Network Operators that have to enforce the

   In many jurisdictions there are legal consequences for the Operator
   not complying with the blocking order.

   Technically the blocking can be implemented using techniques that
   have been adapted over time as new technologies have been introduced.

   Historically depending on the content of the list the technique have
   been based on DNS or proxy blocking.

   DNS is effective on Domains (the whole domain is blocked), while
   proxy is effective either on Domain (for encrypted traffic) or URL
   (for unencrypted traffic).

   Given that nowadays the vast majority of traffic is encrypted, the
   capability of blocking based on URL is limited to a small portion of
   traffic and proxy blocking is as effective as that based on the DNS.

   Theoretically DNS blocking would be the preferred option for
   operators given the more limited investments necessary to implement
   blocking of the Domains, but given the increased usage of external
   encrypted DNS services DNS blocking is becoming less effective and
   operators need to use SNI analysis as well in order to fulfil legal

5.3.  Implications

   The adoption of ECH will cause additional problems and limit the
   possibility of implementing operators fulfilling their legal blocking
   obligations, exposing the population to illegal content related to
   crimes such as Child Sex Abuse Material (CSAM), malware and other
   malicious content, and possibly even content deemed to be detrimental
   to National Security.

   In addition, operators that do not fulfil their legal obligations may
   be exposed to legal or regulatory remedies.

6.  General issues

6.1.  Threat Detection

   [RFC8404] identifies a number of issues arising from increased
   encryption of data, some of which apply to ECH.  For example, it
   notes that an early trigger for DDoS mitigation involves
   distinguishing attacker traffic from legitimate user traffic; this
   become more difficult if traffic sources are obscured.

   The various indicators of compromise (IoCs) are documented in
   [I-D.draft-ietf-opsec-indicators-of-compromise], which also describes
   how they are used effectively in cyber defence.  For example, section
   4.1.1 of the document describes the importance of IoCs as part of a
   defence- in-depth strategy; in this context, SNI is just one of the
   range of indicators that can be used to build up a resilient defence
   (see section 3.1 in the same document on IoC types and the 'pyramid
   of pain').

   In the same Internet-Draft, section 6.1 expands on the importance of
   the defence in depth strategy.  In particular, it explains the role
   that domains and IP addresses can play, especially where end-point
   defences are compromised or ineffective, or where endpoint security
   isn't possible, such as in BYOD, IoT and legacy environments.  SNI
   data plays a role here, in particular where DNS data is unavailable
   because it has been encrypted; if SNI data is lost too, alongside
   DNS, defences are weakened and the attack surface increased.

6.2.  Endpoint security limits

   (Editorial note: Elaborate on endpoint security complications as
   [I-D.draft-taddei-smart-cless-introduction] as well as [MAGECART]
   [MITB] [MITB-MITRE] [MALVERTISING] showed that in some cases, the
   only way to detect an attack is through the use of network-based
   security.  The loss of visibility of the SNI data will make it much
   harder to detect attacks.  The endpoints components (operating
   system, applications, browsers, etc.) cannot be judge and jury.)

6.3.  Network management

   (Editorial note: this is a placeholder for future issues)

6.4.  Future operational deployment issues due to the introduction of
      the Client Facing servers themselves

   (Editorial note: this is a placeholder for future issues;

   *  Consolidation considerations - the use of ECH may accelerate the
      move of content away from standalone servers and on to CDNs,
      reducing infrastructure resilience.

   *  What happens if Client Facing servers are controlled by malicious

   *  The Client Facing servers are acting as a new category of
      middleboxes.  In this shift left movement, until the attack
      surface is minimal and complexities are removed, you have to rely
      on third parties for inspection.  In these conditions, on which
      basis can they be more trusted than any other middleboxes?  Is
      this creating a concentration problem?


6.5.  Migration issues

   (Editorial note: this is a placeholder for future issues;

   *  If ECH is enforced what are the solutions to all the above
      problems and what are the migration paths?


7.  Potential further development of this work

7.1.  Potential development of this document.

   This section lists potential development of this work in particular
   for the General Issues section.

   *  There are need for further clarifications from the ECH draft, e.g.
      The link between the Client Facing and the backend servers are not
      clear enough and need further description.  It can't be just 'left
      to the implementation'.  The action is still underway and feedback
      to the TLS working group will be provided.

   *  Will there be any impact to the DNS by adding so many new RRs?

7.2.  Potential development outside of the scope of this document

   This document infers a number of ideas that could be relevant for
   other groups and in other deliverables.  In particular regarding what
   type of solutions could be considered

   *  There is a need to address the apparent disconnect between user
      privacy and security, it should be possible to provide both rather
      than one compromising the other.

   *  What prevents a Client Facing server providing security solutions
      to protect the data path?

   *  Given some of the many challenges there is the opportunity to
      review the current ECH proposal from the perspective of a
      respectful inspection protocol.

8.  Conclusion

   Access to SNI data is sometimes necessary in order for institutions,
   including those in the education and finance sectors, to discharge
   their compliance obligations.  The introduction of ECH in client
   software poses operational challenges that could be overcome on
   devices owned by those institutions if policy settings are supported
   within the software that allows the ECH functionality to be disabled.

   (Editorial note: these two below paragraph need revision towards the
   end of the development of this draft)

   Third-party devices pose an additional challenge, primarily because
   the use of ECH will render transparent proxies inoperable.  The most
   likely solution is that institutions will require the installation of
   full proxies and certificates on those devices before they are
   allowed to be connected to the host networks.  They may alternatively
   determine that such an approach is impractical and instead withdraw
   the ability for network access by third-party devices.

   An additional option that warrants further consideration is the
   development of a standard that allows a network to declare its policy
   regarding ECH and other such developments.  Clients would then have
   the option to continue in setting up a connection if they are happy
   to accept those policies, or to disconnect and try alternative
   network options if not.  Such a standard is outside of the scope of
   this document but may provide a mechanism that allows the interests
   and preferences of client software, end-users and network operators
   to be balanced.

9.  Security Considerations

   In addition to introducing new operational and financial issues, the
   introduction of SNI encryption poses new challenges for threat
   detection which this document outlines.

   This I-D should help improve security in deployments of ECH.

10.  IANA Considerations

   This document has no IANA actions.

11.  Acknowledgment

   In memory of Simon Edwards who passed away in the night of 8th-9th of
   January 2023.

   In addition to the authors, this document is the product of an
   informal group of experts including the people listed in the
   Contributors list in Appendix.

12.  Appendix A - Initial data illustrating SNI unreliability

12.1.  Data collections

   In this appendix a couple data sets were collected from SSL Session
   logs from a Symantec SSLV.  The goal was to see how prevalent are TLS
   sessions being established where the Server Name Indicator (SNI) was
   incorrect as compared to the Subject Alternative Name (SAN) contained
   within the Server Certificate.  Applications and browsers that are
   establishing these mismatched connections have TLS Hygiene issues.
   In other words these sessions are being improperly established.  None
   of the traffic was for nefarious means.  However, an improperly
   defined SNI can be used to fool inspection devices to bypass security
   rules and measures.

   The first set was based on consumer traffic which includes Internet
   of Things, Social Media, and Corporate access traffic.  The dataset
   of session log entries were over 63K event entries over a 24 hour

   The second dataset was from a telecommunications customer with Proxy
   Offloading.  The log entries were for a period of 24 hours and
   contained over 5M log events entries.  Since this customer is using a
   Symantec Edge Proxy aligned with SSLV.  The session data is for
   explicit clients.  Guest or Internet of things type traffic is a much
   lower percentage of total traffic.  However the existence of
   Mismatched SNIs persisted.

12.2.  Consumer Network Traffic

   For consumer based network traffic Mismatched SNIs were very
   prevalent.  Actually out of new sessions the majority of the sessions
   were with mismatched SNIs as compared to properly matched SNIs.
   These were the result of many many short lived TLS sessions that
   persistently 'phone home'. 22% of all traffic was mismatched as
   compared to only 4% was properly matched.  The rest of the log
   activity was non session related.

   The services that were in the top 20 offenders for mismatched SNIs
   includes Google, Apple, Adware, IoT.  Google DNS dominated the counts
   at 5.3% of all mismatched sessions.  A close second at 4.8% was
   Samsung Smart things.  But, if I add up common services like Google,
   Apple, Adware, and the remainder: 13%, 9.8%, 8%, and 10%

   For matched traffic Amazon Alexa stood tall at 25%, with a runner up
   for Broadcom Cloud Proxy at 7.9%. Both Google and Apple did have a
   minority of properly established sessions.  In other words some of
   their stuff is clean.

12.3.  Corporate Customer Traffic

   Since the traffic was proxy traffic the session hygiene was much
   better.  The majority of new TLS sessions were properly established
   with matching SNIs/SANs.  The vast majority of this traffic was VPN
   based.  Likely masking out consumer like traffic invisible within the
   VPN tunnels.  For instance there was 0.8% traffic
   reported.  But, there was a small percentage ~1.3% Google API traffic
   that was mismatched.  I would have expected more search engine

   Looking across the distribution of domain names for mismatched
   sessions. 29.6% of the traffic was related to Corporate applications.
   These applications could be updated and corrected.  The next runner
   up coming in at 7.4% was for Akamai which also could be updated.
   Office applications and the remainder each individually accounted for
   2.4% of the traffic.

12.4.  Take away observations

   *  IoT and API based traffic is by far the largest offender as
      compared to Browser based initiated sessions.

   *  Long-lived TLS session counts were dwarfed by the chatter of the
      API calls using short lived yet pervasively reporting.  For
      instance there were new sessions at a rate of every 20 seconds per
      IoT device.

   *  The consumer mismatched sessions were all using TLS v1.3.  Which
      reaffirmed the need to decrypt TLS v1.3 traffic.  These sessions,
      if established without TLS interception, may have gone unreported
      by NGFW, which makes policy decisions on SNI vs SAN.  Conversely,
      the corporate traffic was a close split between TLS v1.3 and v1.2.

   *  The presence of VPN tunnels masked a clearer picture of the
      corporate traffic usage.

   *  SNI Mismatches are more prevalent in the wild than first thought.

   *  SNI Mismatches has a side cause which policy rules have to be
      enumerated twice for category matching.  And the second category
      matching is more intensive since it has to enumerate the entire
      SAN list which can be very large.

   *  Fixing the session hygiene for corporate owned applications could
      potentially improve performance of the security stack.

13.  References

13.1.  Normative References

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

13.2.  Informative References

              Spezzati, S., Robinson, M., and L. Beyoud, "Wall Street's
              Record Fines Over WhatsApp Use Were Years in the Making",
              16 August 2022, <

   [CIPA]     FCC, "Children's Internet Protection Act (CIPA)", 30
              December 2019, <

   [Coroner]  Henderson, "Prevention of future deaths report", 26
              November 2021, <

              419 Consulting, "Encrypted Client Hello - Notes from an
              ECH Roundtable", 18 August 2021,

              Paine, K., Whitehouse, O., Sellwood, J., and A. S,
              "Indicators of Compromise (IoCs) and Their Role in Attack
              Defence", Work in Progress, Internet-Draft, draft-ietf-
              opsec-indicators-of-compromise-04, 3 February 2023,

              Cam-Winget, N., Wang, E., Danyliw, R., and R. DuToit,
              "Impact of TLS 1.3 to Operational Network Security
              Practices", Work in Progress, Internet-Draft, draft-ietf-
              opsec-ns-impact-04, 26 January 2021,

              Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-16, 6 April 2023,

              McFadden, M., "BCP72 - A Problem Statement", Work in
              Progress, Internet-Draft, draft-mcfadden-smart-threat-
              changes-04, 22 January 2022,

              Taddei, A., Wueest, C., Roundy, K. A., and D. Lazanski,
              "Capabilities and Limitations of an Endpoint-only Security
              Solution", Work in Progress, Internet-Draft, draft-taddei-
              smart-cless-introduction-03, 13 July 2020,

   [KCSE]     DfE, "Keeping children safe in education 2021", 1 November
              2021, <>.

              Neyret, A. and Autorité des Marchés Financiers, "La
              cybercriminalité boursière – définition, cas et
              perspectives", 10 October 2019, <https://www.amf-

              Deloitte, "Beneath the surface of a cyberattack – A deeper
              look at business impacts", 2016,

              September 2022, <

   [MAGECART] Wikipedia, "Magecart", 3 April 2022,

              Wikipedia, "Malvertising", 2 June 2022,

   [MITB]     OWASP, "Man-in-the-browser attack", n.d.,

              MITRE, "Browser Session Hijacking - T1185", 25 February
              2022, <>.

   [NIST-DID] NIST, "Glossary - defense-in-depth", n.d.,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018,

   [RFC8744]  Huitema, C., "Issues and Requirements for Server Name
              Identification (SNI) Encryption in TLS", RFC 8744,
              DOI 10.17487/RFC8744, July 2020,

   [RFC8890]  Nottingham, M., "The Internet is for End Users", RFC 8890,
              DOI 10.17487/RFC8890, August 2020,

              Symantec, a Division of Broadcom Software Group,
              "SolarWinds (Sunburst) Attack What You Need to Know",
              December 2020, <


   Eric Chien

   Eric contributed to the analysis of the Man in the Browser attacks.

   Gianpaolo Scalone

   Contributed the research on the conflicts of ECH with local
   legislations to block.

   Daniel Engberg
   Skandinaviska Enskilda Banken AB (SEB)

   Validate the issues for his organization.

   Celine Leroy
   Eight Advisory

   Thank you to Céline for her work on cybersecurity financial impacts
   on enterprises.

   Daniel Engberg
   Skandinaviska Enskilda Banken AB (SEB)

   Validate the issues for his organization.

   Gianpiero Tavano

   Review the text, provided feedback and reminded us on the budgetary

   Roelof duToit

   Roelof contributed many things including research, former I-D, text,
   the newly setup github, etc.

   Diego Lopez

   Diego contributed in several aspects including MCPs.

   Gary Tomic

   Gary contributed many things including research, keep us on scope,
   critique for when issues where not impacted by ECH as we initially

   Bob Blair

   Bob contributed to several reviews, many calls, and the whole
   appendix A.

Authors' Addresses

   Andrew Campling
   419 Consulting Limited
   Email: Andrew.Campling@419.Consulting
   URI:   https://www.419.Consulting/

   Paul Vixie
   Red Barn

   David Wright
   UK Safer Internet Centre

   Arnaud Taddei
   1320 Ridder Park Dr
   San Jose, CA 95131
   United States of America
   Phone: 41795061129

   Simon Edwards
   1320 Ridder Park Dr
   San Jose, CA 95131
   United States of America