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Versions: 00                                                            
Transport Layer Security                                        T. Lemon
Internet-Draft                                             Nominum, Inc.
Intended status: Standards Track                            June 6, 2016
Expires: December 8, 2016

                      Blocked Site Alerts for TLS


   Hosts connecting to the Internet should generally be able to connect
   to all available services.  However, as a matter of policy, need or
   preference, some services may be blocked by the network.  TLS
   correctly treats attempts to communicate the reason for such blockage
   to the client as an attack.  This memo describes a safe way for hosts
   to be notified using the TLS alert mechanism that a connection has
   been blocked by the network.

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 December 8, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Meanings of Alert Descriptions  . . . . . . . . . . . . . . .   4
     3.1.  Captive Portal  . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Malicious Site  . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Policy Violation  . . . . . . . . . . . . . . . . . . . .   4
     3.4.  Account Attention Requested . . . . . . . . . . . . . . .   4
     3.5.  Account Attention Required  . . . . . . . . . . . . . . .   4
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   There are a number of situations in which a connection from a
   particular host to a particular service on the Internet may be
   prohibited by policy, or may be blocked in order to redirect the user
   to a captive portal login.  In current practice, such connections,
   particularly HTTP connections, are usually terminated on some sort of
   HTTP proxy that presents a web page notifying the user as to what
   happened, and possibly offering some way to address whatever problem
   has come up.  For instance, with a captive portal, the user may be
   directed to log in.

   Such HTTP proxies are performing what can accurately be described as
   a man-in-the-middle attack.  Whether the purpose is benign or
   malicious, TLS[1] detects such attacks and, rightly, prevents them.

   Unfortunately, TLS's correct behavior in this situation creates a
   usability problem.  There is no way to notify the user as to what
   went wrong.  This is a problem not only with HTTP connections, but
   also with other TLS-based connections, such as secure IMAP
   connections: users of captive portals are generally familiar with the
   phenomenon of having to reset the mail client after logging in to the
   captive portal, because it has concluded that the network is not
   usable as a result of detecting an invalid certificate.

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   One way to address this is to simply tell the user to click through
   the security warning.  This is of course a disastrously bad idea,
   because it trains the end user to automatically permit genuinely
   malicious attacks.

   There is no reasonable basis for trusting a proxy engaging in a MiTM
   attack of this sort.  It would be very unsafe, for example, to
   provide a TLS extension that could be used by the proxy to convey any
   sort of server-generated status message or URL, because these would
   present a valuable attack surface.

   However, the TLS protocol begins as a plaintext communication.  A
   plaintext response to the initial TLS Client Hello message can
   include an Alert response indicating that the connection is not
   permitted.  Alert response codes contain no information generated by
   the server: they simply contain a status code and an indication as to
   whether the alert is fatal or just a warning.

   Because they provide no mechanism for a malicious attacker to trick
   the end user into clicking on a malicious URL, or any way to tell a
   careful lie to the end user, TLS alerts would seem to be a viable
   means of providing the client with sufficient information to present
   a useful error message without compromising the security of the end

   This document defines a set of TLS alert descriptions to indicate
   each of the common reasons why a network service provider might block
   a particular connection.

2.  Applicability

   Alert descriptions defined in this document are intended to be used
   in alert messages marked fatal.  If a server sends an alert using any
   of the codes defined in this document which is marked as a warning,
   the client will detect a MiTM attack once the connection progresses
   to the point where the server certificate can be checked.

   TLS clients receiving any of the alert descriptions documented here
   may present a message in a user interface describing the result code
   that was received.  TLS clients without user interfaces may log a
   message indicating that such an alert was received.  In either case,
   clients should limit the rate at which such messages that are
   presented, to avoid denial of service or resource exhaustion.

   Connections not directly initiated by a user should not result in a
   message being displayed in the user interface (for example, a
   Javascript XMLHttpRequest that generates such an alert).

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   Because a TLS proxy interposed between the host and the server will
   not know the name of the server to which the host is connecting, it
   may need to depend on the Server Name Indication extension [2] to
   provide different status codes for different servers.

3.  Meanings of Alert Descriptions

3.1.  Captive Portal

   The 'captive_portal' alert description represents a claim by the
   server that the host is connected to a network behind a captive
   portal.  This is a curable condition: the end user may be able to
   register with the captive portal, and subsequently a connection to
   the same server would not be intercepted, and could succeed.  TLS
   clients receiving this code may choose to retry the connection
   periodically, frequently enough that authenticating would provide a
   timely resumption of service.

3.2.  Malicious Site

   The 'malicious_site' alert description represents a claim by the
   server that the host has attempted a connection to a service that is
   known by the network administration to serve malicious content (e.g.,
   malware, phishing, etc.).  This condition is assumed to be a
   permanent failure; although it may be that at some future time the
   same IP address is no longer marked malicious, the particular
   transaction that was attempted is not likely to succeed if retried.

3.3.  Policy Violation

   The 'policy_violation' alert description represents a claim by the
   server that the host has attempted to connect to a site the use of
   which is in violation of local policy.  For example, connecting to a
   porn site from an enterprise network might be a policy violation.

3.4.  Account Attention Requested

   The 'account_attention_requested' alert description represents a
   claim by the server that the network service provider is requesting
   that the end user log in to their account.  This is a temporary
   condition, such that an immediate attempt to reconnect can be
   expected to succeed reaching the correct server.

3.5.  Account Attention Required

   The 'account_attention_required' alert description represents a claim
   by the server that the network service provider is insisting that the
   end user log in to their account.  This is not a temporary condition:

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   until whatever situation has motivated the service provider to place
   this block has been resolved, any further attempt to connect will
   result in the same alert description being returned.

4.  Acknowledgements


5.  IANA Considerations

   The IANA is requested to allocate values for the five new TLS Alert
   descriptions documented here from the TLS Alert Registry.  These are:

   TBD  captive_portal

   TBD  malicious_site

   TBD  policy_violation

   TBD  account_attention_requested

   TBD  account_attention_required

6.  Security Considerations

   This document attempts to avoid creating a channel of attack for a
   malicious attacker.  However, any bit of information, no matter how
   small, can be used as a lever to trick the user into taking some
   action which will create an opportunity for attack.

   The situation prior to introduction of these new alert messages is
   that attackers wanting to trick an end user into taking such an
   action can do one of two things: they can simply block the
   connection, which will result in the user trying to figure out what
   went wrong, or they can send an invalid cert and hope that the user
   clicks through the warnings.

   A captive_portal alert might be used by the operating system as a
   means of directing the end user to log in to a captive portal web
   page.  An attacker knowing the expected behavior of the operating
   system could trigger such an attempt.  However, means of triggering
   such attempts already exist, so this introduces no new opportunity.

   A malicious_site alert has no meaningful user mitigation response
   other than to stop trying to visit that site.  An attacker might
   provide such a response as a way to prevent an end user from
   accessing that site in the future.  To avoid this, TLS clients that
   receive such alerts should not cache them.  An end user might still

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   remember that such a warning was received, and might take it more
   seriously than an invalid cert message.  There is no means to
   mitigate this risk; however, the added value of being able to block
   malicious sites likely outweighs the possibility that a malicious
   attacker could succeed in tricking the user in this way.

   A policy_violation alert might encourage the end user to try to find
   some way around the policy.  The alternative is to block the
   connection entirely, however; this would likely trigger similar
   behavior in the end user, so this does not seem to be a substantial
   additional risk.

   The account_attention_requested and account_attention_required alerts
   could be used to trick the end user into going to a faked version of
   their provider's site that is not secured using TLS and PKI.  The
   user could then be tricked into providing authentication credentials
   or other personal information.  Existing browser mitigation for such
   attacks are likely adequate, but it cannot be denied that there is
   some additional risk in presenting these messages to the end user.
   The messages could be presented along with some advice to the end
   user about checking to make sure that the site is secure, or even
   provide the user with a user interface element to click that brings
   them to a browser window that prevents non-TLS/non-PKI connections
   from succeeding.

7.  Privacy Considerations

   To the extent that HTTP proxies using these alert messages rely on
   the Server Name Indication TLS extension [2], there could be a
   concern that the end user's privacy might be violated if the proxy
   logs the SNI information sent in each request.  However, there is no
   way at present to prevent a passive listener from capturing such
   information, so this does not create a new privacy risk.

8.  References

8.1.  Normative References

   [1]        Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

8.2.  Informative References

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

Author's Address

   Ted Lemon
   Nominum, Inc.
   800 Bridge Parkway
   Redwood City, California  94065
   United States of America

   Phone: +1 650 381 6000
   Email: ted.lemon@nominum.com

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