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Summarizing Current Attacks on TLS and DTLS
draft-ietf-uta-tls-attacks-04

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This is an older version of an Internet-Draft that was ultimately published as RFC 7457.
Authors Yaron Sheffer , Ralph Holz , Peter Saint-Andre
Last updated 2014-10-16 (Latest revision 2014-09-28)
Replaces draft-sheffer-uta-tls-attacks
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Send notices to uta-chairs@tools.ietf.org, draft-ietf-uta-tls-attacks@tools.ietf.org, uta@ietf.org
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draft-ietf-uta-tls-attacks-04
uta                                                           Y. Sheffer
Internet-Draft                                                  Porticor
Intended status: Informational                                   R. Holz
Expires: April 1, 2015                                               TUM
                                                          P. Saint-Andre
                                                                    &yet
                                                      September 28, 2014

              Summarizing Current Attacks on TLS and DTLS
                     draft-ietf-uta-tls-attacks-04

Abstract

   Over the last few years there have been several serious attacks on
   TLS, including attacks on its most commonly used ciphers and modes of
   operation.  This document summarizes these attacks, with the goal of
   motivating generic and protocol-specific recommendations on the usage
   of TLS and DTLS.

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 April 1, 2015.

Copyright Notice

   Copyright (c) 2014 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

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Attacks on TLS  . . . . . . . . . . . . . . . . . . . . . . .   3
   2.1.  SSL Stripping . . . . . . . . . . . . . . . . . . . . . . .   3
   2.2.  STARTTLS Command Injection Attack (CVE-2011-0411) . . . . .   3
   2.3.  BEAST (CVE-2011-3389) . . . . . . . . . . . . . . . . . . .   4
   2.4.  Lucky Thirteen (CVE-2013-0169)  . . . . . . . . . . . . . .   4
   2.5.  Attacks on RC4  . . . . . . . . . . . . . . . . . . . . . .   4
   2.6.  Compression Attacks: CRIME, TIME and BREACH . . . . . . . .   4
   2.7.  Certificate Attacks . . . . . . . . . . . . . . . . . . . .   5
   2.8.  Diffie-Hellman Parameters . . . . . . . . . . . . . . . . .   5
   2.9.  Renegotiation (CVE-2009-3555) . . . . . . . . . . . . . . .   5
   2.10. Triple Handshake (CVE-2014-1295)  . . . . . . . . . . . . .   5
   2.11. Virtual Host Confusion  . . . . . . . . . . . . . . . . . .   6
   2.12. Denial of Service . . . . . . . . . . . . . . . . . . . . .   6
   2.13. Implementation Issues . . . . . . . . . . . . . . . . . . .   6
   3.  Applicability to DTLS . . . . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Appendix: Change Log . . . . . . . . . . . . . . . .  10
   A.1.  draft-ietf-uta-tls-attacks-04 . . . . . . . . . . . . . . .  10
   A.2.  draft-ietf-uta-tls-attacks-03 . . . . . . . . . . . . . . .  10
   A.3.  draft-ietf-uta-tls-attacks-02 . . . . . . . . . . . . . . .  11
   A.4.  draft-ietf-uta-tls-attacks-01 . . . . . . . . . . . . . . .  11
   A.5.  draft-ietf-uta-tls-attacks-00 . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Over the last few years there have been several major attacks on TLS
   [RFC5246], including attacks on its most commonly used ciphers and
   modes of operation.  Details are given in Section 2, but suffice it
   to say that both AES-CBC and RC4, which together make up for most
   current usage, have been seriously attacked in the context of TLS.

   This situation was one of the motivations for the creation of the UTA
   working group, which is tasked with the creation of generic and
   protocol-specific recommendations for the use of TLS and DTLS.

   "Attacks always get better; they never get worse" (ironically, this
   saying is attributed to the NSA).  This list of attacks describes our

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   knowledge as of this writing.  It seems likely that new attacks will
   be invented in the future.

   For a more detailed discussion of the attacks listed here, the
   interested reader is referred to [Attacks-iSec].

2.  Attacks on TLS

   This section lists the attacks that motivated the current
   recommendations.  This is not intended to be an extensive survey of
   TLS's security.

   While there are widely deployed mitigations for some of the attacks
   listed below, we believe that their root causes necessitate a more
   systemic solution.

   When such an identifier exists for an attack, we have included its
   CVE (Common Vulnerabilities and Exposures) ID.  CVE [CVE] is an
   extensive, industry-wide database of software vulnerabilities.

2.1.  SSL Stripping

   Various attacks attempt to remove the use of SSL/TLS altogether, by
   modifying unencrypted protocols that request the use of TLS,
   specifically modifying HTTP traffic and HTML pages as they pass on
   the wire.  These attacks are known collectively as SSL Stripping and
   were first introduced by Moxie Marlinspike [SSL-Stripping].  In the
   context of Web traffic, these attacks are only effective if the
   client initially accesses a Web server using HTTP.  A commonly used
   mitigation is HTTP Strict Transport Security (HSTS) [RFC6797].

2.2.  STARTTLS Command Injection Attack (CVE-2011-0411)

   Similarly, there are attacks on the transition between unprotected
   and TLS-protected traffic.  A number of IETF application protocols
   have used an application-level command, usually STARTTLS, to upgrade
   a clear-text connection to use TLS.  Multiple implementations of
   STARTTLS had a flaw where an application-layer input buffer retained
   commands that were pipelined with the STARTTLS command, such that
   commands received prior to TLS negotiation are executed after TLS
   negotiation.  This problem is resolved by requiring the application-
   level command input buffer to be empty before negotiating TLS.  Note
   that this flaw lives in the application layer code and does not
   impact the TLS protocol directly.

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2.3.  BEAST (CVE-2011-3389)

   The BEAST attack [BEAST] uses issues with the TLS 1.0 implementation
   of CBC (that is, the predictable initialization vector) to decrypt
   parts of a packet, and specifically to decrypt HTTP cookies when HTTP
   is run over TLS.

2.4.  Lucky Thirteen (CVE-2013-0169)

   A consequence of the MAC-then-encrypt design in all current versions
   of TLS is the existence of padding oracle attacks [Padding-Oracle].
   A recent incarnation of these attacks is the Lucky Thirteen attack
   [CBC-Attack], a timing side-channel attack that allows the attacker
   to decrypt arbitrary ciphertext.

   The Lucky Thirteen attack can be mitigated by using authenticated
   encryption like AES-GCM [RFC5288] or encrypt-then-mac
   [I-D.ietf-tls-encrypt-then-mac] instead of the TLS default of MAC-
   then-encrypt.

2.5.  Attacks on RC4

   The RC4 algorithm [RC4] has been used with TLS (and previously, SSL)
   for many years.  RC4 has long been known to have a variety of
   cryptographic weaknesses, e.g.  [RC4-Attack-Pau], [RC4-Attack-Man],
   [RC4-Attack-FMS].  Recent cryptanalysis results [RC4-Attack-AlF]
   exploit biases in the RC4 keystream to recover repeatedly encrypted
   plaintexts.

   These recent results are on the verge of becoming practically
   exploitable; currently they require 2^26 sessions or 13x2^30
   encryptions.  As a result, RC4 can no longer be seen as providing a
   sufficient level of security for TLS sessions.  For further details,
   the reader is referred to [I-D.ietf-tls-prohibiting-rc4].

2.6.  Compression Attacks: CRIME, TIME and BREACH

   The CRIME attack [CRIME] (CVE-2012-4929) allows an active attacker to
   decrypt ciphertext (specifically, cookies) when TLS is used with TLS
   level compression.

   The TIME attack [TIME] and the later BREACH attack [BREACH]
   (CVE-2013-3587, though the number has not been officially allocated)
   both make similar use of HTTP-level compression to decrypt secret
   data passed in the HTTP response.  We note that compression of the
   HTTP message body is much more prevalent than compression at the TLS
   level.

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   The former attack can be mitigated by disabling TLS compression.  We
   are not aware of mitigations at the TLS protocol level to the latter
   attack, and so application-level mitigations are needed (see
   [BREACH]).  For example, implementations of HTTP that use CSRF tokens
   will need to randomize them even when the recommendations of
   [I-D.ietf-uta-tls-bcp] are adopted.

2.7.  Certificate Attacks

   There have been several practical attacks on TLS when used with RSA
   certificates (the most common use case).  These include
   [Bleichenbacher98] and [Klima03].  While the Bleichenbacher attack
   has been mitigated in TLS 1.0, the Klima attack that relies on a
   version-check oracle is only mitigated by TLS 1.1.

   The use of RSA certificates often involves exploitable timing issues
   [Brumley03] (CVE-2003-0147), unless the implementation takes care to
   explicitly eliminate them.

   A recent certificate fuzzing tool [Brubaker2014using] uncovered
   numerous vulnerabilities in different TLS libraries, related to
   certificate validation.

2.8.  Diffie-Hellman Parameters

   TLS allows the definition of ephemeral Diffie-Hellman and Elliptic
   Curve Diffie-Hellman parameters in its respective key exchange modes.
   This results in an attack detailed in [Cross-Protocol].  In addition,
   clients that do not properly verify the received parameters are
   exposed to man in the middle (MITM) attacks.  Unfortunately the TLS
   protocol does not require this verification, see [RFC6989] for the
   IPsec analogy.

2.9.  Renegotiation (CVE-2009-3555)

   A major attack on the TLS renegotiation mechanism applies to all
   current versions of the protocol.  The attack and the TLS extension
   that resolves it are described in [RFC5746].

2.10.  Triple Handshake (CVE-2014-1295)

   The triple handshake attack [[TRIPLE-HS, add the reference when
   published]] enables the attacker to cause two TLS connections to
   share keying material.  This leads to a multitude of attacks, e.g.
   Man-in-the-Middle, breaking safe renegotiation and breaking channel
   binding via TLS Exporter [RFC5705] or "tls-unique" [RFC5929].

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2.11.  Virtual Host Confusion

   A recent article [Delignat14] describes a security issue whereby
   SSLv3 fallback and improper handling of session caches on the server
   side can be abused by an attacker to establish a malicious connection
   to a virtual host other than originally intended and approved by the
   server.  This attack is especially serious in performance critical
   environments where sharing of SSLv3 session caches is very common.

2.12.  Denial of Service

   Server CPU power has progressed over the years so that TLS can now be
   turned on by default.  However the risk of malicious clients and
   coordinated groups of clients ("botnets") mounting denial of service
   attacks is still very real.  TLS adds another vector for
   computational attacks, since a client can easily (with little
   computational effort) force the server to expend relatively large
   computational work.  It is known that such attacks have in fact been
   mounted.

2.13.  Implementation Issues

   Even when the protocol is fully specified, there are very common
   issues that often plague implementations.  In particular, when
   integrating into higher-level protocols, TLS and its PKI-based
   authentication are sometimes the source of misunderstandings and
   implementation "shortcuts".  An extensive survey of these issues can
   be found in [Georgiev2012].

   o  Implementations may omit validation of the server certificate
      altogether.  For example, this is true of the default
      implementation of HTTP client libraries in Python 2 (see e.g.
      CVE-2013-2191).

   o  Implementations may not validate the server identity.  This
      validation typically amounts to matching the protocol-level server
      name with the certificate's Subject Alternative Name field.  Note:
      historically, although incorrect, this information is also often
      found in the Common Name part of the Distinguished Name instead.

   o  Implementations may be validating the certificate chain
      incorrectly or not at all, or using an incorrect or outdated trust
      anchor list.

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3.  Applicability to DTLS

   DTLS [RFC4347] [RFC6347] is an adaptation of TLS for UDP datagrams.

   With respect to the attacks described in the current document, DTLS
   1.0 is equivalent to TLS 1.1.  The only exception is RC4 which is
   disallowed in DTLS.  DTLS 1.2 is equivalent to TLS 1.2.

4.  Security Considerations

   This document describes protocol attacks in an informational manner,
   and in itself does not have any security implications.  Its companion
   documents certainly do.

5.  IANA Considerations

   This document requires no IANA actions.  [Note to RFC Editor: please
   remove this whole section before publication.]

6.  Acknowledgments

   We would like to thank Stephen Farrell, Simon Josefsson, John
   Mattsson, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom Ritter and
   Rich Salz for their review of this document.  We thank Andrei Popov
   for contributing text on RC4, Kohei Kasamatsu for text on Lucky13,
   Ilari Liusvaara for text on attacks and on DTLS, Aaron Zauner for
   text on virtual host confusion, Chris Newman for text on STARTTLS
   command injection.

   The document was prepared using the lyx2rfc tool, created by Nico
   Williams.

7.  Informative References

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

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

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              August 2008.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, March 2010.

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   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, February 2010.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, July 2010.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797, November 2012.

   [RFC6989]  Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
              Tests for the Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 6989, July 2013.

   [I-D.ietf-uta-tls-bcp]
              Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of TLS and DTLS", draft-
              ietf-uta-tls-bcp-01 (work in progress), June 2014.

   [I-D.ietf-tls-prohibiting-rc4]
              Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf-
              tls-prohibiting-rc4-00 (work in progress), July 2014.

   [I-D.ietf-tls-encrypt-then-mac]
              Gutmann, P., "Encrypt-then-MAC for TLS and DTLS", draft-
              ietf-tls-encrypt-then-mac-03 (work in progress), July
              2014.

   [CVE]      MITRE, , "Common Vulnerabilities and Exposures",
              <https://cve.mitre.org/>.

   [CBC-Attack]
              AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
              the TLS and DTLS Record Protocols", IEEE Symposium on
              Security and Privacy , 2013.

   [BEAST]    Rizzo, J. and T. Duong, "Browser Exploit Against SSL/TLS",
              2011, <http://packetstormsecurity.com/files/105499/
              Browser-Exploit-Against-SSL-TLS.html>.

   [CRIME]    Rizzo, J. and T. Duong, "The CRIME Attack", EKOparty
              Security Conference 2012, 2012.

   [BREACH]   Prado, A., Harris, N., and Y. Gluck, "The BREACH Attack",
              2013, <http://breachattack.com/>.

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   [TIME]     Be'ery, T. and A. Shulman, "A Perfect CRIME? Only TIME
              Will Tell", Black Hat Europe 2013, 2013,
              <https://media.blackhat.com/eu-13/briefings/Beery/bh-
              eu-13-a-perfect-crime-beery-wp.pdf>.

   [RC4]      Schneier, B., "Applied Cryptography: Protocols,
              Algorithms, and Source Code in C, 2nd Ed.", 1996.

   [RC4-Attack-FMS]
              Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
              Key Scheduling Algorithm of RC4", Selected Areas in
              Cryptography , 2001.

   [RC4-Attack-AlF]
              AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,
              and J. Schuldt, "On the Security of RC4 in TLS", Usenix
              Security Symposium 2013, 2013, <https://www.usenix.org/
              conference/usenixsecurity13/security-rc4-tls>.

   [Georgiev2012]
              Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
              D., and V. Shmatikov, "The most dangerous code in the
              world: validating SSL certificates in non-browser
              software", 2012,
              <http://doi.acm.org/10.1145/2382196.2382204>.

   [Attacks-iSec]
              Sarkar, P. and S. Fitzgerald, "Attacks on SSL, a
              comprehensive study of BEAST, CRIME, TIME, BREACH, Lucky13
              and RC4 biases", 8 2013, <https://www.isecpartners.com/
              media/106031/ssl_attacks_survey.pdf>.

   [Padding-Oracle]
              Vaudenay, S., "Security Flaws Induced by CBC Padding
              Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002,
              2002, <http://www.iacr.org/cryptodb/archive/2002/
              EUROCRYPT/2850/2850.pdf>.

   [Cross-Protocol]
              Mavrogiannopoulos, N., Vercauteren, F., Velichkov, V., and
              B. Preneel, "A cross-protocol attack on the TLS protocol",
              2012, <http://doi.acm.org/10.1145/2382196.2382206>.

   [RC4-Attack-Pau]
              Paul, G. and S. Maitra, "Permutation after RC4 key
              scheduling reveals the secret key.", 2007,
              <http://dblp.uni-trier.de/db/conf/sacrypt/
              sacrypt2007.html#PaulM07>.

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   [RC4-Attack-Man]
              Mantin, I. and A. Shamir, "A practical attack on broadcast
              RC4", 2001.

   [SSL-Stripping]
              Marlinspike, M., "SSL Stripping", February 2009,
              <http://www.thoughtcrime.org/software/sslstrip/>.

   [Bleichenbacher98]
              Bleichenbacher, D., "Chosen ciphertext attacks against
              protocols based on the RSA encryption standard pkcs1",
              1998.

   [Klima03]  Klima, V., Pokorny, O., and T. Rosa, "Attacking RSA-based
              sessions in SSL/TLS", 2003.

   [Brumley03]
              Brumley, D. and D. Boneh, "Remote timing attacks are
              practical", 2003.

   [Brubaker2014using]
              Brubaker, C., Jana, S., Ray, B., Khurshid, S., and V.
              Shmatikov, "Using frankencerts for automated adversarial
              testing of certificate validation in SSL/TLS
              implementations", 2014.

   [Delignat14]
              Delignat-Lavaud, A. and K. Bhargavan, "Virtual Host
              Confusion: Weaknesses and Exploits", Black Hat 2014, 2014.

Appendix A.  Appendix: Change Log

   Note to RFC Editor: please remove this section before publication.

A.1.  draft-ietf-uta-tls-attacks-04

   o  Implemented AD review comments.

A.2.  draft-ietf-uta-tls-attacks-03

   o  Implemented WG Last Call comments.

   o  Virtual host confusion.

   o  STARTTLS command injection.

   o  Added CVE numbers.

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A.3.  draft-ietf-uta-tls-attacks-02

   o  Added implementation issues ("most dangerous code"),
      renegotiation, triple handshake.

   o  Added text re: mitigation of Lucky13.

   o  Added applicability to DTLS.

A.4.  draft-ietf-uta-tls-attacks-01

   o  Added SSL Stripping, attacks related to certificates, Diffie
      Hellman parameters and denial of service.

   o  Expanded on RC4 attacks, thanks to Andrei Popov.

A.5.  draft-ietf-uta-tls-attacks-00

   o  Initial version, extracted from draft-sheffer-tls-bcp-01.

Authors' Addresses

   Yaron Sheffer
   Porticor
   29 HaHarash St.
   Hod HaSharon  4501303
   Israel

   Email: yaronf.ietf@gmail.com

   Ralph Holz
   Technische Universitaet Muenchen
   Boltzmannstr. 3
   Garching  85748
   Germany

   Email: holz@net.in.tum.de

   Peter Saint-Andre
   &yet
   P.O. Box 787
   Parker, CO  80134
   USA

   Email: peter@andyet.com

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