draft-ietf-uta-tls-attacks-05.txt   rfc7457.txt 
uta Y. Sheffer Internet Engineering Task Force (IETF) Y. Sheffer
Internet-Draft Porticor Request for Comments: 7457 Porticor
Intended status: Informational R. Holz Category: Informational R. Holz
Expires: April 26, 2015 TUM ISSN: 2070-1721 Technische Universitaet Muenchen
P. Saint-Andre P. Saint-Andre
&yet &yet
October 23, 2014 February 2015
Summarizing Known Attacks on TLS and DTLS Summarizing Known Attacks on Transport Layer Security (TLS)
draft-ietf-uta-tls-attacks-05 and Datagram TLS (DTLS)
Abstract Abstract
Over the last few years there have been several serious attacks on Over the last few years, there have been several serious attacks on
Transport Layer Security (TLS), including attacks on its most Transport Layer Security (TLS), including attacks on its most
commonly used ciphers and modes of operation. This document commonly used ciphers and modes of operation. This document
summarizes these attacks, with the goal of motivating generic and summarizes these attacks, with the goal of motivating generic and
protocol-specific recommendations on the usage of TLS and Datagram protocol-specific recommendations on the usage of TLS and Datagram
TLS (DTLS). TLS (DTLS).
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
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 This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
This Internet-Draft will expire on April 26, 2015. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7457.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction ....................................................3
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Attacks on TLS ..................................................3
2. Attacks on TLS . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. SSL Stripping ..............................................3
2.1. SSL Stripping . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. STARTTLS Command Injection Attack (CVE-2011-0411) ..........4
2.2. STARTTLS Command Injection Attack (CVE-2011-0411) . . . . . 3 2.3. BEAST (CVE-2011-3389) ......................................4
2.3. BEAST (CVE-2011-3389) . . . . . . . . . . . . . . . . . . . 4 2.4. Padding Oracle Attacks .....................................4
2.4. Padding Oracle Attacks . . . . . . . . . . . . . . . . . . 4 2.5. Attacks on RC4 .............................................5
2.5. Attacks on RC4 . . . . . . . . . . . . . . . . . . . . . . 4 2.6. Compression Attacks: CRIME, TIME, and BREACH ...............5
2.6. Compression Attacks: CRIME, TIME and BREACH . . . . . . . . 5 2.7. Certificate and RSA-Related Attacks ........................5
2.7. Certificate and RSA-Related Attacks . . . . . . . . . . . . 5 2.8. Theft of RSA Private Keys ..................................6
2.8. Theft of RSA Private Keys . . . . . . . . . . . . . . . . . 6 2.9. Diffie-Hellman Parameters ..................................6
2.9. Diffie-Hellman Parameters . . . . . . . . . . . . . . . . . 6 2.10. Renegotiation (CVE-2009-3555) .............................6
2.10. Renegotiation (CVE-2009-3555) . . . . . . . . . . . . . . . 6 2.11. Triple Handshake (CVE-2014-1295) ..........................6
2.11. Triple Handshake (CVE-2014-1295) . . . . . . . . . . . . . 6 2.12. Virtual Host Confusion ....................................7
2.12. Virtual Host Confusion . . . . . . . . . . . . . . . . . . 7 2.13. Denial of Service .........................................7
2.13. Denial of Service . . . . . . . . . . . . . . . . . . . . . 7 2.14. Implementation Issues .....................................7
2.14. Implementation Issues . . . . . . . . . . . . . . . . . . . 7 2.15. Usability .................................................8
2.15. Usability . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. Applicability to DTLS ...........................................8
3. Applicability to DTLS . . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations .........................................8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. Informative References ..........................................8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 Acknowledgements ..................................................13
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses ................................................13
7. Informative References . . . . . . . . . . . . . . . . . . . 9
Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . 12
A.1. draft-ietf-uta-tls-attacks-05 . . . . . . . . . . . . . . . 13
A.2. draft-ietf-uta-tls-attacks-04 . . . . . . . . . . . . . . . 13
A.3. draft-ietf-uta-tls-attacks-03 . . . . . . . . . . . . . . . 13
A.4. draft-ietf-uta-tls-attacks-02 . . . . . . . . . . . . . . . 13
A.5. draft-ietf-uta-tls-attacks-01 . . . . . . . . . . . . . . . 13
A.6. draft-ietf-uta-tls-attacks-00 . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
Over the last few years there have been several major attacks on Over the last few years, there have been several major attacks on TLS
Transport Layer Security (TLS) [RFC5246], including attacks on its [RFC5246], including attacks on its most commonly used ciphers and
most commonly used ciphers and modes of operation. Details are given modes of operation. Details are given in Section 2, but a quick
in Section 2, but a quick summary is that both AES-CBC and RC4, which summary is that both AES-CBC and RC4, which together make up for most
together make up for most current usage, have been seriously attacked current usage, have been seriously attacked in the context of TLS.
in the context of TLS.
This situation was one of the motivations for the creation of the UTA This situation was one of the motivations for the creation of the UTA
working group, which was tasked with the creation of generic and working group, which was tasked with the creation of generic and
protocol-specific recommendations for the use of TLS along with protocol-specific recommendations for the use of TLS and DTLS
Datagram TLS (DTLS) [RFC6347] (unless otherwise noted under [RFC6347] (unless otherwise noted under Section 3, all of the
Section 3, all of the information provided in this document applies information provided in this document applies to DTLS).
to DTLS).
"Attacks always get better; they never get worse" (ironically, this There is an old saying attributed, ironically enough, to the US
saying is attributed to the U.S. National Security Agency, the NSA). National Security Agency (NSA): "Attacks always get better; they
This attacks summarized in this document reflect our knowledge as of never get worse." Unfortunately, that saying is true, so any
this writing. It seems likely that new attacks will be discovered in description of security attacks can only be a snapshot in time.
the future. Therefore this document reflects our knowledge as of this writing.
It seems likely that new attacks will be discovered in the future.
For a more detailed discussion of the attacks listed here, the For a more detailed discussion of the attacks listed here, the
interested reader is referred to [Attacks-iSec]. interested reader is referred to [Attacks-iSec].
2. Attacks on TLS 2. Attacks on TLS
This section lists the attacks that motivated the current This section lists the attacks that motivated the current
recommendations in [I-D.ietf-uta-tls-bcp]. This list is not intended recommendations in [SECURE-TLS]. This list is not intended to be an
to be an extensive survey of the security of TLS. extensive survey of the security of TLS.
While there are widely deployed mitigations for some of the attacks While there are widely deployed mitigations for some of the attacks
listed below, we believe that their root causes necessitate a more listed below, we believe that their root causes necessitate a more
systematic solution, which we have attempted to develop in systematic solution, which we have attempted to develop in
[I-D.ietf-uta-tls-bcp]. [SECURE-TLS].
When an identifier exists for an attack, we have included its CVE When an identifier exists for an attack, we have included its Common
(Common Vulnerabilities and Exposures) ID. CVE [CVE] is an Vulnerabilities and Exposures (CVE) ID. CVE [CVE] is an extensive,
extensive, industry-wide database of software vulnerabilities. industry-wide database of software vulnerabilities.
2.1. SSL Stripping 2.1. SSL Stripping
Various attacks attempt to remove the use of SSL/TLS altogether, by Various attacks attempt to remove the use of Secure Socket Layer /
modifying unencrypted protocols that request the use of TLS, Transport Layer Security (SSL/TLS) altogether by modifying
specifically modifying HTTP traffic and HTML pages as they pass on unencrypted protocols that request the use of TLS, specifically
the wire. These attacks are known collectively as SSL Stripping (a modifying HTTP traffic and HTML pages as they pass on the wire.
form of the more generic "downgrade attack") and were first These attacks are known collectively as "SSL Stripping" (a form of
introduced by Moxie Marlinspike [SSL-Stripping]. In the context of the more generic "downgrade attack") and were first introduced by
Web traffic, these attacks are only effective if the client initially Moxie Marlinspike [SSL-Stripping]. In the context of Web traffic,
accesses a Web server using HTTP. A commonly used mitigation is HTTP these attacks are only effective if the client initially accesses a
Strict Transport Security (HSTS) [RFC6797]. Web server using HTTP. A commonly used mitigation is HTTP Strict
Transport Security (HSTS) [RFC6797].
2.2. STARTTLS Command Injection Attack (CVE-2011-0411) 2.2. STARTTLS Command Injection Attack (CVE-2011-0411)
Similarly, there are attacks on the transition between unprotected Similarly, there are attacks on the transition between unprotected
and TLS-protected traffic. A number of IETF application protocols and TLS-protected traffic. A number of IETF application protocols
have used an application-level command, usually STARTTLS, to upgrade have used an application-level command, usually STARTTLS, to upgrade
a clear-text connection to use TLS. Multiple implementations of a cleartext connection to use TLS. Multiple implementations of
STARTTLS had a flaw where an application-layer input buffer retained STARTTLS had a flaw where an application-layer input buffer retained
commands that were pipelined with the STARTTLS command, such that commands that were pipelined with the STARTTLS command, such that
commands received prior to TLS negotiation are executed after TLS commands received prior to TLS negotiation are executed after TLS
negotiation. This problem is resolved by requiring the application- negotiation. This problem is resolved by requiring the application-
level command input buffer to be empty before negotiating TLS. Note level command input buffer to be empty before negotiating TLS. Note
that this flaw lives in the application layer code and does not that this flaw lives in the application layer code and does not
impact the TLS protocol directly. impact the TLS protocol directly.
STARTLS and similar mechanisms are vulnerable to downgrade attacks STARTTLS and similar mechanisms are vulnerable to downgrade attacks,
whereby the attacker simply removes the STARTTLS indication from the whereby the attacker simply removes the STARTTLS indication from the
(unprotected) request. This cannot be mitigated unless HSTS-like (unprotected) request. This cannot be mitigated unless HSTS-like
solutions are added. solutions are added.
2.3. BEAST (CVE-2011-3389) 2.3. BEAST (CVE-2011-3389)
The BEAST attack [BEAST] uses issues with the TLS 1.0 implementation The BEAST attack [BEAST] uses issues with the TLS 1.0 implementation
of CBC (that is, the predictable initialization vector) to decrypt of Cipher Block Chaining (CBC) (that is, the predictable
parts of a packet, and specifically to decrypt HTTP cookies when HTTP initialization vector) to decrypt parts of a packet, and specifically
is run over TLS. to decrypt HTTP cookies when HTTP is run over TLS.
2.4. Padding Oracle Attacks 2.4. Padding Oracle Attacks
A consequence of the MAC-then-encrypt design in all current versions A consequence of the MAC-then-encrypt design in all current versions
of TLS is the existence of padding oracle attacks [Padding-Oracle]. of TLS is the existence of padding oracle attacks [Padding-Oracle].
A recent incarnation of these attacks is the Lucky Thirteen attack A recent incarnation of these attacks is the Lucky Thirteen attack
(CVE-2013-0169) [CBC-Attack], a timing side-channel attack that (CVE-2013-0169) [CBC-Attack], a timing side-channel attack that
allows the attacker to decrypt arbitrary ciphertext. allows the attacker to decrypt arbitrary ciphertext.
The Lucky Thirteen attack can be mitigated by using authenticated The Lucky Thirteen attack can be mitigated by using authenticated
encryption like AES-GCM [RFC5288] or encrypt-then-mac encryption like AES-GCM [RFC5288] or encrypt-then-MAC [RFC7366]
[I-D.ietf-tls-encrypt-then-mac] instead of the TLS default of MAC- instead of the TLS default of MAC-then-encrypt.
then-encrypt.
An even newer variant of the padding oracle attack, one that does not An even newer variant of the padding oracle attack, one that does not
use timing information, is the POODLE attack (CVE-2014-3566) [POODLE] use timing information, is the POODLE attack (CVE-2014-3566) [POODLE]
on SSL 3.0. This attack has no known mitigation. on SSL 3.0. This attack has no known mitigation.
2.5. Attacks on RC4 2.5. Attacks on RC4
The RC4 algorithm [RC4] has been used with TLS (and previously, SSL) 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 for many years. RC4 has long been known to have a variety of
cryptographic weaknesses, e.g. [RC4-Attack-Pau], [RC4-Attack-Man], cryptographic weaknesses, e.g., [RC4-Attack-Pau], [RC4-Attack-Man],
[RC4-Attack-FMS]. Recent cryptanalysis results [RC4-Attack-AlF] and [RC4-Attack-FMS]. Recent cryptanalysis results [RC4-Attack-AlF]
exploit biases in the RC4 keystream to recover repeatedly encrypted exploit biases in the RC4 keystream to recover repeatedly encrypted
plaintexts. plaintexts.
These recent results are on the verge of becoming practically These recent results are on the verge of becoming practically
exploitable; currently they require 2^26 sessions or 13x2^30 exploitable; currently they require 2^26 sessions or 13x2^30
encryptions. As a result, RC4 can no longer be seen as providing a encryptions. As a result, RC4 can no longer be seen as providing a
sufficient level of security for TLS sessions. For further details, sufficient level of security for TLS sessions. For further details,
the reader is referred to [I-D.ietf-tls-prohibiting-rc4] and the the reader is referred to [CIPHER-SUITES] and the references it
references it cites. cites.
2.6. Compression Attacks: CRIME, TIME and BREACH 2.6. Compression Attacks: CRIME, TIME, and BREACH
The CRIME attack [CRIME] (CVE-2012-4929) allows an active attacker to The CRIME attack [CRIME] (CVE-2012-4929) allows an active attacker to
decrypt ciphertext (specifically, cookies) when TLS is used with TLS decrypt ciphertext (specifically, cookies) when TLS is used with TLS-
level compression. level compression.
The TIME attack [TIME] and the later BREACH attack [BREACH] (CVE- The TIME attack [TIME] and the later BREACH attack [BREACH] (CVE-
2013-3587, though the number has not been officially allocated) both 2013-3587, though the number has not been officially allocated) both
make similar use of HTTP-level compression to decrypt secret data make similar use of HTTP-level compression to decrypt secret data
passed in the HTTP response. We note that compression of the HTTP passed in the HTTP response. We note that compression of the HTTP
message body is much more prevalent than compression at the TLS message body is much more prevalent than compression at the TLS
level. level.
The former attack can be mitigated by disabling TLS compression. We The TIME attack can be mitigated by disabling TLS compression. We
are not aware of mitigations at the TLS protocol level to the latter are not aware of mitigations at the TLS protocol level to the BREACH
attack, and so application-level mitigations are needed (see attack, and so application-level mitigations are needed (see
[BREACH]). For example, implementations of HTTP that use CSRF tokens [BREACH]). For example, implementations of HTTP that use Cross-Site
will need to randomize them. Even the best practices and Request Forgery (CSRF) tokens will need to randomize them. Even the
recommendations from [I-D.ietf-uta-tls-bcp] are insufficient to best practices and recommendations from [SECURE-TLS] are insufficient
thwart this attack. to thwart this attack.
2.7. Certificate and RSA-Related Attacks 2.7. Certificate and RSA-Related Attacks
There have been several practical attacks on TLS when used with RSA There have been several practical attacks on TLS when used with RSA
certificates (the most common use case). These include certificates (the most common use case). These include
[Bleichenbacher98] and [Klima03]. While the Bleichenbacher attack [Bleichenbacher98] and [Klima03]. While the Bleichenbacher attack
has been mitigated in TLS 1.0, the Klima attack relies on a version- has been mitigated in TLS 1.0, the Klima attack, which relies on a
check oracle is only mitigated by TLS 1.1. version-check oracle, is only mitigated by TLS 1.1.
The use of RSA certificates often involves exploitable timing issues The use of RSA certificates often involves exploitable timing issues
[Brumley03] (CVE-2003-0147), unless the implementation takes care to [Brumley03] (CVE-2003-0147), unless the implementation takes care to
explicitly eliminate them. explicitly eliminate them.
A recent certificate fuzzing tool [Brubaker2014using] uncovered A recent certificate fuzzing tool [Brubaker2014using] uncovered
numerous vulnerabilities in different TLS libraries, related to numerous vulnerabilities in different TLS libraries related to
certificate validation. certificate validation.
2.8. Theft of RSA Private Keys 2.8. Theft of RSA Private Keys
When TLS is used with most non-Diffie Hellman cipher suites, it is When TLS is used with most non-Diffie-Hellman cipher suites, it is
sufficient to obtain the server's private key in order to decrypt any sufficient to obtain the server's private key in order to decrypt any
sessions (past and future) that were initiated with that server. sessions (past and future) that were initiated with that server.
This technique is used, for example, by the popular Wireshark network This technique is used, for example, by the popular Wireshark network
sniffer to inspect TLS-protected connections. sniffer to inspect TLS-protected connections.
It is known that stolen (or otherwise obtained) private keys have It is known that stolen (or otherwise obtained) private keys have
been used as part of large-scale monitoring [RFC7258] of certain been used as part of large-scale monitoring [RFC7258] of certain
servers. servers.
Such attacks can be mitigated by better protecting the private key, Such attacks can be mitigated by better protecting the private key,
e.g. using OS protections or dedicated hardware. Even more effective e.g., using OS protections or dedicated hardware. Even more
is the use of cipher suites that offer "forward secrecy", the effective is the use of cipher suites that offer "forward secrecy",
property that revealing a secret such as a private key does not the property where revealing a secret such as a private key does not
expose past or future sessions to a passive attacker. expose past or future sessions to a passive attacker.
2.9. Diffie-Hellman Parameters 2.9. Diffie-Hellman Parameters
TLS allows the definition of ephemeral Diffie-Hellman and Elliptic TLS allows the definition of ephemeral Diffie-Hellman (DH) and
Curve Diffie-Hellman parameters in its respective key exchange modes. Elliptic Curve Diffie-Hellman parameters in its respective key
This results in an attack detailed in [Cross-Protocol]. Using exchange modes. This results in an attack detailed in
predefined DH groups, as proposed in [Cross-Protocol]. Using predefined DH groups, as proposed in
[I-D.ietf-tls-negotiated-ff-dhe], would mitigate this attack. [FFDHE-TLS], would mitigate this attack.
In addition, clients that do not properly verify the received In addition, clients that do not properly verify the received
parameters are exposed to man in the middle (MITM) attacks. parameters are exposed to man-in-the-middle (MITM) attacks.
Unfortunately the TLS protocol does not mandate this verification Unfortunately, the TLS protocol does not mandate this verification
(see [RFC6989] for analogous information for IPsec). (see [RFC6989] for analogous information for IPsec).
2.10. Renegotiation (CVE-2009-3555) 2.10. Renegotiation (CVE-2009-3555)
A major attack on the TLS renegotiation mechanism applies to all A major attack on the TLS renegotiation mechanism applies to all
current versions of the protocol. The attack and the TLS extension current versions of the protocol. The attack and the TLS extension
that resolves it are described in [RFC5746]. that resolves it are described in [RFC5746].
2.11. Triple Handshake (CVE-2014-1295) 2.11. Triple Handshake (CVE-2014-1295)
The triple handshake attack [BhargavanDFPS14] enables the attacker to The triple handshake attack [BhargavanDFPS14] enables the attacker to
cause two TLS connections to share keying material. This leads to a cause two TLS connections to share keying material. This leads to a
multitude of attacks, e.g. Man-in-the-Middle, breaking safe multitude of attacks, e.g., man-in-the-middle, breaking safe
renegotiation, and breaking channel binding via TLS Exporter renegotiation, and breaking channel binding via TLS Exporter
[RFC5705] or "tls-unique" [RFC5929]. [RFC5705] or "tls-unique" [RFC5929].
2.12. Virtual Host Confusion 2.12. Virtual Host Confusion
A recent article [Delignat14] describes a security issue whereby A recent article [Delignat14] describes a security issue whereby
SSLv3 fallback and improper handling of session caches on the server SSLv3 fallback and improper handling of session caches on the server
side can be abused by an attacker to establish a malicious connection side can be abused by an attacker to establish a malicious connection
to a virtual host other than the one originally intended and approved to a virtual host other than the one originally intended and approved
by the server. This attack is especially serious in performance by the server. This attack is especially serious in performance
critical environments where sharing of SSLv3 session caches is very critical environments where sharing of SSLv3 session caches is very
common. common.
2.13. Denial of Service 2.13. Denial of Service
Server CPU power has progressed over the years so that TLS can now be 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 turned on by default. However, the risk of malicious clients and
coordinated groups of clients ("botnets") mounting denial of service coordinated groups of clients ("botnets") mounting denial-of-service
attacks is still very real. TLS adds another vector for attacks is still very real. TLS adds another vector for
computational attacks, since a client can easily (with little computational attacks, since a client can easily (with little
computational effort) force the server to expend relatively large computational effort) force the server to expend relatively large
computational work. It is known that such attacks have in fact been computational work. It is known that such attacks have in fact been
mounted. mounted.
2.14. Implementation Issues 2.14. Implementation Issues
Even when the protocol is properly specified, this does not guarantee Even when the protocol is properly specified, this does not guarantee
the security of implementations. In fact there are very common the security of implementations. In fact, there are very common
issues that often plague TLS implementations. In particular, when issues that often plague TLS implementations. In particular, when
integrating into higher-level protocols, TLS and its PKI-based integrating into higher-level protocols, TLS and its PKI-based
authentication are sometimes the source of misunderstandings and authentication are sometimes the source of misunderstandings and
implementation "shortcuts". An extensive survey of these issues can implementation "shortcuts". An extensive survey of these issues can
be found in [Georgiev2012]. be found in [Georgiev2012].
o Implementations might omit validation of the server certificate o Implementations might omit validation of the server certificate
altogether. For example, this is true of the default altogether. For example, this is true of the default
implementation of HTTP client libraries in Python 2 (see e.g. implementation of HTTP client libraries in Python 2 (e.g., CVE-
CVE-2013-2191). 2013-2191).
o Implementations might not validate the server identity. This o Implementations might not validate the server identity. This
validation typically amounts to matching the protocol-level server validation typically amounts to matching the protocol-level server
name with the certificate's Subject Alternative Name field. Note: name with the certificate's Subject Alternative Name field. Note:
this same information is often also found in the Common Name part this same information is often also found in the Common Name part
of the Distinguished Name, and some validators incorrectly of the Distinguished Name, and some validators incorrectly
retrieve it from there instead of from the Subject Alternative retrieve it from there instead of from the Subject Alternative
Name. Name.
o Implementations might validate the certificate chain incorrectly o Implementations might validate the certificate chain incorrectly
skipping to change at page 8, line 14 skipping to change at page 8, line 14
An implementation attack of a different kind, one that exploits a An implementation attack of a different kind, one that exploits a
simple coding mistake (bounds check), is the Heartbleed attack (CVE- simple coding mistake (bounds check), is the Heartbleed attack (CVE-
2014-0160) that affected a wide swath of the Internet when it was 2014-0160) that affected a wide swath of the Internet when it was
discovered in April 2014. discovered in April 2014.
2.15. Usability 2.15. Usability
Many TLS endpoints, such as browsers and mail clients, allow the user Many TLS endpoints, such as browsers and mail clients, allow the user
to explicitly accept an invalid server certificate. This often takes to explicitly accept an invalid server certificate. This often takes
the form of a UI dialog (e.g., "do you accept this server?") and the form of a UI dialog (e.g., "do you accept this server?"), and
users have been conditioned to respond in the affirmative in order to users have been conditioned to respond in the affirmative in order to
allow the connection to take place. allow the connection to take place.
This user behavior is used by (arguably legitimate) "SSL proxies" This user behavior is used by (arguably legitimate) "SSL proxies"
that decrypt and re-encrypt the TLS connection in order to enforce that decrypt and re-encrypt the TLS connection in order to enforce
local security policy. It is also abused by attackers whose goal is local security policy. It is also abused by attackers whose goal is
to gain access to the encrypted information. to gain access to the encrypted information.
Mitigation is complex and will probably involve a combination of Mitigation is complex and will probably involve a combination of
protocol mechanisms (HSTS, certificate pinning protocol mechanisms (HSTS, certificate pinning [KEY-PINNING]), and
[I-D.ietf-websec-key-pinning]) and very careful UI design. very careful UI design.
3. Applicability to DTLS 3. Applicability to DTLS
DTLS [RFC4347] [RFC6347] is an adaptation of TLS for UDP. DTLS [RFC4347] [RFC6347] is an adaptation of TLS for UDP.
With respect to the attacks described in the current document, DTLS 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 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. disallowed in DTLS. DTLS 1.2 is equivalent to TLS 1.2.
4. IANA Considerations 4. Security Considerations
This document requires no IANA actions. [Note to RFC Editor: please
remove this whole section before publication.]
5. Security Considerations
This document describes protocol attacks in an informational manner, This document describes protocol attacks in an informational manner
and in itself does not have any security implications. Its companion and in itself does not have any security implications. Its companion
documents, especially [I-D.ietf-uta-tls-bcp], certainly do. documents, especially [SECURE-TLS], certainly do.
6. Acknowledgments
We would like to thank Stephen Farrell, Simon Josefsson, John
Mattsson, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom Ritter,
Rich Salz and Meral Shirazipour for their feedback on 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, and Chris Newman for
text on STARTTLS command injection.
During IESG review, Richard Barnes, Barry Leiba, and Kathleen
Moriarty caught several issues that needed to be addressed.
The authors gratefully acknowledge the assistance of Leif Johansson
and Orit Levin as the working group chairs and Pete Resnick as the
sponsoring Area Director.
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.
[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 5. Informative References
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an [Attacks-iSec]
Attack", BCP 188, RFC 7258, May 2014. Sarkar, P. and S. Fitzgerald, "Attacks on SSL, a
comprehensive study of BEAST, CRIME, TIME, BREACH, Lucky13
and RC4 biases", August 2013,
<https://www.isecpartners.com/media/106031/
ssl_attacks_survey.pdf>.
[I-D.ietf-uta-tls-bcp] [BEAST] Rizzo, J. and T. Duong, "Browser Exploit Against SSL/TLS",
Sheffer, Y., Holz, R., and P. Saint-Andre, 2011, <http://packetstormsecurity.com/files/105499/
"Recommendations for Secure Use of TLS and DTLS", draft- Browser-Exploit-Against-SSL-TLS.html>.
ietf-uta-tls-bcp-05 (work in progress), October 2014.
[I-D.ietf-tls-prohibiting-rc4] [BREACH] Prado, A., Harris, N., and Y. Gluck, "The BREACH Attack",
Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf- 2013, <http://breachattack.com/>.
tls-prohibiting-rc4-00 (work in progress), July 2014.
[I-D.ietf-tls-encrypt-then-mac] [BhargavanDFPS14]
Gutmann, P., "Encrypt-then-MAC for TLS and DTLS", draft- Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
ietf-tls-encrypt-then-mac-03 (work in progress), July A., and P. Strub, "Triple handshakes and cookie cutters:
2014. breaking and fixing authentication over tls", 2014,
<https://secure-resumption.com/tlsauth.pdf>.
[I-D.ietf-tls-negotiated-ff-dhe] [Bleichenbacher98]
Gillmor, D., "Negotiated Finite Field Diffie-Hellman Bleichenbacher, D., "Chosen Ciphertext Attacks Against
Ephemeral Parameters for TLS", draft-ietf-tls-negotiated- Protocols Based on the RSA Encryption Standard PKCS #1",
ff-dhe-02 (work in progress), October 2014. 1998, <http://archiv.infsec.ethz.ch/education/fs08/secsem/
Bleichenbacher98.pdf>.
[I-D.ietf-websec-key-pinning] [Brubaker2014using]
Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning Brubaker, C., Jana, S., Ray, B., Khurshid, S., and V.
Extension for HTTP", draft-ietf-websec-key-pinning-21 Shmatikov, "Using Frankencerts for Automated Adversarial
(work in progress), October 2014. Testing of Certificate Validation in SSL/TLS
Implementations", 2014,
<https://www.cs.utexas.edu/~shmat/shmat_oak14.pdf>.
[CVE] MITRE, , "Common Vulnerabilities and Exposures", [Brumley03]
<https://cve.mitre.org/>. Brumley, D. and D. Boneh, "Remote Timing Attacks are
Practical", 2003,
<http://crypto.stanford.edu/~dabo/papers/ssl-timing.pdf>.
[CBC-Attack] [CBC-Attack]
AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
the TLS and DTLS Record Protocols", IEEE Symposium on the TLS and DTLS Record Protocols", IEEE Symposium on
Security and Privacy , 2013. Security and Privacy, 2013, <http://www.ieee-security.org/
TC/SP2013/papers/4977a526.pdf>.
[BEAST] Rizzo, J. and T. Duong, "Browser Exploit Against SSL/TLS",
2011, <http://packetstormsecurity.com/files/105499/
Browser-Exploit-Against-SSL-TLS.html>.
[POODLE] Moeller, B., Duong, T., and K. Kotowicz, "This POODLE [CIPHER-SUITES]
Bites:Exploiting the SSL 3.0 Fallback", 2014, Popov, A., "Prohibiting RC4 Cipher Suites", Work in
<https://www.openssl.org/~bodo/ssl-poodle.pdf>. Progress, draft-ietf-tls-prohibiting-rc4-01, October 2014.
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", EKOparty [CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", EKOparty
Security Conference 2012, 2012. Security Conference, 2012.
[BREACH] Prado, A., Harris, N., and Y. Gluck, "The BREACH Attack",
2013, <http://breachattack.com/>.
[TIME] Be'ery, T. and A. Shulman, "A Perfect CRIME? Only TIME [CVE] MITRE, "Common Vulnerabilities and Exposures",
Will Tell", Black Hat Europe 2013, 2013, <https://cve.mitre.org/>.
<https://media.blackhat.com/eu-13/briefings/Beery/bh-eu-
13-a-perfect-crime-beery-wp.pdf>.
[RC4] Schneier, B., "Applied Cryptography: Protocols, [Cross-Protocol]
Algorithms, and Source Code in C, 2nd Ed.", 1996. Mavrogiannopoulos, N., Vercauteren, F., Velichkov, V., and
B. Preneel, "A cross-protocol attack on the TLS protocol",
Proceedings of the 2012 ACM Conference in Computer and
Communications Security, pages 62-72, 2012,
<http://doi.acm.org/10.1145/2382196.2382206>.
[RC4-Attack-FMS] [Delignat14]
Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the Delignat-Lavaud, A. and K. Bhargavan, "Virtual Host
Key Scheduling Algorithm of RC4", Selected Areas in Confusion: Weaknesses and Exploits", Black Hat 2014, 2014,
Cryptography , 2001. <https://bh.ht.vc/vhost_confusion.pdf>.
[RC4-Attack-AlF] [FFDHE-TLS]
AlFardan, N., Bernstein, D., Paterson, K., Poettering, B., Gillmor, D., "Negotiated Finite Field Diffie-Hellman
and J. Schuldt, "On the Security of RC4 in TLS", Usenix Ephemeral Parameters for TLS", Work in Progress,
Security Symposium 2013, 2013, draft-ietf-tls-negotiated-ff-dhe-05, December 2014.
<https://www.usenix.org/conference/usenixsecurity13/
security-rc4-tls>.
[Georgiev2012] [Georgiev2012]
Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh, Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
D., and V. Shmatikov, "The most dangerous code in the D., and V. Shmatikov, "The most dangerous code in the
world: validating SSL certificates in non-browser world: validating SSL certificates in non-browser
software", 2012, software", Proceedings of the 2012 ACM conference on
Computer and Communications Security, pages 38-49, 2012,
<http://doi.acm.org/10.1145/2382196.2382204>. <http://doi.acm.org/10.1145/2382196.2382204>.
[Attacks-iSec] [KEY-PINNING]
Sarkar, P. and S. Fitzgerald, "Attacks on SSL, a Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
comprehensive study of BEAST, CRIME, TIME, BREACH, Lucky13 Extension for HTTP", Work in Progress,
and RC4 biases", 8 2013, draft-ietf-websec-key-pinning-21, October 2014.
<https://www.isecpartners.com/media/106031/
ssl_attacks_survey.pdf>. [Klima03] Klima, V., Pokorny, O., and T. Rosa, "Attacking RSA-based
Sessions in SSL/TLS", 2003,
<https://eprint.iacr.org/2003/052.pdf>.
[POODLE] Moeller, B., Duong, T., and K. Kotowicz, "This POODLE
Bites: Exploiting the SSL 3.0 Fallback", September 2014,
<https://www.openssl.org/~bodo/ssl-poodle.pdf>.
[Padding-Oracle] [Padding-Oracle]
Vaudenay, S., "Security Flaws Induced by CBC Padding Vaudenay, S., "Security Flaws Induced by CBC Padding
Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002, Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002,
2002, <http://www.iacr.org/cryptodb/archive/2002/ 2002, <http://www.iacr.org/cryptodb/archive/2002/
EUROCRYPT/2850/2850.pdf>. EUROCRYPT/2850/2850.pdf>.
[Cross-Protocol] [RC4] Schneier, B., "Applied Cryptography: Protocols,
Mavrogiannopoulos, N., Vercauteren, F., Velichkov, V., and Algorithms, and Source Code in C", Second Edition, October
B. Preneel, "A cross-protocol attack on the TLS protocol", 1996.
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>.
[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] [RC4-Attack-AlF]
Brubaker, C., Jana, S., Ray, B., Khurshid, S., and V. AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,
Shmatikov, "Using frankencerts for automated adversarial and J. Schuldt, "On the Security of RC4 in TLS", Usenix
testing of certificate validation in SSL/TLS Security Symposium 2013, August 2013,
implementations", 2014. <https://www.usenix.org/conference/usenixsecurity13/
security-rc4-tls>.
[Delignat14] [RC4-Attack-FMS]
Delignat-Lavaud, A. and K. Bhargavan, "Virtual Host Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
Confusion: Weaknesses and Exploits", Black Hat 2014, 2014. Key Scheduling Algorithm of RC4", Selected Areas in
Cryptography, August 2001,
<http://www.crypto.com/papers/others/rc4_ksaproc.pdf>.
[BhargavanDFPS14] [RC4-Attack-Man]
Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti, Mantin, I. and A. Shamir, "A Practical Attack on Broadcast
A., and P. Strub, "Triple handshakes and cookie cutters: RC4", April 2001,
breaking and fixing authentication over tls", 2014, <http://saluc.engr.uconn.edu/refs/stream_cipher/
<http://prosecco.gforge.inria.fr/pubs/ mantin01attackRC4.pdf>.
triple-handshakes-and-cookie-cutters-sp14.pdf>.
Appendix A. Appendix: Change Log [RC4-Attack-Pau]
Paul, G. and S. Maitra, "Permutation After RC4 Key
Scheduling Reveals the Secret Key", August 2007,
<http://dblp.uni-trier.de/db/conf/sacrypt/
sacrypt2007.html#PaulM07>.
Note to RFC Editor: please remove this section before publication. [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006,
<http://www.rfc-editor.org/info/rfc4347>.
A.1. draft-ietf-uta-tls-attacks-05 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
o Implemented Gen-ART and IESG reviews. [RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008, <http://www.rfc-editor.org/info/rfc5288>.
A.2. draft-ietf-uta-tls-attacks-04 [RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, March 2010,
<http://www.rfc-editor.org/info/rfc5705>.
o Implemented AD review comments. [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>.
A.3. draft-ietf-uta-tls-attacks-03 [RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010,
<http://www.rfc-editor.org/info/rfc5929>.
o Implemented WG Last Call comments. [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012,
<http://www.rfc-editor.org/info/rfc6347>.
o Virtual host confusion. [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012,
<http://www.rfc-editor.org/info/rfc6797>.
o STARTTLS command injection. [RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013,
<http://www.rfc-editor.org/info/rfc6989>.
o Added CVE numbers. [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014,
<http://www.rfc-editor.org/info/rfc7258>.
A.4. draft-ietf-uta-tls-attacks-02 [RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, September 2014,
<http://www.rfc-editor.org/info/rfc7366>.
o Added implementation issues ("most dangerous code"), [SECURE-TLS]
renegotiation, triple handshake. Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of TLS and DTLS", Work in
Progress, draft-ietf-uta-tls-bcp-08, December 2014.
o Added text re: mitigation of Lucky13. [SSL-Stripping]
Marlinspike, M., "sslstrip", February 2009,
<http://www.thoughtcrime.org/software/sslstrip/>.
o Added applicability to DTLS. [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>.
A.5. draft-ietf-uta-tls-attacks-01 Acknowledgements
o Added SSL Stripping, attacks related to certificates, Diffie We would like to thank Stephen Farrell, Simon Josefsson, John
Hellman parameters and denial of service. Mattsson, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom Ritter,
Rich Salz, and Meral Shirazipour for their feedback on 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, and Chris Newman for
text on STARTTLS command injection. Ralph Holz gratefully
acknowledges the support of NICTA (National ICT of Australia) in the
preparation of this document.
o Expanded on RC4 attacks, thanks to Andrei Popov. During IESG review, Richard Barnes, Barry Leiba, and Kathleen
Moriarty caught several issues that needed to be addressed.
A.6. draft-ietf-uta-tls-attacks-00 The authors gratefully acknowledge the assistance of Leif Johansson
and Orit Levin as the working group chairs and Pete Resnick as the
sponsoring Area Director.
o Initial version, extracted from draft-sheffer-tls-bcp-01. The document was prepared using the lyx2rfc tool, created by Nico
Williams.
Authors' Addresses Authors' Addresses
Yaron Sheffer Yaron Sheffer
Porticor Porticor
29 HaHarash St. 29 HaHarash St.
Hod HaSharon 4501303 Hod HaSharon 4501303
Israel Israel
Email: yaronf.ietf@gmail.com EMail: yaronf.ietf@gmail.com
Ralph Holz Ralph Holz
Technische Universitaet Muenchen Technische Universitaet Muenchen
Boltzmannstr. 3 Boltzmannstr. 3
Garching 85748 Garching 85748
Germany Germany
Email: holz@net.in.tum.de EMail: holz@net.in.tum.de
Peter Saint-Andre Peter Saint-Andre
&yet &yet
Email: peter@andyet.com EMail: peter@andyet.com
URI: https://andyet.com/ URI: https://andyet.com/
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