|Title||Summarizing Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)
|Author||Y. Sheffer, R. Holz, P. Saint-Andre
Internet Engineering Task Force (IETF) Y. Sheffer
Request for Comments: 7457 Porticor
Category: Informational R. Holz
ISSN: 2070-1721 Technische Universitaet Muenchen
Summarizing Known Attacks on Transport Layer Security (TLS)
and Datagram TLS (DTLS)
Over the last few years, there have been several serious attacks on
Transport Layer Security (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 Datagram
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
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.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction ....................................................3
2. Attacks on TLS ..................................................3
2.1. SSL Stripping ..............................................3
2.2. STARTTLS Command Injection Attack (CVE-2011-0411) ..........4
2.3. BEAST (CVE-2011-3389) ......................................4
2.4. Padding Oracle Attacks .....................................4
2.5. Attacks on RC4 .............................................5
2.6. Compression Attacks: CRIME, TIME, and BREACH ...............5
2.7. Certificate and RSA-Related Attacks ........................5
2.8. Theft of RSA Private Keys ..................................6
2.9. Diffie-Hellman Parameters ..................................6
2.10. Renegotiation (CVE-2009-3555) .............................6
2.11. Triple Handshake (CVE-2014-1295) ..........................6
2.12. Virtual Host Confusion ....................................7
2.13. Denial of Service .........................................7
2.14. Implementation Issues .....................................7
2.15. Usability .................................................8
3. Applicability to DTLS ...........................................8
4. Security Considerations .........................................8
5. Informative References ..........................................8
Authors' Addresses ................................................13
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 a quick
summary is 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 was tasked with the creation of generic and
protocol-specific recommendations for the use of TLS and DTLS
[RFC6347] (unless otherwise noted under Section 3, all of the
information provided in this document applies to DTLS).
There is an old saying attributed, ironically enough, to the US
National Security Agency (NSA): "Attacks always get better; they
never get worse." Unfortunately, that saying is true, so any
description of security attacks can only be a snapshot in time.
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
interested reader is referred to [Attacks-iSec].
2. Attacks on TLS
This section lists the attacks that motivated the current
recommendations in [SECURE-TLS]. This list is not intended to be an
extensive survey of the security of TLS.
While there are widely deployed mitigations for some of the attacks
listed below, we believe that their root causes necessitate a more
systematic solution, which we have attempted to develop in
When an identifier exists for an attack, we have included its Common
Vulnerabilities and Exposures (CVE) ID. CVE [CVE] is an extensive,
industry-wide database of software vulnerabilities.
2.1. SSL Stripping
Various attacks attempt to remove the use of Secure Socket Layer /
Transport Layer Security (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" (a form of
the more generic "downgrade attack") 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 cleartext 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.
STARTTLS and similar mechanisms are vulnerable to downgrade attacks,
whereby the attacker simply removes the STARTTLS indication from the
(unprotected) request. This cannot be mitigated unless HSTS-like
solutions are added.
2.3. BEAST (CVE-2011-3389)
The BEAST attack [BEAST] uses issues with the TLS 1.0 implementation
of Cipher Block Chaining (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. Padding Oracle Attacks
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
(CVE-2013-0169) [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 [RFC7366]
instead of the TLS default of MAC-then-encrypt.
An even newer variant of the padding oracle attack, one that does not
use timing information, is the POODLE attack (CVE-2014-3566) [POODLE]
on SSL 3.0. This attack has no known mitigation.
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],
and [RC4-Attack-FMS]. Recent cryptanalysis results [RC4-Attack-AlF]
exploit biases in the RC4 keystream to recover repeatedly encrypted
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 [CIPHER-SUITES] and the references it
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-
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
The TIME attack can be mitigated by disabling TLS compression. We
are not aware of mitigations at the TLS protocol level to the BREACH
attack, and so application-level mitigations are needed (see
[BREACH]). For example, implementations of HTTP that use Cross-Site
Request Forgery (CSRF) tokens will need to randomize them. Even the
best practices and recommendations from [SECURE-TLS] are insufficient
to thwart this attack.
2.7. Certificate and RSA-Related 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, which 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
2.8. Theft of RSA Private Keys
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
sessions (past and future) that were initiated with that server.
This technique is used, for example, by the popular Wireshark network
sniffer to inspect TLS-protected connections.
It is known that stolen (or otherwise obtained) private keys have
been used as part of large-scale monitoring [RFC7258] of certain
Such attacks can be mitigated by better protecting the private key,
e.g., using OS protections or dedicated hardware. Even more
effective is the use of cipher suites that offer "forward secrecy",
the property where revealing a secret such as a private key does not
expose past or future sessions to a passive attacker.
2.9. Diffie-Hellman Parameters
TLS allows the definition of ephemeral Diffie-Hellman (DH) and
Elliptic Curve Diffie-Hellman parameters in its respective key
exchange modes. This results in an attack detailed in
[Cross-Protocol]. Using predefined DH groups, as proposed in
[FFDHE-TLS], would mitigate this attack.
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 mandate this verification
(see [RFC6989] for analogous information for IPsec).
2.10. 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].
2.11. Triple Handshake (CVE-2014-1295)
The triple handshake attack [BhargavanDFPS14] 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].
2.12. 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 the one originally intended and approved
by the server. This attack is especially serious in performance
critical environments where sharing of SSLv3 session caches is very
2.13. 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
2.14. Implementation Issues
Even when the protocol is properly specified, this does not guarantee
the security of implementations. In fact, there are very common
issues that often plague TLS 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 might omit validation of the server certificate
altogether. For example, this is true of the default
implementation of HTTP client libraries in Python 2 (e.g., CVE-
o Implementations might 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:
this same information is often also found in the Common Name part
of the Distinguished Name, and some validators incorrectly
retrieve it from there instead of from the Subject Alternative
o Implementations might validate the certificate chain incorrectly
or not at all, or use an incorrect or outdated trust anchor list.
An implementation attack of a different kind, one that exploits a
simple coding mistake (bounds check), is the Heartbleed attack (CVE-
2014-0160) that affected a wide swath of the Internet when it was
discovered in April 2014.
Many TLS endpoints, such as browsers and mail clients, allow the user
to explicitly accept an invalid server certificate. This often takes
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
allow the connection to take place.
This user behavior is used by (arguably legitimate) "SSL proxies"
that decrypt and re-encrypt the TLS connection in order to enforce
local security policy. It is also abused by attackers whose goal is
to gain access to the encrypted information.
Mitigation is complex and will probably involve a combination of
protocol mechanisms (HSTS, certificate pinning [KEY-PINNING]), and
very careful UI design.
3. Applicability to DTLS
DTLS [RFC4347] [RFC6347] is an adaptation of TLS for UDP.
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, especially [SECURE-TLS], certainly do.
5. Informative References
Sarkar, P. and S. Fitzgerald, "Attacks on SSL, a
comprehensive study of BEAST, CRIME, TIME, BREACH, Lucky13
and RC4 biases", August 2013,
[BEAST] Rizzo, J. and T. Duong, "Browser Exploit Against SSL/TLS",
[BREACH] Prado, A., Harris, N., and Y. Gluck, "The BREACH Attack",
Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
A., and P. Strub, "Triple handshakes and cookie cutters:
breaking and fixing authentication over tls", 2014,
Bleichenbacher, D., "Chosen Ciphertext Attacks Against
Protocols Based on the RSA Encryption Standard PKCS #1",
Brubaker, C., Jana, S., Ray, B., Khurshid, S., and V.
Shmatikov, "Using Frankencerts for Automated Adversarial
Testing of Certificate Validation in SSL/TLS
Brumley, D. and D. Boneh, "Remote Timing Attacks are
AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
the TLS and DTLS Record Protocols", IEEE Symposium on
Security and Privacy, 2013, <http://www.ieee-security.org/
Popov, A., "Prohibiting RC4 Cipher Suites", Work in
Progress, draft-ietf-tls-prohibiting-rc4-01, October 2014.
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", EKOparty
Security Conference, 2012.
[CVE] MITRE, "Common Vulnerabilities and Exposures",
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,
Delignat-Lavaud, A. and K. Bhargavan, "Virtual Host
Confusion: Weaknesses and Exploits", Black Hat 2014, 2014,
Gillmor, D., "Negotiated Finite Field Diffie-Hellman
Ephemeral Parameters for TLS", Work in Progress,
draft-ietf-tls-negotiated-ff-dhe-05, December 2014.
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", Proceedings of the 2012 ACM conference on
Computer and Communications Security, pages 38-49, 2012,
Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", Work in Progress,
draft-ietf-websec-key-pinning-21, October 2014.
[Klima03] Klima, V., Pokorny, O., and T. Rosa, "Attacking RSA-based
Sessions in SSL/TLS", 2003,
[POODLE] Moeller, B., Duong, T., and K. Kotowicz, "This POODLE
Bites: Exploiting the SSL 3.0 Fallback", September 2014,
Vaudenay, S., "Security Flaws Induced by CBC Padding
Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002,
[RC4] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C", Second Edition, October
AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,
and J. Schuldt, "On the Security of RC4 in TLS", Usenix
Security Symposium 2013, August 2013,
Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
Key Scheduling Algorithm of RC4", Selected Areas in
Cryptography, August 2001,
Mantin, I. and A. Shamir, "A Practical Attack on Broadcast
RC4", April 2001,
Paul, G. and S. Maitra, "Permutation After RC4 Key
Scheduling Reveals the Secret Key", August 2007,
[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, <http://www.rfc-editor.org/info/rfc5288>.
[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
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013,
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014,
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, September 2014,
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.
Marlinspike, M., "sslstrip", February 2009,
[TIME] Be'ery, T. and A. Shulman, "A Perfect CRIME? Only TIME
Will Tell", Black Hat Europe 2013, 2013,
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. Ralph Holz gratefully
acknowledges the support of NICTA (National ICT of Australia) in the
preparation of this document.
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
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