Internet DRAFT - draft-pettersen-tls-version-rollback-removal
draft-pettersen-tls-version-rollback-removal
Network Working Group Y. Pettersen
Internet-Draft
Intended status: Informational February 13, 2014
Expires: August 17, 2014
Managing and removing automatic version rollback in TLS Clients
draft-pettersen-tls-version-rollback-removal-03
Abstract
Ever since vendors started deploying TLS 1.0 clients, these clients
have had to handle server implementations that do not tolerate the
TLS version supported by the client, usually by automatically
signaling an older supported version instead. Such version rollbacks
represent a potential security hazard, if the older version should
become vulnerable to attacks. The same history repeated when TLS
Extensions were introduced, as some servers would not negotiate with
clients that sent these protocol extensions, forcing clients to
reduce protocol functionality in order to maintain interoperability.
This document outlines a procedure to help clients decide when they
may use version rollback to maintain interoperability with legacy
servers, under what conditions the clients should not allow version
rollbacks, such as when the server has indicated support for the TLS
Renegotiation Information extension. The intention of this procedure
is to limit the use of automatic version rollback to legacy servers
and eventually eliminate its use.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 17, 2014.
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Copyright Notice
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1. Introduction
When vendors of Transport Layer Security (TLS) clients intially
developed and released TLS 1.0 [RFC2246] clients, they quickly
discovered that not all Secure Sockets Layer (SSL) v3 [RFC6101]
servers were willing to accept or complete handshakes with the TLS
clients. The reasons for this varied across various server
implementations, such as not accepting versions higher than SSL v3,
and various errors in the implementation of the handshake, e.g.,
expecting the RSA Premaster Secret's version field to match the
selected version, not the signaled version.
Given the scope of the problem of getting servers fixed, in order to
provide a good user experience for their customers, vendors elected
instead to restart the connection and signal the older protocol
version as the highest supported version in such cases.
This process was repeated when TLS Extensions[RFC6066], TLS 1.1
[RFC4346] and TLS 1.2 [RFC5246] were introduced, as clients had to
disable these features to be able to connect with servers that did
not tolerate them.
As a consequence, clients are not just vulnerable to a version
rollback attack; in the event that a vulnerability in older protocol
versions should be discovered, they are intentionally designed to be
vulnerable to such attacks by automatically performing a version
rollback whenever something goes wrong with the current TLS
handshake.
While it would be preferable that clients do not perform version
rollbacks, it is presently not practical to forbid it entirely, but
there are ways to limit the use of rollbacks, and eventually phase
out the usage completely.
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This document presents a procedure for selecting when to allow a
version rollback and how to implement it, in order to maintain
interoperability with legacy servers, as well as when to not allow
version rollbacks.
The main factor for deciding not to allow version rollbacks is
whether the server supports the TLS Renegotiation Information
Extension[RFC5746]. [RFC5746] specifically reminds implementors that
servers MUST correctly handle clients that support TLS Extensions and
/or new TLS versions than supported by the server. For the most
part, server vendors have adhered to this, as (per July 2012) less
than 0.14% of servers with Renegotiation Information extension
support (70.6%) do not adhere to this requirement, compared to 4.5%
among servers that does not support this extension.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Managing Version Rollbacks
When a TLS client initially connects to a TLS server, it exchanges a
number messages with the server in order to establish the encrypted
connection:
o Sending the Client Hello, which identifies the client's higest
supported version, supported extensions, and cryptographic
parameters
o Receiving the Server Hello, which identifies the server's selected
version, supported extensions, cryptographic paramaters
o Exchange of more messages to negotiate the encryption keys, and
other parameters
o Each sends a Finished message to the other, showing that the
negotiation succeeded, after which the secure connection is active
Each step of this negotiation sequence can fail for various reasons,
until the Finished messages have been sent and verified. The
failures can be indicated with Alert codes or by just shutting down
the connection. Frequently, many of these failures are due to
incorrect implementation on either end.
This tendency toward implementation issues leading to connection
failures have caused most client vendors to adopt a policy of
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retrying with older versions of the protocol, in case the failure was
caused by version-specific problems in the server.
2.1. Version Rollback Sequence
When establishing a TLS connection to a server with unknown
capabilities, a client SHOULD use the following sequence, advancing
to the next step if the connection attempt fails.
1. If the client supports TLS 1.1 or higher, it SHOULD send a Client
Hello indicating this highest version and include all supported
extensions. The version of the Record Protocol SHOULD at most be
TLS 1.0
2. If step 1 failed, and the server either did not indicate a
supported version or this version was TLS 1.0 or below, send a
Client Hello indicating TLS 1.0 as the highest version and
include all supported extensions. If this fails, the client MAY
remove extensions in a separate connection attempt before
considering this step to have failed.
3. If step 2 failed, and the server either did not indicate a
supported version or this version was SSL v3, send a Client Hello
indicating SSL v3 as the highest version, without sending TLS
Extensions.
In each step, the client MUST indicate support for the TLS
Renegotiation Information Extension, using the
TLS_EMPTY_RENEGOTIATION_INFO_SCSV cipher suite value specified by
[RFC5746] if TLS Extensions are not sent in the Client Hello.
The client MUST NOT roll back to an older version than the server has
indicated, even if the connection handshake failed. That is, if the
server indicates support for TLS 1.1, but the connection fails, then
the client MUST NOT attempt to connect to the server using TLS 1.0,
but allow the connection to fail.
2.2. Version Recovery
Once a connection is established and the client has received the
Server Hello, it MUST check the response to determine if the server
sends the TLS Renegotiation Information (RI) extension, and then
decide how to proceed:
o If the server did not return the RI extension, the client can
continue the handshake as normal and MAY continue version
rollbacks as described in Section 2.1 if the connection fails.
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o If the server did return the RI extension, and the client
indicated its highest supported version, with extensions, (first
step in Section 2.1) in the Client Hello, the client can continue
the handshake as normal but MUST NOT permit version rollbacks, in
case the connection fails, but instead allow the connection to
fail.
o If the server did return the RI extension, but the client was
indicating an older TLS version as its highest supported version,
or without TLS Extensions, the client MUST terminate the
connection, reestablish it, and send a Client Hello that signals
the highest supported version, and includes extensions, and it
MUST NOT permit a failure to trigger a new version rollback
sequence, but instead end the attempt to establish the connection.
The reason for not allowing version rollbacks if the server supports
the RI extension is that such servers MUST accept that clients
indicate a higher supported version than they do, and they MUST
support or tolerate clients that send TLS Extensions. It must be
presumed that, if such a handshake fails, it is because the
connection is being subjected to a active version downgrade attack,
not that the server has been incorrectly implemented in this respect.
3. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
4. Security Considerations
Allowing automatic version rollbacks exposes the TLS connection
between the client and server to significant risk if the older
version that gets negotiated is vulnerable to an attack that allows
the transmitted information to leak.
The use of automatic version rollbacks should be limited to
connections to servers that require it for interoperability reasons
and be prohibited for any other servers. While it is impractial to
discover which servers truly need such consideration, this document
specifies the presence of the TLS Renegotiation Information extension
as a proxy indication that the server does not require such
interoperability considerations.
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5. Acknowledgements
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
August 2011.
6.2. Informative References
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
Author's Address
Yngve N. Pettersen
Email: yngve@spec-work.net
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