Network Working Group Y. Pettersen
Intended status: Informational February 13, 2014
Expires: August 17, 2014
Managing and removing automatic version rollback in TLS Clients
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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 17, 2014.
Pettersen Expires August 17, 2014 [Page 1]Internet-Draft Version rollback management February 2014Copyright Notice
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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
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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