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Stop supporting fallback between SSH versions.

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The UI now only has "1" and "2" options for SSH protocol version, which
behave like the old "1 only" and "2 only" options; old
SSH-N-with-fallback settings are interpreted as SSH-N-only.

This prevents any attempt at a protocol downgrade attack.
Most users should see no difference; those poor souls who still have to
work with SSH-1 equipment now have to explicitly opt in.
This commit is contained in:
Jacob Nevins
2016-03-28 20:23:57 +01:00
parent 43f1aa01cd
commit 16dfefcbde
9 changed files with 90 additions and 43 deletions
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36
doc/config.but
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@ -1667,7 +1667,7 @@ Keepalives are only supported in Telnet and SSH; the Rlogin and Raw
protocols offer no way of implementing them. (For an alternative, see
\k{config-tcp-keepalives}.)
Note that if you are using \i{SSH-1} and the server has a bug that makes
Note that if you are using SSH-1 and the server has a bug that makes
it unable to deal with SSH-1 ignore messages (see
\k{config-ssh-bug-ignore1}), enabling keepalives will have no effect.
@ -2267,30 +2267,28 @@ client end. Likewise, data sent by PuTTY to the server is compressed
first and the server decompresses it at the other end. This can help
make the most of a low-\i{bandwidth} connection.
\S{config-ssh-prot} \q{Preferred \i{SSH protocol version}}
\S{config-ssh-prot} \q{\i{SSH protocol version}}
\cfg{winhelp-topic}{ssh.protocol}
This allows you to select whether you would prefer to use \i{SSH protocol
version 1} or \I{SSH-2}version 2, and whether to permit falling back
to the other version.
This allows you to select whether to use \i{SSH protocol version 2}
or the older \I{SSH-1}version 1.
With the settings \q{1} and \q{2}, PuTTY will attempt to use protocol 1
if the server you connect to does not offer protocol 2, and vice versa.
You should normally leave this at the default of \q{2}. As well as
having fewer features, the older SSH-1 protocol is no longer
developed, has many known cryptographic weaknesses, and is generally
not considered to be secure. PuTTY's protocol 1 implementation is
provided mainly for compatibility, and is no longer being enhanced.
If you select \q{1 only} or \q{2 only} here, PuTTY will only connect
if the server you connect to offers the SSH protocol version you
have specified.
If a server offers both versions, prefer \q{2}. If you have some
server or piece of equipment that only talks SSH-1, select \q{1}
here, and do not treat the resulting connection as secure.
You should normally leave this at the default, \q{2 only}. The older
SSH-1 protocol is no longer developed, has many known cryptographic
weaknesses, and is generally not considered to be secure. If you
permit use of SSH-1 by selecting \q{2} instead of \q{2 only}, an
active attacker can force downgrade to SSH-1 even if the server
you're connecting to supports SSH-2.
PuTTY's protocol 1 implementation is provided mainly for
compatibility, and is no longer being enhanced.
PuTTY will not automatically fall back to the other version of the
protocol if the server turns out not to match your selection here;
instead, it will put up an error message and abort the connection.
This prevents an active attacker downgrading an intended SSH-2
connection to SSH-1.
\S{config-ssh-sharing} Sharing an SSH connection between PuTTY tools