Copssh - OpenSSH for Windows

Copssh version 7.4.0 installers come with OpenSSH 8.4p(link is external) and LibreSSL 3.1.4(link is external). Cygwin and GNU tools are also upgraded.

 Future deprecation notice

=========================

It is now possible to perform chosen-prefix attacks against the
SHA-1 algorithm for less than USD$50K. For this reason, we will be
disabling the "ssh-rsa" public key signature algorithm by default in a
near-future release.

This algorithm is unfortunately still used widely despite the
existence of better alternatives, being the only remaining public key
signature algorithm specified by the original SSH RFCs.

The better alternatives include:

 * The RFC8332 RSA SHA-2 signature algorithms rsa-sha2-256/512. These
   algorithms have the advantage of using the same key type as
   "ssh-rsa" but use the safe SHA-2 hash algorithms. These have been
   supported since OpenSSH 7.2 and are already used by default if the
   client and server support them.

 * The ssh-ed25519 signature algorithm. It has been supported in
   OpenSSH since release 6.5.

 * The RFC5656 ECDSA algorithms: ecdsa-sha2-nistp256/384/521. These
   have been supported by OpenSSH since release 5.7.

To check whether a server is using the weak ssh-rsa public key
algorithm, for host authentication, try to connect to it after
removing the ssh-rsa algorithm from ssh(1)'s allowed list:

    ssh -oHostKeyAlgorithms=-ssh-rsa user@host

If the host key verification fails and no other supported host key
types are available, the server software on that host should be
upgraded.

We intend to enable UpdateHostKeys by default in the next OpenSSH
release. This will assist the client by automatically migrating to
better algorithms. Users may consider enabling this option manually.

 "SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and
    Application to the PGP Web of Trust" Leurent, G and Peyrin, T
    (2020) https://eprint.iacr.org/2020/014.pdf

Security
========

 * ssh-agent(1): restrict ssh-agent from signing web challenges for
   FIDO/U2F keys.

   When signing messages in ssh-agent using a FIDO key that has an
   application string that does not start with "ssh:", ensure that the
   message being signed is one of the forms expected for the SSH protocol
   (currently public key authentication and sshsig signatures).

   This prevents ssh-agent forwarding on a host that has FIDO keys
   attached granting the ability for the remote side to sign challenges
   for web authentication using those keys too.

   Note that the converse case of web browsers signing SSH challenges is
   already precluded because no web RP can have the "ssh:" prefix in the
   application string that we require.

 * ssh-keygen(1): Enable FIDO 2.1 credProtect extension when generating
   a FIDO resident key.

   The recent FIDO 2.1 Client to Authenticator Protocol introduced a
   "credProtect" feature to better protect resident keys. We use this
   option to require a PIN prior to all operations that may retrieve
   a resident key from a FIDO token.