OpenSSH 8.4 was released on 2020-09-27. It is available from the
mirrors listed at https://www.openssh.com/.

OpenSSH is a 100% complete SSH protocol 2.0 implementation and
includes sftp client and s

OpenSSH 8.4 was released on 2020-09-27. It is available from the
mirrors listed at https://www.openssh.com/.

OpenSSH is a 100% complete SSH protocol 2.0 implementation and
includes sftp client and server support.

Once again, we would like to thank the OpenSSH community for their
continued support of the project, especially those who contributed
code or patches, reported bugs, tested snapshots or donated to the
project. More information on donations may be found at:
https://www.openssh.com/donations.html

Future deprecation notice
=========================

It is now possible[1] 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.

[1] "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.

Potentially-incompatible changes
================================

This release includes a number of changes that may affect existing
configurations:

* For FIDO/U2F support, OpenSSH recommends the use of libfido2 1.5.0
or greater. Older libraries have limited support at the expense of
disabling particular features. These include resident keys, PIN-
required keys and multiple attached tokens.

* ssh-keygen(1): the format of the attestation information optionally
recorded when a FIDO key is generated has changed. It now includes
the authenticator data needed to validate attestation signatures.

* The API between OpenSSH and the FIDO token middleware has changed
and the SSH_SK_VERSION_MAJOR version has been incremented as a
result. Third-party middleware libraries must support the current
API version (7) to work with OpenSSH 8.4.

* The portable OpenSSH distribution now requires automake to rebuild
the configure script and supporting files. This is not required when
simply building portable OpenSSH from a release tar file.

Changes since OpenSSH 8.3
=========================

New features
------------

* ssh(1), ssh-keygen(1): support for FIDO keys that require a PIN for
each use. These keys may be generated using ssh-keygen using a new
"verify-required" option. When a PIN-required key is used, the user
will be prompted for a PIN to complete the signature operation.

* sshd(8): authorized_keys now supports a new "verify-required"
option to require FIDO signatures assert that the token verified
that the user was present before making the signature. The FIDO
protocol supports multiple methods for user-verification, but
currently OpenSSH only supports PIN verification.

* sshd(8), ssh-keygen(1): add support for verifying FIDO webauthn
signatures. Webauthn is a standard for using FIDO keys in web
browsers. These signatures are a slightly different format to plain
FIDO signatures and thus require explicit support.

* ssh(1): allow some keywords to expand shell-style ${ENV}
environment variables. The supported keywords are CertificateFile,
ControlPath, IdentityAgent and IdentityFile, plus LocalForward and
RemoteForward when used for Unix domain socket paths. bz#3140

* ssh(1), ssh-agent(1): allow some additional control over the use of
ssh-askpass via a new $SSH_ASKPASS_REQUIRE environment variable,
including forcibly enabling and disabling its use. bz#69

* ssh(1): allow ssh_config(5)'s AddKeysToAgent keyword accept a time
limit for keys in addition to its current flag options. Time-
limited keys will automatically be removed from ssh-agent after
their expiry time has passed.

* scp(1), sftp(1): allow the -A flag to explicitly enable agent
forwarding in scp and sftp. The default remains to not forward an
agent, even when ssh_config enables it.

* ssh(1): add a '%k' TOKEN that expands to the effective HostKey of
the destination. This allows, e.g., keeping host keys in individual
files using "UserKnownHostsFile ~/.ssh/known_hosts.d/%k". bz#1654

* ssh(1): add %-TOKEN, environment variable and tilde expansion to
the UserKnownHostsFile directive, allowing the path to be
completed by the configuration (e.g. bz#1654)

* ssh-keygen(1): allow "ssh-add -d -" to read keys to be deleted
from stdin. bz#3180

* sshd(8): improve logging for MaxStartups connection throttling.
sshd will now log when it starts and stops throttling and periodically
while in this state. bz#3055

Bugfixes
--------

* ssh(1), ssh-keygen(1): better support for multiple attached FIDO
tokens. In cases where OpenSSH cannot unambiguously determine which
token to direct a request to, the user is now required to select a
token by touching it. In cases of operations that require a PIN to
be verified, this avoids sending the wrong PIN to the wrong token
and incrementing the token's PIN failure counter (tokens
effectively erase their keys after too many PIN failures).

* sshd(8): fix Include before Match in sshd_config; bz#3122

* ssh(1): close stdin/out/error when forking after authentication
completes ("ssh -f ...") bz#3137

* ssh(1), sshd(8): limit the amount of channel input data buffered,
avoiding peers that advertise large windows but are slow to read
from causing high memory consumption.

* ssh-agent(1): handle multiple requests sent in a single write() to
the agent.

* sshd(8): allow sshd_config longer than 256k

* sshd(8): avoid spurious "Unable to load host key" message when sshd
load a private key but no public counterpart

* ssh(1): prefer the default hostkey algorithm list whenever we have
a hostkey that matches its best-preference algorithm.

* sshd(1): when ordering the hostkey algorithms to request from a
server, prefer certificate types if the known_hosts files contain a key
marked as a @cert-authority; bz#3157

* ssh(1): perform host key fingerprint comparisons for the "Are you
sure you want to continue connecting (yes/no/[fingerprint])?"
prompt with case sensitivity.

* sshd(8): ensure that address/masklen mismatches in sshd_config
yield fatal errors at daemon start time rather than later when
they are evaluated.

* ssh-keygen(1): ensure that certificate extensions are lexically
sorted. Previously if the user specified a custom extension then
the everything would be in order except the custom ones. bz#3198

* ssh(1): also compare username when checking for JumpHost loops.
bz#3057

* ssh-keygen(1): preserve group/world read permission on known_hosts
files across runs of "ssh-keygen -Rf /path". The old behaviour was
to remove all rights for group/other. bz#3146

* ssh-keygen(1): Mention the [-a rounds] flag in the ssh-keygen
manual page and usage().

* sshd(8): explicitly construct path to ~/.ssh/rc rather than
relying on it being relative to the current directory, so that it
can still be found if the shell startup changes its directory.
bz#3185

* sshd(8): when redirecting sshd's log output to a file, undo this
redirection after the session child process is forked(). Fixes
missing log messages when using this feature under some
circumstances.

* sshd(8): start ClientAliveInterval bookkeeping before first pass
through select() loop; fixed theoretical case where busy sshd may
ignore timeouts from client.

* ssh(1): only reset the ServerAliveInterval check when we receive
traffic from the server and ignore traffic from a port forwarding
client, preventing a client from keeping a connection alive when
it should be terminated. bz#2265

* ssh-keygen(1): avoid spurious error message when ssh-keygen
creates files outside ~/.ssh

* sftp-client(1): fix off-by-one error that caused sftp downloads to
make one more concurrent request that desired. This prevented using
sftp(1) in unpipelined request/response mode, which is useful when
debugging. bz#3054

* ssh(1), sshd(8): handle EINTR in waitfd() and timeout_connect()
helpers. bz#3071

* ssh(1), ssh-keygen(1): defer creation of ~/.ssh until we attempt to
write to it so we don't leave an empty .ssh directory when it's not
needed. bz#3156

* ssh(1), sshd(8): fix multiplier when parsing time specifications
when handling seconds after other units. bz#3171

Portability
-----------

* sshd(8): always send any PAM account messages. If the PAM account
stack returns any messages, always send them to the user and not
just if the check succeeds. bz#2049

* Implement some backwards compatibility for libfido2 libraries
older than 1.5.0. Note that use of an older library will result
in the loss of certain features including resident key support,
PIN support and support for multiple attached tokens.

* configure fixes for XCode 12

* gnome-ssh-askpass3: ensure the "close" button is not focused by
default for SSH_ASKPASS_PROMPT=none prompts. Avoids space/enter
accidentally dismissing FIDO touch notifications.

* gnome-ssh-askpass3: allow some control over textarea colour via
$GNOME_SSH_ASKPASS_FG_COLOR and $GNOME_SSH_ASKPASS_BG_COLOR
environment variables.

* sshd(8): document another PAM spec problem in a frustrated comment

* sshd(8): support NetBSD's utmpx.ut_ss address field. bz#960

* Add the ssh-sk-helper binary and its manpage to the RPM spec file

* Detect the Frankenstein monster of Linux/X32 and allow the sandbox
to function there. bz#3085

show more ...

OpenSSH 8.3 was released on 2020-05-27. It is available from the
mirrors listed at https://www.openssh.com/.

OpenSSH is a 100% complete SSH protocol 2.0 implementation and
includes sftp client and s

OpenSSH 8.3 was released on 2020-05-27. It is available from the
mirrors listed at https://www.openssh.com/.

OpenSSH is a 100% complete SSH protocol 2.0 implementation and
includes sftp client and server support.

Once again, we would like to thank the OpenSSH community for their
continued support of the project, especially those who contributed
code or patches, reported bugs, tested snapshots or donated to the
project. More information on donations may be found at:
https://www.openssh.com/donations.html

Future deprecation notice
=========================

It is now possible[1] 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.

A future release of OpenSSH will enable UpdateHostKeys by default
to allow the client to automatically migrate to better algorithms.
Users may consider enabling this option manually. Vendors of devices
that implement the SSH protocol should ensure that they support the
new signature algorithms for RSA keys.

[1] "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
========

* scp(1): when receiving files, scp(1) could be become desynchronised
if a utimes(2) system call failed. This could allow file contents
to be interpreted as file metadata and thereby permit an adversary
to craft a file system that, when copied with scp(1) in a
configuration that caused utimes(2) to fail (e.g. under a SELinux
policy or syscall sandbox), transferred different file names and
contents to the actual file system layout.

Exploitation of this is not likely as utimes(2) does not fail under
normal circumstances. Successful exploitation is not silent - the
output of scp(1) would show transfer errors followed by the actual
file(s) that were received.

Finally, filenames returned from the peer are (since openssh-8.0)
matched against the user's requested destination, thereby
disallowing a successful exploit from writing files outside the
user's selected target glob (or directory, in the case of a
recursive transfer). This ensures that this attack can achieve no
more than a hostile peer is already able to achieve within the scp
protocol.

Potentially-incompatible changes
================================

This release includes a number of changes that may affect existing
configurations:

* sftp(1): reject an argument of "-1" in the same way as ssh(1) and
scp(1) do instead of accepting and silently ignoring it.

Changes since OpenSSH 8.2
=========================

The focus of this release is bug fixing.

New Features
------------

* sshd(8): make IgnoreRhosts a tri-state option: "yes" to ignore
rhosts/shosts, "no" allow rhosts/shosts or (new) "shosts-only"
to allow .shosts files but not .rhosts.

* sshd(8): allow the IgnoreRhosts directive to appear anywhere in a
sshd_config, not just before any Match blocks; bz3148

* ssh(1): add %TOKEN percent expansion for the LocalFoward and
RemoteForward keywords when used for Unix domain socket forwarding.
bz#3014

* all: allow loading public keys from the unencrypted envelope of a
private key file if no corresponding public key file is present.

* ssh(1), sshd(8): prefer to use chacha20 from libcrypto where
possible instead of the (slower) portable C implementation included
in OpenSSH.

* ssh-keygen(1): add ability to dump the contents of a binary key
revocation list via "ssh-keygen -lQf /path" bz#3132

Bugfixes
--------

* ssh(1): fix IdentitiesOnly=yes to also apply to keys loaded from
a PKCS11Provider; bz#3141

* ssh-keygen(1): avoid NULL dereference when trying to convert an
invalid RFC4716 private key.

* scp(1): when performing remote-to-remote copies using "scp -3",
start the second ssh(1) channel with BatchMode=yes enabled to
avoid confusing and non-deterministic ordering of prompts.

* ssh(1), ssh-keygen(1): when signing a challenge using a FIDO token,
perform hashing of the message to be signed in the middleware layer
rather than in OpenSSH code. This permits the use of security key
middlewares that perform the hashing implicitly, such as Windows
Hello.

* ssh(1): fix incorrect error message for "too many known hosts
files." bz#3149

* ssh(1): make failures when establishing "Tunnel" forwarding
terminate the connection when ExitOnForwardFailure is enabled;
bz#3116

* ssh-keygen(1): fix printing of fingerprints on private keys and add
a regression test for same.

* sshd(8): document order of checking AuthorizedKeysFile (first) and
AuthorizedKeysCommand (subsequently, if the file doesn't match);
bz#3134

* sshd(8): document that /etc/hosts.equiv and /etc/shosts.equiv are
not considered for HostbasedAuthentication when the target user is
root; bz#3148

* ssh(1), ssh-keygen(1): fix NULL dereference in private certificate
key parsing (oss-fuzz #20074).

* ssh(1), sshd(8): more consistency between sets of %TOKENS are
accepted in various configuration options.

* ssh(1), ssh-keygen(1): improve error messages for some common
PKCS#11 C_Login failure cases; bz#3130

* ssh(1), sshd(8): make error messages for problems during SSH banner
exchange consistent with other SSH transport-layer error messages
and ensure they include the relevant IP addresses bz#3129

* various: fix a number of spelling errors in comments and debug/error
messages

* ssh-keygen(1), ssh-add(1): when downloading FIDO2 resident keys
from a token, don't prompt for a PIN until the token has told us
that it needs one. Avoids double-prompting on devices that
implement on-device authentication.

* sshd(8), ssh-keygen(1): no-touch-required FIDO certificate option
should be an extension, not a critical option.

* ssh(1), ssh-keygen(1), ssh-add(1): offer a better error message
when trying to use a FIDO key function and SecurityKeyProvider is
empty.

* ssh-add(1), ssh-agent(8): ensure that a key lifetime fits within
the values allowed by the wire format (u32). Prevents integer
wraparound of the timeout values. bz#3119

* ssh(1): detect and prevent trivial configuration loops when using
ProxyJump. bz#3057.

Portability
-----------

* Detect systems where signals flagged with SA_RESTART will interrupt
select(2). POSIX permits implementations to choose whether
select(2) will return when interrupted with a SA_RESTART-flagged
signal, but OpenSSH requires interrupting behaviour.

* Several compilation fixes for HP/UX and AIX.

* On platforms that do not support setting process-wide routing
domains (all excepting OpenBSD at present), fail to accept a
configuration attempts to set one at process start time rather than
fatally erroring at run time. bz#3126

* Improve detection of egrep (used in regression tests) on platforms
that offer a poor default one (e.g. Solaris).

* A number of shell portability fixes for the regression tests.

* Fix theoretical infinite loop in the glob(3) replacement
implementation.

* Fix seccomp sandbox compilation problems for some Linux
configurations bz#3085

* Improved detection of libfido2 and some compilation fixes for some
configurations when --with-security-key-builtin is selected.

show more ...

OpenSSH 8.2/8.2p1 (2020-02-14)
OpenSSH 8.2 was released on 2020-02-14. It is available from the
mirrors listed at https://www.openssh.com/.

OpenSSH is a 100% complete SSH protocol 2.0 implementation

OpenSSH 8.2/8.2p1 (2020-02-14)
OpenSSH 8.2 was released on 2020-02-14. It is available from the
mirrors listed at https://www.openssh.com/.

OpenSSH is a 100% complete SSH protocol 2.0 implementation and
includes sftp client and server support.

Once again, we would like to thank the OpenSSH community for their
continued support of the project, especially those who contributed
code or patches, reported bugs, tested snapshots or donated to the
project. More information on donations may be found at:
https://www.openssh.com/donations.html

Future deprecation notice
=========================

It is now possible[1] to perform chosen-prefix attacks against the
SHA-1 hash algorithm for less than USD$50K. For this reason, we will
be disabling the "ssh-rsa" public key signature algorithm that depends
on SHA-1 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.

A future release of OpenSSH will enable UpdateHostKeys by default
to allow the client to automatically migrate to better algorithms.
Users may consider enabling this option manually.

[1] "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(1), sshd(8), ssh-keygen(1): this release removes the "ssh-rsa"
(RSA/SHA1) algorithm from those accepted for certificate signatures
(i.e. the client and server CASignatureAlgorithms option) and will
use the rsa-sha2-512 signature algorithm by default when the
ssh-keygen(1) CA signs new certificates.

Certificates are at special risk to the aforementioned SHA1
collision vulnerability as an attacker has effectively unlimited
time in which to craft a collision that yields them a valid
certificate, far more than the relatively brief LoginGraceTime
window that they have to forge a host key signature.

The OpenSSH certificate format includes a CA-specified (typically
random) nonce value near the start of the certificate that should
make exploitation of chosen-prefix collisions in this context
challenging, as the attacker does not have full control over the
prefix that actually gets signed. Nonetheless, SHA1 is now a
demonstrably broken algorithm and futher improvements in attacks
are highly likely.

OpenSSH releases prior to 7.2 do not support the newer RSA/SHA2
algorithms and will refuse to accept certificates signed by an
OpenSSH 8.2+ CA using RSA keys unless the unsafe algorithm is
explicitly selected during signing ("ssh-keygen -t ssh-rsa").
Older clients/servers may use another CA key type such as
ssh-ed25519 (supported since OpenSSH 6.5) or one of the
ecdsa-sha2-nistp256/384/521 types (supported since OpenSSH 5.7)
instead if they cannot be upgraded.

Potentially-incompatible changes
================================

This release includes a number of changes that may affect existing
configurations:

* ssh(1), sshd(8): the above removal of "ssh-rsa" from the accepted
CASignatureAlgorithms list.

* ssh(1), sshd(8): this release removes diffie-hellman-group14-sha1
from the default key exchange proposal for both the client and
server.

* ssh-keygen(1): the command-line options related to the generation
and screening of safe prime numbers used by the
diffie-hellman-group-exchange-* key exchange algorithms have
changed. Most options have been folded under the -O flag.

* sshd(8): the sshd listener process title visible to ps(1) has
changed to include information about the number of connections that
are currently attempting authentication and the limits configured
by MaxStartups.

* ssh-sk-helper(8): this is a new binary. It is used by the FIDO/U2F
support to provide address-space isolation for token middleware
libraries (including the internal one). It needs to be installed
in the expected path, typically under /usr/libexec or similar.

Changes since OpenSSH 8.1
=========================

This release contains some significant new features.

FIDO/U2F Support
----------------

This release adds support for FIDO/U2F hardware authenticators to
OpenSSH. U2F/FIDO are open standards for inexpensive two-factor
authentication hardware that are widely used for website
authentication. In OpenSSH FIDO devices are supported by new public
key types "ecdsa-sk" and "ed25519-sk", along with corresponding
certificate types.

ssh-keygen(1) may be used to generate a FIDO token-backed key, after
which they may be used much like any other key type supported by
OpenSSH, so long as the hardware token is attached when the keys are
used. FIDO tokens also generally require the user explicitly authorise
operations by touching or tapping them.

Generating a FIDO key requires the token be attached, and will usually
require the user tap the token to confirm the operation:

$ ssh-keygen -t ecdsa-sk -f ~/.ssh/id_ecdsa_sk
Generating public/private ecdsa-sk key pair.
You may need to touch your security key to authorize key generation.
Enter file in which to save the key (/home/djm/.ssh/id_ecdsa_sk):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/djm/.ssh/id_ecdsa_sk
Your public key has been saved in /home/djm/.ssh/id_ecdsa_sk.pub

This will yield a public and private key-pair. The private key file
should be useless to an attacker who does not have access to the
physical token. After generation, this key may be used like any other
supported key in OpenSSH and may be listed in authorized_keys, added
to ssh-agent(1), etc. The only additional stipulation is that the FIDO
token that the key belongs to must be attached when the key is used.

FIDO tokens are most commonly connected via USB but may be attached
via other means such as Bluetooth or NFC. In OpenSSH, communication
with the token is managed via a middleware library, specified by the
SecurityKeyProvider directive in ssh/sshd_config(5) or the
$SSH_SK_PROVIDER environment variable for ssh-keygen(1) and
ssh-add(1). The API for this middleware is documented in the sk-api.h
and PROTOCOL.u2f files in the source distribution.

OpenSSH includes a middleware ("SecurityKeyProvider=internal") with
support for USB tokens. It is automatically enabled in OpenBSD and may
be enabled in portable OpenSSH via the configure flag
--with-security-key-builtin. If the internal middleware is enabled
then it is automatically used by default. This internal middleware
requires that libfido2 (https://github.com/Yubico/libfido2) and its
dependencies be installed. We recommend that packagers of portable
OpenSSH enable the built-in middleware, as it provides the
lowest-friction experience for users.

Note: FIDO/U2F tokens are required to implement the ECDSA-P256
"ecdsa-sk" key type, but hardware support for Ed25519 "ed25519-sk" is
less common. Similarly, not all hardware tokens support some of the
optional features such as resident keys.

The protocol-level changes to support FIDO/U2F keys in SSH are
documented in the PROTOCOL.u2f file in the OpenSSH source
distribution.

There are a number of supporting changes to this feature:

* ssh-keygen(1): add a "no-touch-required" option when generating
FIDO-hosted keys, that disables their default behaviour of
requiring a physical touch/tap on the token during authentication.
Note: not all tokens support disabling the touch requirement.

* sshd(8): add a sshd_config PubkeyAuthOptions directive that
collects miscellaneous public key authentication-related options
for sshd(8). At present it supports only a single option
"no-touch-required". This causes sshd to skip its default check for
FIDO/U2F keys that the signature was authorised by a touch or press
event on the token hardware.

* ssh(1), sshd(8), ssh-keygen(1): add a "no-touch-required" option
for authorized_keys and a similar extension for certificates. This
option disables the default requirement that FIDO key signatures
attest that the user touched their key to authorize them, mirroring
the similar PubkeyAuthOptions sshd_config option.

* ssh-keygen(1): add support for the writing the FIDO attestation
information that is returned when new keys are generated via the
"-O write-attestation=/path" option. FIDO attestation certificates
may be used to verify that a FIDO key is hosted in trusted
hardware. OpenSSH does not currently make use of this information,
beyond optionally writing it to disk.

FIDO2 resident keys
-------------------

FIDO/U2F OpenSSH keys consist of two parts: a "key handle" part stored
in the private key file on disk, and a per-device private key that is
unique to each FIDO/U2F token and that cannot be exported from the
token hardware. These are combined by the hardware at authentication
time to derive the real key that is used to sign authentication
challenges.

For tokens that are required to move between computers, it can be
cumbersome to have to move the private key file first. To avoid this
requirement, tokens implementing the newer FIDO2 standard support
"resident keys", where it is possible to effectively retrieve the key
handle part of the key from the hardware.

OpenSSH supports this feature, allowing resident keys to be generated
using the ssh-keygen(1) "-O resident" flag. This will produce a
public/private key pair as usual, but it will be possible to retrieve
the private key part from the token later. This may be done using
"ssh-keygen -K", which will download all available resident keys from
the tokens attached to the host and write public/private key files
for them. It is also possible to download and add resident keys
directly to ssh-agent(1) without writing files to the file-system
using "ssh-add -K".

Resident keys are indexed on the token by the application string and
user ID. By default, OpenSSH uses an application string of "ssh:" and
an empty user ID. If multiple resident keys on a single token are
desired then it may be necessary to override one or both of these
defaults using the ssh-keygen(1) "-O application=" or "-O user="
options. Note: OpenSSH will only download and use resident keys whose
application string begins with "ssh:"

Storing both parts of a key on a FIDO token increases the likelihood
of an attacker being able to use a stolen token device. For this
reason, tokens should enforce PIN authentication before allowing
download of keys, and users should set a PIN on their tokens before
creating any resident keys.

Other New Features
------------------

* sshd(8): add an Include sshd_config keyword that allows including
additional configuration files via glob(3) patterns. bz2468

* ssh(1)/sshd(8): make the LE (low effort) DSCP code point available
via the IPQoS directive; bz2986,

* ssh(1): when AddKeysToAgent=yes is set and the key contains no
comment, add the key to the agent with the key's path as the
comment. bz2564

* ssh-keygen(1), ssh-agent(1): expose PKCS#11 key labels and X.509
subjects as key comments, rather than simply listing the PKCS#11
provider library path. PR138

* ssh-keygen(1): allow PEM export of DSA and ECDSA keys; bz3091

* ssh(1), sshd(8): make zlib compile-time optional, available via the
Makefile.inc ZLIB flag on OpenBSD or via the --with-zlib configure
option for OpenSSH portable.

* sshd(8): when clients get denied by MaxStartups, send a
notification prior to the SSH2 protocol banner according to
RFC4253 section 4.2.

* ssh(1), ssh-agent(1): when invoking the $SSH_ASKPASS prompt
program, pass a hint to the program to describe the type of
desired prompt. The possible values are "confirm" (indicating
that a yes/no confirmation dialog with no text entry should be
shown), "none" (to indicate an informational message only), or
blank for the original ssh-askpass behaviour of requesting a
password/phrase.

* ssh(1): allow forwarding a different agent socket to the path
specified by $SSH_AUTH_SOCK, by extending the existing ForwardAgent
option to accepting an explicit path or the name of an environment
variable in addition to yes/no.

* ssh-keygen(1): add a new signature operations "find-principals" to
look up the principal associated with a signature from an allowed-
signers file.

* sshd(8): expose the number of currently-authenticating connections
along with the MaxStartups limit in the process title visible to
"ps".

Bugfixes
--------

* sshd(8): make ClientAliveCountMax=0 have sensible semantics: it
will now disable connection killing entirely rather than the
current behaviour of instantly killing the connection after the
first liveness test regardless of success. bz2627

* sshd(8): clarify order of AllowUsers / DenyUsers vs AllowGroups /
DenyGroups in the sshd(8) manual page. bz1690

* sshd(8): better describe HashKnownHosts in the manual page. bz2560

* sshd(8): clarify that that permitopen=/PermitOpen do no name or
address translation in the manual page. bz3099

* sshd(8): allow the UpdateHostKeys feature to function when
multiple known_hosts files are in use. When updating host keys,
ssh will now search subsequent known_hosts files, but will add
updated host keys to the first specified file only. bz2738

* All: replace all calls to signal(2) with a wrapper around
sigaction(2). This wrapper blocks all other signals during the
handler preventing races between handlers, and sets SA_RESTART
which should reduce the potential for short read/write operations.

* sftp(1): fix a race condition in the SIGCHILD handler that could
turn in to a kill(-1); bz3084

* sshd(8): fix a case where valid (but extremely large) SSH channel
IDs were being incorrectly rejected. bz3098

* ssh(1): when checking host key fingerprints as answers to new
hostkey prompts, ignore whitespace surrounding the fingerprint
itself.

* All: wait for file descriptors to be readable or writeable during
non-blocking connect, not just readable. Prevents a timeout when
the server doesn't immediately send a banner (e.g. multiplexers
like sslh)

* sshd_config(5): document the sntrup4591761x25519-sha512@tinyssh.org
key exchange algorithm. PR#151

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