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What makes the Linux community special?

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Linux

In 2021, there are more reasons why people love Linux than ever before. In this series, I'll share 21 different reasons to use Linux. The community is a cornerstone reason to use Linux.

Many a Linux user has said that the key feature of Linux is its community. That might seem strange to a new user because "community" is a pretty popular term these days. There are actual jobs for building and managing communities. With communities seemingly a dime a dozen, what makes the Linux community unique?

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OpenSSH 8.5

OpenSSH 8.5 was released on 2021-03-03. 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.

In the SSH protocol, the "ssh-rsa" signature scheme uses the SHA-1
hash algorithm in conjunction with the RSA public key algorithm.
OpenSSH will disable this signature scheme by default in the near
future.

Note that the deactivation of "ssh-rsa" signatures does not necessarily
require cessation of use for RSA keys. In the SSH protocol, keys may be
capable of signing using multiple algorithms. In particular, "ssh-rsa"
keys are capable of signing using "rsa-sha2-256" (RSA/SHA256),
"rsa-sha2-512" (RSA/SHA512) and "ssh-rsa" (RSA/SHA1). Only the last of
these is being turned off by default.

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 that is still
enabled by default.

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 RFC8709 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.

This release enables the UpdateHostKeys option by default to assist
the client by automatically migrating to better algorithms.

[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): fixed a double-free memory corruption that was
   introduced in OpenSSH 8.2 . We treat all such memory faults as
   potentially exploitable. This bug could be reached by an attacker
   with access to the agent socket.

   On modern operating systems where the OS can provide information
   about the user identity connected to a socket, OpenSSH ssh-agent
   and sshd limit agent socket access only to the originating user
   and root. Additional mitigation may be afforded by the system's
   malloc(3)/free(3) implementation, if it detects double-free
   conditions.

   The most likely scenario for exploitation is a user forwarding an
   agent either to an account shared with a malicious user or to a
   host with an attacker holding root access.

 * Portable sshd(8): Prevent excessively long username going to PAM.
   This is a mitigation for a buffer overflow in Solaris' PAM username
   handling (CVE-2020-14871), and is only enabled for Sun-derived PAM
   implementations.  This is not a problem in sshd itself, it only
   prevents sshd from being used as a vector to attack Solaris' PAM.
   It does not prevent the bug in PAM from being exploited via some
   other PAM application. GHPR#212


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

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

 * ssh(1), sshd(8): this release changes the first-preference signature
   algorithm from ECDSA to ED25519.

 * ssh(1), sshd(8): set the TOS/DSCP specified in the configuration
   for interactive use prior to TCP connect. The connection phase of
   the SSH session is time-sensitive and often explicitly interactive.
   The ultimate interactive/bulk TOS/DSCP will be set after
   authentication completes.

 * ssh(1), sshd(8): remove the pre-standardization cipher
   rijndael-cbc@lysator.liu.se. It is an alias for aes256-cbc before
   it was standardized in RFC4253 (2006), has been deprecated and
   disabled by default since OpenSSH 7.2 (2016) and was only briefly
   documented in ssh.1 in 2001.

 * ssh(1), sshd(8): update/replace the experimental post-quantum
   hybrid key exchange method based on Streamlined NTRU Prime coupled
   with X25519.

   The previous sntrup4591761x25519-sha512@tinyssh.org method is
   replaced with sntrup761x25519-sha512@openssh.com. Per its
   designers, the sntrup4591761 algorithm was superseded almost two
   years ago by sntrup761.

   (note this both the updated method and the one that it replaced are
   disabled by default)

 * ssh(1): disable CheckHostIP by default. It provides insignificant
   benefits while making key rotation significantly more difficult,
   especially for hosts behind IP-based load-balancers.

Changes since OpenSSH 8.4
=========================

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

 * ssh(1): this release enables UpdateHostkeys by default subject to
   some conservative preconditions:
    - The key was matched in the UserKnownHostsFile (and not in the
      GlobalKnownHostsFile).
    - The same key does not exist under another name.
    - A certificate host key is not in use.
    - known_hosts contains no matching wildcard hostname pattern.
    - VerifyHostKeyDNS is not enabled.
    - The default UserKnownHostsFile is in use.

   We expect some of these conditions will be modified or relaxed in
   future.

 * ssh(1), sshd(8): add a new LogVerbose configuration directive for
   that allows forcing maximum debug logging by file/function/line
   pattern-lists.

 * ssh(1): when prompting the user to accept a new hostkey, display
   any other host names/addresses already associated with the key.

 * ssh(1): allow UserKnownHostsFile=none to indicate that no
   known_hosts file should be used to identify host keys.

 * ssh(1): add a ssh_config KnownHostsCommand option that allows the
   client to obtain known_hosts data from a command in addition to
   the usual files.

 * ssh(1): add a ssh_config PermitRemoteOpen option that allows the
   client to restrict the destination when RemoteForward is used
   with SOCKS.

 * ssh(1): for FIDO keys, if a signature operation fails with a
   "incorrect PIN" reason and no PIN was initially requested from the
   user, then request a PIN and retry the operation. This supports
   some biometric devices that fall back to requiring PIN when reading
   of the biometric failed, and devices that require PINs for all
   hosted credentials.

 * sshd(8): implement client address-based rate-limiting via new
   sshd_config(5) PerSourceMaxStartups and PerSourceNetBlockSize
   directives that provide more fine-grained control on a per-origin
   address basis than the global MaxStartups limit.

Bugfixes
--------

 * ssh(1): Prefix keyboard interactive prompts with "(user@host)" to
   make it easier to determine which connection they are associated
   with in cases like scp -3, ProxyJump, etc. bz#3224

 * sshd(8): fix sshd_config SetEnv directives located inside Match
   blocks. GHPR#201

 * ssh(1): when requesting a FIDO token touch on stderr, inform the
   user once the touch has been recorded.

 * ssh(1): prevent integer overflow when ridiculously large
   ConnectTimeout values are specified, capping the effective value
   (for most platforms) at 24 days. bz#3229

 * ssh(1): consider the ECDSA key subtype when ordering host key
   algorithms in the client.

 * ssh(1), sshd(8): rename the PubkeyAcceptedKeyTypes keyword to
   PubkeyAcceptedAlgorithms. The previous name incorrectly suggested
   that it control allowed key algorithms, when this option actually
   specifies the signature algorithms that are accepted. The previous
   name remains available as an alias. bz#3253

 * ssh(1), sshd(8): similarly, rename HostbasedKeyTypes (ssh) and
   HostbasedAcceptedKeyTypes (sshd) to HostbasedAcceptedAlgorithms.

 * sftp-server(8): add missing lsetstat@openssh.com documentation
   and advertisement in the server's SSH2_FXP_VERSION hello packet.

 * ssh(1), sshd(8): more strictly enforce KEX state-machine by
   banning packet types once they are received. Fixes memleak caused
   by duplicate SSH2_MSG_KEX_DH_GEX_REQUEST (oss-fuzz #30078).

 * sftp(1): allow the full range of UIDs/GIDs for chown/chgrp on 32bit
   platforms instead of being limited by LONG_MAX. bz#3206

 * Minor man page fixes (capitalization, commas, etc.) bz#3223

 * sftp(1): when doing an sftp recursive upload or download of a
   read-only directory, ensure that the directory is created with
   write and execute permissions in the interim so that the transfer
   can actually complete, then set the directory permission as the
   final step. bz#3222

 * ssh-keygen(1): document the -Z, check the validity of its argument
   earlier and provide a better error message if it's not correct.
   bz#2879

 * ssh(1): ignore comments at the end of config lines in ssh_config,
   similar to what we already do for sshd_config. bz#2320

 * sshd_config(5): mention that DisableForwarding is valid in a
   sshd_config Match block. bz3239

 * sftp(1): fix incorrect sorting of "ls -ltr" under some
   circumstances. bz3248.

 * ssh(1), sshd(8): fix potential integer truncation of (unlikely)
   timeout values. bz#3250

 * ssh(1): make hostbased authentication send the signature algorithm
   in its SSH2_MSG_USERAUTH_REQUEST packets instead of the key type.
   This make HostbasedAcceptedAlgorithms do what it is supposed to -
   filter on signature algorithm and not key type.

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

 * sshd(8): add a number of platform-specific syscalls to the Linux
   seccomp-bpf sandbox. bz#3232 bz#3260

 * sshd(8): remove debug message from sigchld handler that could cause
   deadlock on some platforms. bz#3259

 * Sync contrib/ssh-copy-id with upstream.

 * unittests: add a hostname function for systems that don't have it.
   Some systems don't have a hostname command (it's not required by
   POSIX). The do have uname -n (which is), but not all of those have
   it report the FQDN.

Checksums:
==========

 - SHA1 (openssh-8.5.tar.gz) = 04cae43c389fb411227c01219e4eb46e3113f34e
 - SHA256 (openssh-8.5.tar.gz) = 5qB2CgzNG4io4DmChTjHgCWqRWvEOvCKJskLdJCz+SU=

 - SHA1 (openssh-8.5p1.tar.gz) = 72eadcbe313b07b1dd3b693e41d3cd56d354e24e
 - SHA256 (openssh-8.5p1.tar.gz) = 9S8/QdQpqpkY44zyAK8iXM3Y5m8FLaVyhwyJc3ZG7CU=

Please note that the SHA256 signatures are base64 encoded and not
hexadecimal (which is the default for most checksum tools). The PGP
key used to sign the releases is available from the mirror sites:
https://cdn.openbsd.org/pub/OpenBSD/OpenSSH/RELEASE_KEY.asc

Please note that the OpenPGP key used to sign releases has been
rotated for this release. The new key has been signed by the previous
key to provide continuity.

Reporting Bugs:
===============

- Please read https://www.openssh.com/report.html
  Security bugs should be reported directly to openssh@openssh.com
Read more

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