OpenSSH 8.2 Released With FIDO/U2F Hardware Authentication

OpenSSH
OpenSSH

OpenSSH 8.2 Adds Supports FIDO/U2F Hardware Authentication.

OpenSSH is a software for secure networking utilities based on the Secure Shell protocol for remote login. It encrypts all traffic to eliminate eavesdropping, connection hijacking, and other attacks.

In addition, OpenSSH provides a large suite of secure tunneling capabilities, several authentication methods, and sophisticated configuration options.

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

Future deprecation notice

It is now possible 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 [email protected]

 

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.

“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, sshd: the above removal of “ssh-rsa” from the accepted CASignatureAlgorithms list.
  • ssh, sshd: this release removes diffie-hellman-group14-sha1 from the default key exchange proposal for both the client and server.
  • ssh-keygen: 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: 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: 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, 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 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: 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: add a sshd_config PubkeyAuthOptions directive that collects miscellaneous public key authentication-related options for sshd. 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, sshd, ssh-keygen: 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: 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 “-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 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 “-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: add an Include sshd_config keyword that allows including additional configuration files via glob patterns.

ssh/sshd: make the LE (low effort) DSCP code point available via the IPQoS directive.

ssh: 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, 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.

ssh-keygen: allow PEM export of DSA and ECDSA keys.

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

ssh, 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: 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: add a new signature operations “find-principals” to look up the principal associated with a signature from an allowed-signers file.

sshd: expose the number of currently-authenticating connections along with the MaxStartups limit in the process title visible to “ps”.

Bugfixes

sshd: 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.

sshd: clarify order of AllowUsers / DenyUsers vs AllowGroups /
DenyGroups in the sshd manual page.

sshd: better describe HashKnownHosts in the manual page.

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

sshd: 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.

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

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

ssh: 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: document the [email protected]
key exchange algorithm.

Portability

 sshd: multiple adjustments to the Linux seccomp sandbox:

  • Non-fatally deny IPC syscalls in sandbox
  • Allow clock_gettime64() in sandbox (MIPS / glibc >= 2.31)
  • Allow clock_nanosleep_time64 in sandbox (ARM)
  • Allow clock_nanosleep() in sandbox (recent glibc)

Explicit check for memmem declaration and fix up declaration if the system headers lack it.

Checksums:

– SHA1 (openssh-8.2.tar.gz) = 77584c22fbb89269398acdf53c1e554400584ba8
– SHA256 (openssh-8.2.tar.gz) = UttLaaSYXVK1O65cYvyQzyQ5sCfuJ4Lwrs8zNsPrluQ=

– SHA1 (openssh-8.2p1.tar.gz) = d1ab35a93507321c5db885e02d41ce1414f0507c
– SHA256 (openssh-8.2p1.tar.gz) = Q5JRUebPbO4UUBkMDpr03Da0HBJzdhnt/4vOvf9k5nE=

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 as RELEASE_KEY.asc from the mirror sites.

Reporting Bugs:

If you want to submit Security bugs, then you should be reported directly to [email protected]

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