Public bug reported: The OpenSSH client defaults to picking hmac-md5, which is based on the demonstrably insecure MD5 algorithm:
faux@wilf:~% ssh -v localhost true 2>&1 | grep hmac debug1: kex: server->client aes128-ctr hmac-md5 none debug1: kex: client->server aes128-ctr hmac-md5 none MD5 has had practical vulnerabilities for around eight years, and its use is highly discouraged. SHA1 is a supported alternative, and is supported by the packaged openssh-server, and many other ssh implementations. MD5 is selected as man ssh_config suggests the default algorithms are, in order of preference (most preferred first): hmac-md5,hmac-sha1,umac...@openssh.com,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 Please append: MACs hmac-sha1,hmac-md5,umac...@openssh.com,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 ...to /etc/ssh/ssh_config, such that the client will prefer SHA-1: faux@wilf:~% ssh -v localhost true 2>&1 | grep hmac debug1: kex: server->client aes128-ctr hmac-sha1 none debug1: kex: client->server aes128-ctr hmac-sha1 none This should have no compatibility concerns as MD5 is still a supported algorithm. Note that non-privileged users can override this setting either way on a per-connection basis by specifying MACs in ~/.ssh/config. ProblemType: Bug DistroRelease: Ubuntu 11.10 Package: openssh-client 1:5.8p1-7ubuntu1 ProcVersionSignature: Ubuntu 3.0.0-16.28-generic 3.0.17 Uname: Linux 3.0.0-16-generic x86_64 NonfreeKernelModules: nvidia ApportVersion: 1.23-0ubuntu4 Architecture: amd64 CheckboxSubmission: b0d31efda01870980e2e5a89390b685c CheckboxSystem: 6ce041aeed0a2c17b3343b66d157175d Date: Thu Feb 16 13:59:43 2012 EcryptfsInUse: Yes InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007) ProcEnviron: PATH=(custom, user) LANG=en_GB.UTF-8 SHELL=/bin/zsh RelatedPackageVersions: ssh-askpass N/A libpam-ssh N/A keychain N/A ssh-askpass-gnome 1:5.8p1-7ubuntu1 SSHClientVersion: OpenSSH_5.8p1 Debian-7ubuntu1, OpenSSL 1.0.0e 6 Sep 2011 SourcePackage: openssh UpgradeStatus: Upgraded to oneiric on 2011-05-03 (289 days ago) modified.conffile..etc.ssh.ssh.config: [modified] mtime.conffile..etc.ssh.ssh.config: 2012-02-16T13:59:11.376423 ** Affects: openssh (Ubuntu) Importance: Undecided Status: New ** Tags: amd64 apport-bug oneiric running-unity -- You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to openssh in Ubuntu. https://bugs.launchpad.net/bugs/933480 Title: Picks hmac-md5 over hmac-sha1 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/933480/+subscriptions -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs