[Bug 933480] [NEW] Picks hmac-md5 over hmac-sha1

[Chris West]

Public bug reported:

The OpenSSH client defaults to picking hmac-md5, which is based on the
demonstrably insecure MD5 algorithm:

faux at 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-64 at openssh.com,hmac-ripemd160,hmac-sha1-96,hmac-md5-96

Please append:
    MACs hmac-sha1,hmac-md5,umac-64 at openssh.com,hmac-ripemd160,hmac-sha1-96,hmac-md5-96

...to /etc/ssh/ssh_config, such that the client will prefer SHA-1:

faux at 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

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https://bugs.launchpad.net/bugs/933480

Title:
  Picks hmac-md5 over hmac-sha1

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