This bug was fixed in the package openssh - 1:7.6p1-4ubuntu0.7
---------------
openssh (1:7.6p1-4ubuntu0.7) bionic; urgency=medium
* d/p/fix-connect-timeout-overflow.patch: prevent ConnectTimeout overflow.
(LP: #1903516)
[ Sergio Durigan Junior ]
* d/p/lp1966591-upstream-preserve-group-world-read-permission-on-kno.patch:
Preserve group/world read permissions on known_hosts. (LP: #1966591)
-- Athos Ribeiro <athos.ribe...@canonical.com> Wed, 30 Mar 2022
10:17:14 -0300
** Changed in: openssh (Ubuntu Bionic)
Status: Fix Committed => Fix Released
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https://bugs.launchpad.net/bugs/1966591
Title:
ssh-keygen -R changes known_hosts file permissions (mode)
Status in portable OpenSSH:
Unknown
Status in openssh package in Ubuntu:
Fix Released
Status in openssh source package in Bionic:
Fix Released
Status in openssh source package in Focal:
Fix Released
Status in openssh source package in Impish:
Fix Released
Status in openssh source package in Jammy:
Fix Released
Bug description:
[Impact]
When using "ssh-keygen -R" to remove a host from "known_hosts" the
command changes permissions on the file. This can cause problems
particularly when used on the global "known_hosts" file
(/etc/ssh/ssh_known_hosts), because then only root can read it.
Programs running non-interactively as non-root users suddenly fail to
SSH and it's not immediately obvious why.
[Test Plan]
The problem happens on Bionic and Focal.
$ lxc launch ubuntu-daily:focal openssh-bug1966591
$ lxc shell openssh-bug1966591
# ssh-keyscan github.com > test_known_hosts
# chmod 644 test_known_hosts
# ssh-keygen -R github.com -f test_known_hosts
# stat test_known_hosts
...
Access: (0600/-rw-------) ...
...
[Where problems could occur]
The upstream patch is very simple and it is unlikely that it will
cause any regressions. An indirect problem that could occur is that
users might expect to see a more strict set of permissions on a
"known_hosts" file after using "ssh-keygen -R", but arguably this is
not defined behaviour and should not be relied upon. Of course, there
is always a (very) small risk of introducing problems when rebuilding
packages using newer versions of its dependencies (especially on
Bionic, because it's older).
[Original Description]
When I use ssh-keygen -R to remove a host from known_hosts it changes
permissions on the file. This causes problems particularly when used
on the global known hosts file (/etc/ssh/ssh_known_hosts), because
then only root can read it. Programs running non-interactively as non-
root users suddenly fail to SSH and it's not immediately obvious why.
To reproduce:
$ ssh-keyscan github.com >test_known_hosts
$ chmod 741 test_known_hosts
$ ssh-keygen -R github.com -f test_known_hosts
$ stat test_known_hosts
...
Access: (0600/-rw-------) ...
Expected behavior: file permissions remain unchanged (mode 0741 in
this example).
$ lsb_release -rd
Description: Ubuntu 18.04.6 LTS
Release: 18.04
$ apt-cache policy openssh-client
openssh-client:
Installed: 1:7.6p1-4ubuntu0.6
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