[Bug 1556302] Re: Ubuntu patch to add HOME to env_keep makes custom commands vulnerable by default

[Dan Streetman]

The *downside* of reverting our custom patch is that end-users are used
to all their personal customization of applications from $HOME working;
i.e. currently, when anyone runs vim, emacs, bash, etc. under sudo, any
~/.WHATEVER customization they have will be retained.  This is different
than, essentially, all other UNIXes, and the fix for this bug would undo
that, to put us back in line with all other UNIXes - but would result in
behavior change for users, where e.g. 'sudo vim' would not pick up any
of their ~/.vimrc configuration (or ~/.emacs.d for emacs, etc...)

Thus, this change, if we do make it, probably should only be done to
Eoan and not SRUed.

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https://bugs.launchpad.net/bugs/1556302

Title:
  Ubuntu patch to add HOME to env_keep makes custom commands vulnerable
  by default

Status in sudo package in Ubuntu:
  Confirmed

Bug description:
  I wanted to allow certain users to execute a python script as another user, so I created the following sudoers config:
  Defaults env_reset
  source_user ALL=(target_user) NOPASSWD: /home/target_user/bin/script.py

  This results in a highly insecure Python environment because the
  source user can set HOME and override any Python package by putting
  files in $HOME/.local/lib/python*/site-packages/.

  This should be a safe configuration because the default behaviour (as
  specified in the man page) is that env_reset will replace HOME with
  the target user's home directory. The "env_reset" option even has
  special behaviour for bash which has its own potential environment
  vulnerabilities.

  However there is an Ubuntu-specific patch in the package
  (keep_home_by_default.patch) that makes sudo preserve HOME by default,
  which negates the correct behaviour of "env_reset". It should not be
  necessary to explicitly specify the "always_set_home" option in order
  to negate this patch.

  The patch should be removed and the default /etc/sudoers should
  explicitly add HOME to "env_keep" for the "allow admins to run any
  command as root" entries, to get the desired behaviour without
  creating security issues for other sudoers commands.

  
  Note: for quick reference to anyone coming to this bug, this behavior (of sudo keeping the calling user's $HOME) can be disabled by running 'sudo visudo' and adding this line:

  Defaults        always_set_home

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