Bug 432757

During a review of kdenetwork-filesharing [0] for inclusion in openSUSE Tumbleweed , I found an issue with the plugin's input validation of `authhelper`. It accepts arbitrary strings as user names, which are passed verbatim to `pdbedit`, `smbpasswd` and `usermod` respectively.

The privileged `authhelper` program does not sanitize user-supplied arguments properly. In particular, AuthHelper::createuser() accepts arbitrary strings as user names.

AuthHelper::createuser() calls `['smbpasswd', '-L', '-s', '-D', '0', '-a', user]`, and writes the password to the child's stdin.

Some problematic user name arguments are:

* `-d`: Disables the Samba account for root.
* `-e`: Enables the Samba account for root, if it was intentionally disabled.
* `-n`: User root password set to none.

Remediation:

Validate user names before passing them to the child process. An example validation function can be found in the standard "shadow" package [1]. A simple regular expression `\A[a-z_][a-z0-9_-]*\z` should be sufficient.

[0] https://bugzilla.opensuse.org/show_bug.cgi?id=1175633
[1] https://github.com/shadow-maint/shadow/blob/master/libmisc/chkname.c#L58