Linuxfabrik Monitoring Plugins: Sudoers may be able to obtain privilege escalation via /usr/bin/apt-get arguments

Summary

In the Debian.sudoers file, apt-get is allowed for the nagios user. The full command including the arguments are not enforced and can therefore be choosen arbitrarily. This allows to easily get a root shell as the nagios user:

PoC

By choosing a particular argument, you can get (as a nagios user) a root shell:

 1sudo apt-get update -o APT::Update::Pre-Invoke::="/bin/sh"

Since the nagious user can use sudo to run apt-get as root, the resulting shell is also running as root.

Impact

The vulnerability is a local privilege escalation, impacting users who use the provided sudoers file. It requires that an attacker already compromised the nagios account (which is quite a high barrier to be honest).

Fix

Since only one place where apt-get is currently used (in deb-updates) was found, it should be enough to allow only the specific arguments used there.

Here an example how the line in the sudoers file could look like:

 1                    /usr/lib64/nagios/plugins/strongswan-connections,\
 2                    /usr/lib64/nagios/plugins/systemd-unit,\
 3                    /usr/bin/apt-get update --quiet 2