Note: I'm not sure if it is really a vulnerability, or if it should be an issue instead.

Summary

In the SSH configuration documentation, the sudoer line that you suggested can be escalated to edit any files in the system.

Details

You suggested adding the following lines in the sudoers file:

terraform ALL=(root) NOPASSWD: /usr/bin/tee /var/lib/vz/*

But this is highly insecure as we can escape the folder using ../ and edit any files on the system.

PoC

Using a terraform user with the previously mentioned line in the /etc/sudoers file, I can add a /etc/sudoers.d/sudo file using this command:

echo "ALL=(ALL) NOPASSWD:ALL" | tee /var/lib/vz/../../../etc/sudoers.d/sudo

And then I am full root of the node.

Impact

This breaches the access limits of the Terraform user.

Suggested workaround

Use a strict regex on the command to allow only the names that should be pushed by this user.

Example for cloudinit yaml files:

terraform ALL=(root) NOPASSWD: /usr/bin/tee /var/lib/vz/snippets/[A-Za-z0-9-]*\\.yaml