Hostmaster (Aegir) - Moderately critical - Access bypass, Arbitrary code execution - SA-CONTRIB-2020-031

Project: Hostmaster (Aegir)
Version: 7.x-3.x-dev
Date: 2020-July-29
Security risk: Moderately critical 14∕25
Vulnerability: Access bypass, Arbitrary code execution

Description

Aegir is a powerful hosting system that sits alongside a LAMP or LEMP server to create, deploy and manage Drupal sites.

Given that

• Aegir can use both Apache and Nginx Web servers,
• Apache allows configuration-writing users to escalate their privileges to the superuser root, and
• Aegir's operations are performed by the GNU/Linux user aegir,

It follows that:

• Users with access to the aegir account can escalate their privileges to root.
• Any PHP code submitted through the front-end Web UI via enabling modules (such as PHP, Views PHP, and Computed Field) could be run as root though a cron hook implementation. (Aegir runs cron through the aegir user via Drush.)

This vulnerability is mitigated by the fact that

• an attacker must have access to the aegir account, and
• the Web server must be Apache.

While it was generally assumed that aegir access should only be provided to trusted users (i.e. users who also have access to root), this wasn't explicitly stated. The documentation has since been updated.

Solution

If you're running Aegir and have granted untrusted users access to the aegir account,

1. revoke aegir account access for users who you would not trust with root access,
2. disable any module functionality on the hosted Drupal sites that allows PHP code to be entered on the front-end Web UI. Computed Field, for example, can still be used safely by providing code from the back-end only. (See Stop allowing PHP from being entered on the Web UI for a plan to enforce this.)

We do not recommend switching to an Nginx Web server instead of revoking access. This is because there could be as-yet-unknown privilege-escalation exploits involving Nginx (as with any other piece of software).

Switching to Nginx

While not recommended, if this is something you'd like to do in addition to making the above change, we can offer some information on how to do it.

While there may eventually be a migration path to convert existing Apache installations to Nginx, the recommended approach is currently:

1. Set up a new Aegir installation using Nginx.
2. Remotely import sites from the original Apache server.
3. Decommission the original Apache server.

Also see the Hostmaster (Aegir) project page.