Apotheosis: A GCP Privilege Escalation Tool

Posted by on September 25, 2019

The Principle of Least Privilege

One of the most fundamental principles of information security is the principle of least privilege. This principle states that users should only be given the minimal permissions necessary to do their job. A corollary of the principle of least privilege is that users should only have those privileges while they are actively using them. For especially sensitive actions, users should be able to elevate their privileges within established policies, take sensitive actions, and then return their privilege level to normal to resume normal usage patterns. This is sometimes called privilege bracketing when applied to software, but it’s also useful for human users.

Following this principle reduces the chance of accidental destructive actions due to typos or misunderstandings. It may also provide some protection in case the user’s credentials are stolen, or if the user is tricked into running malicious code. Furthermore, it can be used as a notice to perform additional logging or monitoring of user actions.

In Unix this takes the form of the su command, which allows authorized users to elevate their privileges, take some sensitive actions, and then reduce their permissions. The sudo command is an even more fine-grained approach with the same purpose, as it will elevate privileges for a single command. 

Some cloud providers have features that allow for temporary escalation of privileges. Authorized users can take actions with a role other than the one which is normally assigned to them. The credentials used to assume a role are temporary, so they will expire after a specified amount of time. However, we did not find a built-in solution to achieve the same functionality in Google Cloud Platform (GCP).

Enter Apotheosis

Apotheosis is a tool that is meant to address the issues above. The word apotheosis means the elevation of someone to divine status. It’s possible, and convenient, to give users permanent “godlike” permissions, but this is a violation of the principle of least privilege. This tool will allow us to “apotheosize” users, and then return them to a “mortal” level of privilege when their job duties no longer require additional privileges.

Users or groups can be given “actual permissions” and “eligible permissions”. For example, a user who currently has the owner role may instead be given only the viewer role, and we will call that their “actual permissions”. Then we can give them “eligible permissions” of owner, which will come in the form of the service account token creator role on a service account with the editor or organization admin role.

For this user to elevate their privileges, the Apotheosis command line program will use their GCP credentials to call the REST API to create a short-lived service account token. Then, using that token, Apotheosis will make another REST API call which will grant the requested permissions to the user. Or, alternatively, the permissions may be granted to a specified third party, allowing the Apotheosis user to leverage their eligible permissions to grant actual permissions to another entity. The program will wait for a specified amount of time, remove the requested permissions, and then delete the short-lived service account token.

This process has the following advantages:

Future Additions

Some additional features which may be added to Apotheosis are contingent on the launch of other features, such as conditional IAM. Conditional IAM will allow the use of temporal restrictions on IAM grants, which will make Apotheosis more reliable. With conditional IAM, if Apotheosis is interrupted and does not revoke the granted permissions, they will expire anyway.

The ability to allow restricted permissions granting will be a useful IAM feature as well. Right now a user or service account can be given a role like editor or organization admin, and then can grant any other role in existence. But if it were possible to allow granting a predefined list of roles, that would make Apotheosis useful for a larger set of users. As it is now, Apotheosis is useful for users who have the highest level of eligible privilege, since their access to the Apotheosis service account gives them all the privileges of that service account. That is, the scope of those privileges can be limited to a particular project, folder, or organization, but cannot be restricted to a limited set of actions. At the moment that service account must have one of the few permissions which grant the ability to assign any role to any user. 

Requiring two-factor authentication when using the short-lived service account token feature on a particular service account would be another useful feature. This would require an Apotheosis user to re-authenticate with another factor when escalating privileges.

Open Source

Apotheosis is open source and can be found on Github.

Posted by on September 25, 2019
Category: security Tags: , ,

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