In his 4 years working as a security researcher for Bitdefender’s Cyber Threat Intelligence Labs, Radu Tudorică has proven he has a knack for tinkering with different types of technologies. His research is mainly focused on cloud security and malware reverse engineering, with the goal of better protecting Bitdefender’s customers against both well-known attacks and cutting-edge methodologies.

Abusing GCPW for lateral movement from local to cloud

GCPW (Google Credential Provider for Windows) is a product from Google that helps enterprises better manage their employees’ computers and allows for seamless integration to Google Workspace. 

When installed on a computer and configured to integrate with the company’s Google Workspace, it can be used to let users login using their GWS accounts (so that security policies such as 2FA enforcement are also used for local Windows logins) and it can also be used to deploy MDM (mobile device management) policies to better remotely administrate workstations. 

Based on our research, the seamless integration features of GCPW can be abused by attackers to move laterally from a local computer to the company’s cloud infrastructure and potentially be used to infect all company devices that are using GCPW. 

When logging into a local account administrated by GCPW, the user follows one of two authentication flows: the first one is an integrated browser page that’s presenting the user with an usual Google login screen; the second one is a normal login screen with a password prompt, it occurs when the login session hasn’t expired yet. 

The first login flow, creates a login session with Google and generates an Oauth2 refresh token that has been put temporarily in a registry and encrypted using Windows’s DPAPI. It can be accessed and decrypted by any process running in the user’s context. 

This refresh token is for Chrome’s Oauth2 app and it can be used to derive access tokens (with a lot of privileges) that can be used with Google’s APIs with the same level of access as the victim (making it possible to read emails, files and even perform some GWS administrative operations). 

The first login flow also adds the user’s password into LSA’s secrets storage in an encrypted form. The decryption key is stored by Google, but it can be obtained through an API call to an undocumented service, for which the refresh token previously acquired can be used. This means that if an attacker gains SYSTEM level privileges and can steal a refresh token, the attacker could now get the credentials for the Google account of the user. 

To obtain the refresh token, an attacker would have two options: the first one would be to delete the token handle found in the registry (requires Administrator privileges), then force the user to logout and get the refresh token after the user logs back in; the second one would be to extract the refresh token from the user’s Chrome profile. 

GCPW can also be used as an offensive tool by an attacker if the company has deployed it with its MDM features enabled. If the attacker gains access to an administrative account, then they could put a DownloadInstall MDM task to compromise all company devices with GCPW installed (and gaining SYSTEM level privileges for all devices). 

The study and understanding of cloud security is crucial and companies that integrate cloud platforms within their workflows must make sure that those integrations are not affecting their security posture.