Guest access is limited to approved tenants

Overview

Without limiting guest access to approved tenants, threat actors can exploit unrestricted guest access to establish initial access through compromised external accounts or by creating accounts in untrusted tenants. Organizations can configure an allowlist or blocklist to control B2B collaboration invitations from specific organizations, and without these controls, threat actors can leverage social engineering techniques to obtain invitations from legitimate internal users. Once threat actors gain guest access through unrestricted domains, they can perform discovery activities to enumerate internal resources, users, and applications that guest accounts can access. The compromised guest account then serves as a persistent foothold, allowing threat actors to execute collection activities against accessible SharePoint sites, Teams channels, and other resources granted to guest users. From this position, threat actors can attempt lateral movement by exploiting trust relationships between the compromised tenant and partner organizations, or by leveraging guest permissions to access sensitive data that can be used for further credential compromise or business email compromise attacks.

Remediation

• Configure Domain-Based Allow or Deny Lists

Run it

The check runs as part of the whole assessment, with the ZeroTrustAssessment module installed:

Install-Module ZeroTrustAssessment -Scope CurrentUser
Connect-Maester
Invoke-Maester -Pack ZTA -SkipBuiltIn

Leave out -SkipBuiltIn to run Maester’s own checks in the same run.

About this check

Severity	Medium
Effort to fix	High
Impact on users	Medium
Pillar	Identity
Category	Access control
Product	Entra ID
Feature	External identities
Area	External access and sharing
ATT&CK	T1078.004 Cloud Accounts, T1199 Trusted Relationship
Licence	Free
Tenant	Workforce

Product, feature, area and ATT&CK are Maester’s: the assessment does not record them, so Maester keeps them in the pack’s taxonomy.json.

A Microsoft check, part of the Zero Trust Assessment pack. Text from ZeroTrustAssessment 3.1.0, © Microsoft, MIT licence.