Guests do not own apps in the tenant

Overview

Without restrictions preventing guest users from registering and owning applications, threat actors can exploit external user accounts to establish persistent backdoor access to organizational resources through application registrations that might evade traditional security monitoring. When guest users own applications, compromised guest accounts can be used to exploit guest-owned applications that might have broad permissions. This vulnerability enables threat actors to request access to sensitive organizational data such as emails, files, and user information without the same level of scrutiny for internal user-owned applications.

This attack vector is dangerous because guest-owned applications can be configured to request high-privilege permissions and, once granted consent, provide threat actors with legitimate OAuth tokens. Furthermore, guest-owned applications can serve as command and control infrastructure, so threat actors can maintain access even after the compromised guest account is detected and remediated. Application credentials and permissions might persist independently of the original guest user account, so threat actors can retain access. Guest-owned applications also complicate security auditing and governance efforts, as organizations might have limited visibility into the purpose and security posture of applications registered by external users. These hidden weaknesses in the application lifecycle management make it difficult to assess the true scope of data access granted to non-Microsoft entities through seemingly legitimate application registrations.

Remediation

• Remove guest users as owners from applications and service principals, and implement controls to prevent future guest user application ownership.
• Restrict guest user access permissions

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	Medium
Impact on users	Low
Pillar	Identity
Category	External collaboration
Product	Entra ID
Feature	Applications
Area	Applications and workload identities
ATT&CK	T1098.001 Additional Cloud Credentials, T1199 Trusted Relationship
Licence	Free
Tenant	Workforce, External

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.