Guest access is protected by strong authentication methods

Overview

External user accounts are often used to provide access to business partners who belong to organizations that have a business relationship with your organization. If these accounts are compromised in their organization, attackers can use the valid credentials to gain initial access to your environment, often bypassing traditional defenses due to their legitimacy.

Attackers might gain access with external user accounts, if multifactor authentication (MFA) isn’t universally enforced or if there are exceptions in place. They might also gain access by exploiting the vulnerabilities of weaker MFA methods like SMS and phone calls using social engineering techniques, such as SIM swapping or phishing, to intercept the authentication codes.

Once an attacker gains access to an account without MFA or a session with weak MFA methods, they might attempt to manipulate MFA settings (for example, registering attacker controlled methods) to establish persistence to plan and execute further attacks based on the privileges of the compromised accounts.

Remediation

• Deploy a Conditional Access policy to enforce authentication strength for guests.
• For organizations with a closer business relationship and vetting on their MFA practices, consider deploying cross-tenant access settings to accept the MFA claim.
  • Configure B2B collaboration cross-tenant access settings

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	Medium
Pillar	Identity
Category	External collaboration
Product	Entra ID
Feature	Conditional Access
Area	Authentication
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.