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Introduction:
A recent discovery by security researcher Jan Bakker has revealed a critical contradiction within Microsoft’s Entra ID (Azure AD) security model. The platform’s “immutable” custom security attributes, a cornerstone of access control and policy, may not be as unchangeable as presumed, creating a potential backdoor for privilege escalation and policy bypass. This vulnerability strikes at the heart of cloud identity management, where a single misconfigured attribute can dismantle an organization’s entire security posture.
Learning Objectives:
- Understand the inherent risk in Microsoft Entra’s “immutable” custom security attributes and the potential for privilege escalation.
- Learn to audit your Entra ID tenant for custom security attributes and their associated application policies.
- Master PowerShell and Microsoft Graph API commands to identify, analyze, and mitigate risks associated with this security flaw.
You Should Know:
1. Auditing Existing Custom Security Attributes
The first step is to inventory all custom security attributes in your tenant. These attributes are often used in Application Policies for conditional access and can be a prime target for manipulation.
Verified Command (PowerShell & Microsoft Graph):
Connect to Microsoft Graph (Requires 'Policy.ReadWrite.Authorization' scope)
Connect-MgGraph -Scopes "Policy.Read.All"
Retrieve all custom security attribute definitions
Get-MgDirectoryAttributeSet | ForEach-Object {
$attributeSet = $_
Get-MgDirectoryAttributeSetAttribute -AttributeSetId $attributeSet.Id
}
Step-by-step guide:
This PowerShell script utilizes the Microsoft Graph PowerShell module. After installing the module (Install-Module Microsoft.Graph), you connect with the necessary permissions. The `Get-MgDirectoryAttributeSet` cmdlet fetches all attribute sets (logical groupings), and then for each set, it retrieves the individual attributes. Review the output for any attributes with names like ExcludedCallers, which were automatically created and are at the center of the current issue. Pay close attention to the `id` and `isImmutableSupported` properties.
2. Identifying Application Policies Using These Attributes
Once you have the attribute list, you must find which critical application policies depend on them. A policy using a flawed attribute is a direct security risk.
Verified Command (Microsoft Graph API):
Use Graph Explorer or a token-based request curl -H "Authorization: Bearer <token>" \ "https://graph.microsoft.com/v1.0/policies/authorizationPolicy"
Step-by-step guide:
This direct Microsoft Graph API call fetches the tenant’s authorization policy. You will need an access token with the `Policy.Read.All` permission. The response will contain an `applicationConfigurations` array. Look for objects within this array that have a `definition.applicableTo` property value of “applicableToCustomSecAttribute.” This indicates a policy that relies on the custom security attributes you just audited. Correlate the attribute IDs from the previous step with the policies found here.
3. Assessing User and Service Principal Attribute Assignments
An attribute is only dangerous if it’s assigned. This command checks which users and service principals have been granted these powerful attributes.
Verified Command (PowerShell):
Get a specific user's custom security attributes Get-MgUser -UserId "[email protected]" -Property "customSecurityAttributes" | Select-Object -ExpandProperty CustomSecurityAttributes Get all service principals with custom security attributes (more advanced query) Get-MgServicePrincipal -All | Where-Object { $_.CustomSecurityAttributes -ne $null }
Step-by-step guide:
This is crucial for understanding the blast radius. The first command checks a specific user’s assigned attributes. The second, more complex command attempts to find all service principals (non-human identities) that have any custom security attributes. In a production environment, a service principal with an incorrectly assigned “excluded” attribute could have unintended, high-level permissions. Running these commands helps you map the potential attack paths.
4. The Hypothetical Exploit: Forging an “Immutable” Attribute
While the exact exploit for the discussed flaw is not public, the underlying risk is the potential for an attacker or a misconfigured Microsoft backend process to alter an “immutable” attribute.
Verified Command (Conceptual – Illustrative Graph API Call):
PATCH https://graph.microsoft.com/v1.0/directory/attributeSets/{id}/attributes/{id}
Authorization: Bearer <token>
Content-Type: application/json
{
"isImmutableSupported": false
}
Step-by-step guide:
This is a hypothetical example of the type of call that would represent the core of the vulnerability. If an entity (like a Microsoft backend process fixing the `ExcludedCallers` issue) can execute a PATCH request against an attribute defined as isImmutableSupported: true, it breaks the security contract. In a real-world attack, this could be chained to change a user’s attribute from `excluded: false` to excluded: true, thereby granting them access to a privileged application policy. Monitoring for such PATCH operations in your audit logs is essential.
5. Mitigation via Conditional Access Policy Audit
While the attribute flaw is a platform issue, you can mitigate the risk by ensuring no single policy creates a system-wide failure.
Verified Command (PowerShell):
Get all Conditional Access policies Get-MgIdentityConditionalAccessPolicy | Select-Object DisplayName, State, Conditions
Step-by-step guide:
This command lists all Conditional Access policies. Look for policies that rely heavily on a single custom security attribute for critical access decisions. The goal is to identify “eggs in one basket” scenarios. If a policy named “Break Glass Admin Access” relies solely on a single custom attribute that could be mutated, the risk is extreme. The mitigation is to design policies with multiple, redundant controls (like MFA and device compliance) so that the failure of one control does not completely compromise security.
6. Proactive Monitoring with Unified Audit Logs
You must monitor for any changes to authorization policies and custom security attributes, as these are high-value targets.
Verified Command (PowerShell for Audit Log):
Search audit logs for activities related to 'Authorization Policy' Search-UnifiedAuditLog -Operations "New-AuthorizationPolicy", "Set-AuthorizationPolicy", "Update-AuthorizationPolicy" -StartDate (Get-Date).AddDays(-1) -EndDate (Get-Date)
Step-by-step guide:
This command searches the Microsoft 365 Unified Audit Log for events related to authorization policy changes. You should run this regularly or integrate it into a SIEM. The `-StartDate` and `-EndDate` parameters define the search window. Any result from this query should be treated as a high-severity event and investigated immediately, as it indicates a change to the core authorization fabric of your tenant.
7. Hardening with Privileged Identity Management (PIM)
As a defense-in-depth measure, protect the roles that have the ability to manage custom security attributes and authorization policies.
Verified Command (PowerShell to Check Eligible Roles):
Get PIM eligible role assignments for a specific user Get-MgRoleManagementDirectoryRoleEligibilitySchedule -Filter "principalId eq '<user-id>'"
Step-by-step guide:
This command checks which Azure AD roles a user is eligible for via PIM. The principle of least privilege is paramount. Roles like “Security Administrator” and “Global Administrator” can manage these settings. By requiring Just-In-Time (JIT) access and approval workflows via PIM for these roles, you drastically reduce the attack surface. An attacker or a misconfiguration cannot easily change an attribute if the required admin role is not actively assigned.
What Undercode Say:
- The Illusion of Immutability: Trust, but verify. This incident proves that no cloud security control should be taken at face value. “Immutable” properties defined by a vendor are only as strong as the vendor’s own operational integrity and can be changed by them, creating a hidden risk for all customers.
- The Supply Chain Blind Spot: The most dangerous vulnerabilities are often those introduced by the platform provider itself. Organizations spend immense resources securing their configurations but have zero control over the underlying platform’s code and operational procedures, making them vulnerable to supply chain-style attacks from their most trusted vendors.
This situation is a classic case of a platform-level trust violation. The risk is not from a traditional external hacker but from an authorized, automated backend process designed to fix a problem. The very act of remediation could inadvertently become the exploit vector. Organizations must now question all “immutable” guarantees and build their security architectures with the assumption that any single control, especially one managed by a third party, can fail. This necessitates a layered security model where the compromise of one layer (like a custom attribute) does not lead to a total breach.
Prediction:
This vulnerability represents a subtle but significant shift in the cloud threat landscape. We predict a rise in “remediation-based exploits,” where a vendor’s patch or backend fix inadvertently introduces a new, more subtle security flaw. This will force a new discipline of “post-patch security validation,” where organizations must immediately audit their environments after any major platform update. Furthermore, it will fuel the growth of third-party Cloud Security Posture Management (CSPM) tools that operate under a “zero-trust for the cloud platform” assumption, continuously validating the integrity of core platform services against a known secure baseline, independent of the vendor’s own health status reports.
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IT/Security Reporter URL:
Reported By: Jan Bakker – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
