Microsoft Defender Exploits Show Why M365 Security Assessments Can’t Be Optional

Microsoft Defender Exploits Show Why M365 Security Assessments Matter

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Microsoft 365 is the operational backbone for most modern organizations. Email, Teams, SharePoint, OneDrive, Azure AD/Microsoft Entra ID, endpoint telemetry, collaboration workflows, and identity controls all converge inside the Microsoft ecosystem.

That also makes Microsoft 365 one of the most attractive targets for attackers.

A recent Cyber Press report highlighted CISA’s warning that two Microsoft Defender vulnerabilities were added to the Known Exploited Vulnerabilities catalog after being observed in active exploitation. The flaws include CVE-2026-45498, a Microsoft Defender denial-of-service vulnerability, and CVE-2026-41091, a link-following vulnerability that can allow local privilege escalation. Federal agencies were directed to apply vendor mitigations by June 3, 2026, or discontinue use if mitigations were unavailable.

The lesson for business leaders is direct: even trusted, native Microsoft security controls require validation, configuration review, monitoring, and governance. Having Microsoft 365 does not automatically mean Microsoft 365 is secure.

The Risk Is Bigger Than One Defender Vulnerability

The Microsoft Defender vulnerabilities matter because they target a tool organizations depend on for protection. One flaw can disrupt Defender’s ability to function properly. Another can help an attacker elevate privileges after gaining initial access. NVD lists CVE-2026-45498 as a Microsoft Defender denial-of-service vulnerability and confirms it is in CISA’s Known Exploited Vulnerabilities catalog. NVD also describes CVE-2026-41091 as an improper link resolution vulnerability in Microsoft Defender that allows an authorized attacker to elevate privileges locally, with a CVSS 3.1 score of 7.8.

That creates a real-world attack chain:

An employee falls for a phishing email.
The attacker gains a low-privileged foothold.
A privilege escalation flaw is used to gain deeper control.
Defender disruption limits visibility.
The attacker moves laterally, accesses SharePoint or OneDrive data, abuses mailbox rules, or targets privileged accounts.

This is why Microsoft 365 security cannot be treated as a one-time setup task. Attackers do not need every control to fail. They only need one weak identity policy, one over-permissive admin role, one unmanaged guest relationship, one exposed file-sharing path, or one alerting gap.

Microsoft 365 Is Now an Identity, Data, and Collaboration Attack Surface

For many organizations, Microsoft 365 is where the most sensitive business activity happens. Executives use Exchange Online for confidential communications. Teams channels contain operational discussions. SharePoint and OneDrive store contracts, financial data, legal documents, patient records, customer data, intellectual property, and board materials.

Microsoft’s own threat reporting reinforces why identity security remains central. The 2025 Microsoft Digital Defense Report states that more than 97% of identity attacks are password spray or brute-force attacks, while modern multifactor authentication reduces identity compromise risk by more than 99%.

That does not mean MFA alone solves the problem. Organizations still need to validate:

Which users have excessive privileges
Whether Conditional Access policies are properly scoped
Whether legacy authentication is disabled
Whether risky sign-ins are monitored
Whether guest access is controlled
Whether mailbox forwarding and inbox rules are being abused
Whether SharePoint and OneDrive sharing settings expose sensitive data
Whether third-party applications have excessive API permissions
Whether Microsoft Secure Score reflects actual risk reduction, not just checkbox completion

These are exactly the types of issues that a Microsoft 365 Security Assessment is designed to uncover.

Why Secure Score Alone Is Not Enough

Microsoft Secure Score is useful, but it should not be mistaken for a complete security program. A higher score can indicate progress, but it does not always prove that controls are properly aligned to the organization’s actual risk, regulatory obligations, user behavior, data exposure, or threat model.

A company may improve its Secure Score while still having:

Over-permissive Global Admin accounts
Weak Conditional Access exceptions
Excessive guest access in Teams
Public or anonymous SharePoint links
Incomplete email authentication controls
Poor visibility into third-party OAuth applications
No clear remediation ownership
Limited executive reporting

Security teams need context. Leadership needs prioritization. Auditors need evidence. IT teams need ticket-ready remediation steps.

That is where InfoSight’s M365 Security Assessment becomes valuable.

How InfoSight’s M365 Security Assessment Reduces Microsoft 365 Risk

InfoSight’s Microsoft 365 Security Assessment is built to identify misconfigurations, excessive permissions, collaboration risks, and security control gaps across the Microsoft 365 tenant. The assessment reviews Exchange Online, Teams, SharePoint, OneDrive, and Azure AD/Microsoft Entra ID, combining automated scans with expert-led review to harden the tenant, improve Secure Score, and map findings to frameworks such as NIST 800-53 and CIS Benchmarks.

The assessment process includes three practical phases:

1. Recon and Inventory
InfoSight discovers users, mailboxes, applications, and guest relationships across the tenant. This gives the organization a clear view of who has access, what services are exposed, and where unmanaged risk may exist.

2. Configuration and Permission Audit
The assessment reviews Conditional Access, MFA, Exchange Online, Teams, SharePoint, and API permissions. This is where hidden exposure often appears: excessive roles, weak authentication policies, uncontrolled external sharing, and application permissions that no longer match business need.

3. Threat Simulation and Reporting
InfoSight validates exploitable paths, maps findings to MITRE ATT&CK and NIST, and delivers an executive-ready remediation roadmap. This helps translate technical findings into business risk, remediation priority, and audit-ready evidence.

Real-World Scenario: From One Inbox to Enterprise Exposure

Consider a mid-sized healthcare, financial, or manufacturing organization running Microsoft 365.

An attacker compromises one user through phishing. MFA exists, but Conditional Access exceptions allow access from unmanaged devices. The attacker logs in, sets up mailbox forwarding, searches Teams and SharePoint for financial documents, and finds externally shared folders with sensitive data.

From there, the attacker identifies an over-permissioned user account, abuses application consent, and begins expanding access. If endpoint visibility is degraded or Defender health is not being monitored, the security team may not see the full chain until data has already been accessed or exfiltrated.

An M365 Security Assessment helps expose these weaknesses before the attacker does. It validates whether identity controls, email protections, collaboration permissions, data-sharing policies, and alerting pipelines are working as intended.

What Organizations Should Review Now

The CISA warning should push organizations to look beyond patching alone. Patching Defender vulnerabilities is necessary, but it is not sufficient.

Organizations should also review:

Microsoft Defender health and update status
Endpoint telemetry and alert forwarding
Conditional Access coverage
MFA enforcement, especially for privileged users
Legacy authentication exposure
Azure AD/Microsoft Entra ID administrative roles
Guest users and external collaboration settings
SharePoint and OneDrive anonymous links
Exchange Online forwarding rules and mailbox permissions
Third-party OAuth applications and API permissions
Secure Score gaps that map to actual business risk

This is the difference between assuming Microsoft 365 is secure and proving it.

The Business Case for an M365 Security Assessment

For IT and security leaders, the value is operational. The assessment identifies the Microsoft 365 risks most likely to create real exposure and turns them into prioritized remediation work.

For compliance leaders, the value is evidence. Findings can be mapped to frameworks and control expectations, supporting audit readiness and governance.

For executives and boards, the value is clarity. Instead of abstract technical gaps, leadership receives a business-aligned view of where Microsoft 365 risk exists, what needs to be fixed first, and how remediation reduces exposure.

InfoSight’s assessment deliverables include a comprehensive risk report, prioritized findings, business-impact scoring, ticket-ready remediation tasks, audit trail and change-log support, Secure Score improvement planning, optional continuous monitoring, and compliance mapping to frameworks including NIST 800-53, ISO 27001, HIPAA, and PCI DSS.

Microsoft 365 Security Requires Continuous Validation

The recent Microsoft Defender vulnerabilities are another reminder that security tools, cloud platforms, and identity systems must be continuously validated. Attackers are not waiting for annual reviews. They are actively looking for weak configurations, delayed patching, excessive privileges, unmanaged sharing, and identity paths that let them move deeper into the environment.

Microsoft 365 gives organizations powerful security capabilities. But those capabilities only reduce risk when they are correctly configured, monitored, tested, and aligned to the organization’s operating reality.

InfoSight’s Microsoft 365 Security Assessment helps organizations move from default settings and assumed protection to validated security, prioritized remediation, and measurable risk reduction across the Microsoft 365 environment.

Ready to validate your Microsoft 365 environment before attackers test it for you?

Schedule an InfoSight Microsoft 365 Security Assessment to uncover identity, email, collaboration, data-sharing, and configuration risks across Exchange Online, Teams, SharePoint, OneDrive, and Azure AD/Microsoft Entra ID.