Microsoft 365 Security Update: Blocking Legacy Authentication for Improved Cloud Security

Microsoft’s recent announcement to update security defaults for all Microsoft 365 tenants marks a significant move towards modernizing cloud security and reducing risk exposures for organizations worldwide. Starting in July, the rollout will see Microsoft 365—encompassing platforms such as SharePoint, OneDrive, and Office files—block access via legacy authentication protocols, including RPS (Relying Party Suite) and FPRPC (FrontPage Remote Procedure Call), with full enforcement expected by August. This update aligns with Microsoft’s broader Secure Future Initiative (SFI) and its “Secure by Default” principle, designed to deliver a hardened, baseline security posture for enterprises of all sizes.

Understanding Legacy Authentication and Its Risks​

What Are Legacy Authentication Protocols?​

Legacy authentication protocols refer to outdated forms of verifying user identities—predominantly pre-dating modern, token-based security methods such as OAuth 2.0 or SAML. Notable protocols impacted by the Microsoft 365 update include RPS and FPRPC:

• Relying Party Suite (RPS): Used by older browser or client applications to access cloud resources without modern security.
• FrontPage Remote Procedure Call (FPRPC): Historically used for remote web authoring and Office file manipulation.

Both have been largely replaced in the ecosystem but have persisted due to compatibility with legacy applications and automated tools still in use in some organizations.

Inherent Weaknesses​

Legacy protocols commonly lack support for multi-factor authentication (MFA), are susceptible to brute-force and phishing attacks, and make it difficult for administrators to enforce security policies. According to Microsoft, blocking these protocols “prevents applications that are using outdated methods from accessing SharePoint and OneDrive via browser,” greatly reducing the attack surface presented to bad actors exploiting these weaknesses.

Scope and Timeline of the Update​

Rollout Schedule and Coverage​

Microsoft’s changes will begin mid-July 2025, aiming for global completion by August 2025. The update will be applied by default—no additional licensing required—to all Microsoft 365 tenants, cutting across:

• Microsoft Entra (Azure Active Directory successor)
• Microsoft 365 apps
• SharePoint Online
• OneDrive for Business

Organizations should anticipate automatic changes, without the need for user intervention, to how applications and users authenticate with these resources.

Technical Changes in Authentication Flow​

The new security defaults entail:

• Blocking RPS authentication for SharePoint and OneDrive via legacy browsers
• Blocking FPRPC protocol for Office file opens, halting use of non-modern protocols in Microsoft 365 clients

Microsoft’s rationale references the well-documented vulnerabilities of these legacy methods. Blocking these connections not only obviates an easy vector for brute-force attacks but also constrains attackers’ ability to phish credentials or subvert weak authentication mechanisms.

Impacts on User Experience and IT Admins​

Direct Effects on Users​

Most modern clients and up-to-date browsers already utilize modern authentication. Thus, everyday users on current Office apps or browsers should see little to no disruption. However, workflows or automated tasks that depend on “old school” protocols will fail to connect until updated to support modern authentication.

Potential User Pain Points​

• Older Add-ins and Tools: Legacy Office add-ins or custom integrations using deprecated protocols will lose access.
• Scripts and Automation: Any automated scripts relying on RPS or FPRPC for batch processing, file syncs, or reporting will break unless rewritten.
• Minimal Legacy App Support: Organizations with critical legacy systems may need temporary workarounds or expedited upgrades.

New Admin Consent Policies for Third-Party Apps​

A major security boost comes from changes to application access permissions. Post-update, Microsoft 365 tenants will require explicit admin consent for third-party apps to access files and sites. Users will no longer be able to grant access for apps by themselves, which previously led to inadvertent overexposure of organizational data.
Microsoft will deploy “Microsoft-managed App Consent Policies,” effectively locking out third-party requesters until an admin has reviewed and approved their permission requests. Granular policies can be configured, so approvals are only granted for named users or high-trust groups. This mitigates one of the growing attack vectors—malicious or negligent OAuth applications siphoning sensitive documents and data from cloud-hosted repositories.
For guidance, Microsoft references Entra documentation that enables IT teams to configure these granular consent and access controls, further strengthening the “least privilege” model within organizations.

Critical Analysis: Benefits and Cautions​

Notable Strengths​

1. Improved Security Baseline​

Blocking legacy authentication protocols dramatically shrinks the opportunities for credential-based attacks, which have been a top concern in security incident reports for years. Attacks leveraging weak protocols like basic authentication have accounted for a disproportionate share of successful breaches.

2. Alignment with Compliance and Best Practices​

The move reflects industry shifts towards zero-trust and secure-by-default environments, aiding organizations in meeting regulatory compliance benchmarks for data protection and risk management.

3. Reduced Admin Overhead​

By preventing users from auto-granting access to third-party apps, organizations save time spent remediating overpermissive app access and chasing down shadow IT incidents. The introduction of admin consent workflows further centralizes control, ensuring better oversight.

4. Licensing-Simplified​

Crucially, these security upgrades do not require additional license purchases, removing cost barriers to adoption. Microsoft is clear that all tenants—regardless of size or subscription tier—receive the benefit.

Potential Risks and Considerations​

1. Disruption for Legacy Workflows​

Organizations with embedded legacy tools may find themselves unprepared for sudden access failures. Despite Microsoft’s multi-month rollout window, the company encourages early evaluation of existing apps and workflows for dependencies on soon-to-be-blocked protocols.

2. Migration Risks​

Rushed migration of critical business automation can introduce service gaps or errors, especially in large enterprises with complex heritage systems. Planning and testing must begin well ahead of July’s enforcement window.

3. Limited Flexibility​

There’s limited flexibility for exceptions: Microsoft is not offering a “grace period” or phased opt-out for legacy workflows. This can create immediate issues for regulated industries or specialized businesses with niche requirements.

4. Potential for Increased Admin Load Initially​

The new admin consent requirements, while improving security, could bottleneck productivity if IT departments are not equipped to quickly review and approve third-party app access requests. Preparation, including process documentation and communication to users, is essential.

Microsoft’s Broader “Secure by Default” Direction​

This strategic posture is part of Microsoft’s ongoing Secure Future Initiative (SFI), aimed at recalibrating default cloud settings to meet contemporary cybersecurity challenges. According to Microsoft, these defaults represent “the first step in a broader effort to evaluate and evolve Microsoft 365 defaults through the lens of security best practices.”
Additional measures on the roadmap include:

• Disabling ActiveX Controls: Since early 2025, Microsoft has started disabling all ActiveX controls within Windows versions of Microsoft 365 and Office 2024 to preempt exploitation via embedded or downloadable macros.
• Screenshot Blocking in Teams: Beginning July, Teams will block screenshots during meetings, preventing data leakage from visual eavesdropping.
• Expanded Blocklists for Outlook Attachments: New file types such as .library-ms and .search-ms will be blocked as attachments in Outlook, closing off lesser-known attack routes.

Industry analysts are largely supportive of these changes as essential steps to counter persistent threats and social engineering attempts exploiting outdated technology, even while acknowledging the challenge for IT shops burdened by legacy dependencies.

Recommendations for IT Managers and Security Teams​

Immediate Actions​

• Inventory Your Applications: Use Microsoft’s reporting tools to discover which, if any, users or processes are still leveraging RPS, FPRPC, or other deprecated methods.
• Engage Stakeholders: Communicate early with business unit leaders whose teams might be affected. Prepare clear guidance on what is changing and why.
• Test and Validate Modern Authentication: Where legacy methods are in use, plan and execute migrations, leveraging Microsoft’s documentation to minimize friction.
• Update Internal Documentation: Revise onboarding, training, and support documentation covering file access and integration workflows for end-users.
• Plan for Consent Management: Prepare for increased admin requests by documenting frameworks for app approval, triage processes, and escalation chains.

Long-term Considerations​

• Embrace Zero Trust: Beyond compliance, these changes nudge businesses toward adopting zero-trust architectures, where no user or device is implicitly trusted, and all access is continually verified.
• Review Vendor Relationships: Check with third-party vendors to verify product readiness for modern authentication and admin consent policies, requesting roadmaps and timelines where necessary.
• Monitor for Upcoming Changes: Microsoft has indicated further evolution of defaults and is likely to phase out even more legacy features—remaining vigilant ensures ongoing compliance and minimizes surprise outages.

Broader Industry Context​

Microsoft’s approach is mirrored by similar moves at other cloud vendors: Google, AWS, and others have all intensified efforts to retire legacy protocols, integrate admin controls, and require modern authentication. This convergence underscores a consensus that cloud security must evolve faster than the threat landscape, and default settings should not lag behind best practices due to inertia.
According to multiple independent security advisories and industry reports, credential stuffing—the practice of automated login attempts using weak or reused credentials—remains rampant in environments supporting legacy auth. Organizations are increasingly liable for breaches occurring due to deprecated settings, emphasizing the need for proactive stance.

Caution and Caveats​

While the benefits are substantial and well-justified by industry data, it is essential to approach implementation with careful planning. Microsoft’s message focuses on protection, yet there is limited published data on the number of enterprises still relying on legacy protocols, which means the scope of potential short-term disruption is hard to quantify externally. Some private-sector IT forums report that niche industries with proprietary integrations may struggle with cut-over deadlines, flagging the need for industry-specific consultation or escalation.

Conclusion​

The scheduled blocking of file access through legacy authentication protocols in Microsoft 365 represents a pivotal moment in cloud security evolution. Raising the baseline—by removing historic vulnerabilities and defaulting to administrator-controlled third-party access—fortifies the platform in the face of modern threats. The long-term dividends in risk reduction, compliance, and management efficiency are clear. However, organizations must heed this transition as a call to inventory, test, and modernize their own application estates, avoiding last-minute surprises and service interruptions. For those still running on borrowed time with legacy protocols, the countdown to July’s enforcement has already begun—a proactive, well-coordinated response will ensure the transition brings only benefits and not disruption.
For more technical guidance, IT managers are encouraged to review Microsoft Entra’s support documentation and leverage the company's available tooling for both usage discovery and migration. As Microsoft continues to fine-tune and elevate security defaults, organizations that embrace these changes will find themselves more resilient and better positioned for a cloud-first, threat-aware future.

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Source: BleepingComputer Microsoft 365 to block file access via legacy auth protocols by default