Microsoft continues to reshape the security landscape for Windows users and administrators with a series of strategic changes to how its flagship productivity apps handle potentially risky content. A landmark update is scheduled to roll out between October 2025 and July 2026: Excel will disable external workbook links pointing to file types that are classified as blocked by the Microsoft Office Trust Center. On the surface, this may appear to be a technical nuance, but in reality, it is emblematic of a broader, multi-year transformation of Microsoft’s security philosophy—a journey that underscores both the shifting nature of cyber threats and the company’s response to a rapidly evolving digital ecosystem.
The attack surface presented by Office applications has, for decades, been a favorite among cybercriminals looking to gain a foothold in corporate or personal systems. As collaborative features flourished—from macros to workbook links and add-in support—the potential for abuse grew exponentially. Microsoft’s move to block external workbook links to specific file types is the latest step in a deliberate campaign to curtail these risks.
Historically, Excel users could reference data from a wide range of external files—even those considered high-risk or "blocked" according to the Trust Center's evolving list. These links, while powerful for legitimate workflows, also presented attackers with an alluring vector: by simply having a workbook reference a malicious (or compromised) file, threat actors could initiate data leaks, malware infections, or phishing campaigns without much user intervention. Recent high-profile attacks have demonstrated exactly how such mechanics can be weaponized, and Microsoft’s security telemetry appears to have tipped the balance toward user safety over legacy flexibility.
With the so-called “FileBlockExternalLinks” group policy taking effect in Build 2510, if this policy is left unconfigured, no immediate change will occur. However, starting in October 2025, the default behavior across Microsoft 365 and Office 2024 versions will be to block external workbook links to any file types currently managed by the Trust Center. Attempts to refresh such references will result in a #BLOCKED error or, in some cases, simply fail to retrieve new data, effectively neutering the threat vector.
—administrators can re-enable the ability to refresh or create new references to these blocked file types. This requires careful consideration, as doing so may reintroduce some of the very risks Microsoft is trying to mitigate. Microsoft’s documentation stresses that careful review of dependencies and workflows is essential before making such changes, especially in environments with high sensitivity to data breaches or malware.
Anecdotal evidence from IT forums and Microsoft’s own support channels suggests that “shadow IT” (unofficial or user-built workbooks and macros) frequently make use of interlinked files. In many cases, these dependencies are poorly documented, raising the risk of silent breakage come enforcement day.
By blocking Excel workbook links to risky file types, Microsoft both closes an exploited attack vector and stakes out a philosophical position: Legacy convenience must yield to clear, present security imperatives. Beta feedback and evolving threat data will likely inform further refinement of these controls, but the general principle—building secure defaults and allowing local override sparingly—appears firmly entrenched.
Such measures, layered alongside broader hardening of the Office suite, point to a future in which user safety trumps all but the most foundational compatibility requirements. For end users, the message is clear: Expect the guardrails to get higher with each passing year, and plan your workflows with both convenience and cyber resilience in mind. For IT and security professionals, the imperative is unmistakable—audit now, educate continuously, and embrace the era of secure-by-default productivity.
Source: BleepingComputer Microsoft to disable Excel workbook links to blocked file types
The Evolution of Trust and File Handling in Microsoft Office
The attack surface presented by Office applications has, for decades, been a favorite among cybercriminals looking to gain a foothold in corporate or personal systems. As collaborative features flourished—from macros to workbook links and add-in support—the potential for abuse grew exponentially. Microsoft’s move to block external workbook links to specific file types is the latest step in a deliberate campaign to curtail these risks.Historically, Excel users could reference data from a wide range of external files—even those considered high-risk or "blocked" according to the Trust Center's evolving list. These links, while powerful for legitimate workflows, also presented attackers with an alluring vector: by simply having a workbook reference a malicious (or compromised) file, threat actors could initiate data leaks, malware infections, or phishing campaigns without much user intervention. Recent high-profile attacks have demonstrated exactly how such mechanics can be weaponized, and Microsoft’s security telemetry appears to have tipped the balance toward user safety over legacy flexibility.
How the New Policy Will Work: Blocking Untrusted File Types in Excel Workbooks
Microsoft’s technical approach centers on expanding the File Block Settings in Office, specifically through the introduction of a new policy: “FileBlockExternalLinks.” This policy will give administrators granular control over what happens when Excel workbooks attempt to reference file types that the Trust Center already deems unsafe or unsupported.What Are Blocked File Types?
Excel’s Trust Center maintains a dynamic list of file types that are not considered safe for interaction. These include older Office formats, certain script files, database files (such as .mdb and .accdb), executables (.exe, .bat, .cmd), and newer additions like .library-ms and .search-ms. Many of these have legitimate purposes, but all have been implicated in real-world attack scenarios—from ransomware delivery to credential harvesting.Timeline and Default Behavior
Microsoft 365 customers will see warning messages beginning with Excel Build 2509. These warnings will appear on the business bar whenever a workbook with external links to blocked file types is opened, alerting users that their workflows may soon be disrupted.With the so-called “FileBlockExternalLinks” group policy taking effect in Build 2510, if this policy is left unconfigured, no immediate change will occur. However, starting in October 2025, the default behavior across Microsoft 365 and Office 2024 versions will be to block external workbook links to any file types currently managed by the Trust Center. Attempts to refresh such references will result in a #BLOCKED error or, in some cases, simply fail to retrieve new data, effectively neutering the threat vector.
Administrative Override and Granularity
While the new default state aims to protect the broadest possible user base, Microsoft does offer a granular override path for IT professionals. By editing the Windows registry key at:HKCU\Software\Microsoft\Office\<version>\Excel\Security\FileBlock\FileBlockExternalLinks—administrators can re-enable the ability to refresh or create new references to these blocked file types. This requires careful consideration, as doing so may reintroduce some of the very risks Microsoft is trying to mitigate. Microsoft’s documentation stresses that careful review of dependencies and workflows is essential before making such changes, especially in environments with high sensitivity to data breaches or malware.
Security Risks Addressed by the Policy: What’s at Stake?
The rationale behind this stringent new policy is rooted in a history of real-world attacks. Excel’s ability to reference data externally, while designed for collaboration and efficiency, has just as easily enabled cybercriminals to insert themselves into trusted workflows. Notably:- Phishing Campaigns: Malicious Excel files can include links out to external workbooks hosted on compromised or attacker-controlled servers, tricking users into fetching and executing payloads.
- Data Exfiltration: Automated links to remote databases or files can quietly siphon sensitive data out of corporate environments.
- Malware and Ransomware Delivery: Exploitable file types—such as older Office file formats or scripts—can act as vehicles for advanced malware, exploiting vulnerabilities in Excel’s file handlers or external data functions.
- Persistence and Lateral Movement: Linked workbooks can be leveraged to maintain persistence within an organization, evading simple detection or removal.
The Bigger Picture: Microsoft’s Multi-Year Hardening of the Office Platform
The “FileBlockExternalLinks” policy is not an isolated measure but, rather, a piece of a comprehensive modernization of Office’s security fabric. Since at least 2018, Microsoft has been incrementally closing attack surfaces long exploited by malicious actors. Key milestones in this campaign include:- Antimalware Scan Interface (AMSI) Integration: Enabling Office client apps to scan dynamic code—including macros—for suspicious behavior before execution.
- VBA Macro Blocking: Beginning in 2022, Microsoft started blocking VBA macros from files downloaded from the internet by default. This controversial move was lauded by most security professionals and academically shown to disrupt common malware delivery chains.
- XLM Macro Protection: Excel 4.0 (XLM) macros, an outdated macro language, became a major target of threat actors. Microsoft responded by disabling XLM macros by default and offering administrators custom controls over their usage.
- XLL Add-in Blocking: In response to a surge of attacks using untrusted XLL add-ins, Microsoft delivered a policy-based method to block such add-ins by default for Microsoft 365 tenants.
- Rolling Retirement of VBScript: The company has publicly committed to disabling VBScript across Windows editions, a move that removes an entire class of potential exploits from would-be attackers.
- Blocking New File Types: In tandem with the latest Excel update, Microsoft has expanded the list of file formats blocked as email attachments in Outlook (notably .library-ms and .search-ms), further shrinking attackable surfaces across the Office ecosystem.
- Deactivation of ActiveX Controls: Microsoft has started disabling all ActiveX controls in Windows versions of Microsoft 365 and Office 2024 apps, preemptively closing a frequently abused legacy feature.
Notable Strengths: What This Means for End Users and Admins
Security by Default
The most immediate benefit is a dramatic reduction in the exposure window for all users—even those unaware of technical risk factors. Administrators no longer need to race to configure policies in advance or worry about users inadvertently introducing risks through their own workflow habits. Instead, the default state is broadly defensive, which is a recognized best practice across the industry.Granular Administrator Control
While user-facing configuration is simplified, administrators retain the ability to selectively re-enable risky external links—should critical workflows depend on them—via registry edits and group policy. Microsoft’s rollout documentation encourages a careful audit of all existing workbooks with external references in advance of the policy change, a stance that supports both business continuity and security best practice.Improved Transparency
By surfacing clear warnings on the business bar, Microsoft provides users with advance notice of coming changes. This heads up period gives organizations ample time to retrain users, adjust workflows, and catch edge-case dependencies before the policy is enforced unilaterally.Alignment With Modern Threat Models
The move reflects a realistic assessment of modern attacker tactics. By breaking away from “permit by default,” Microsoft signals a serious commitment to customer safety—even at the risk of initially disrupting some established processes.Potential Risks and Areas of Concern
No policy fix is without trade-offs. Microsoft’s new approach, while security-centric, will inevitably pose challenges for segments of its user base—especially in large, complex environments with deep integration between legacy tools.Workflow Disruption and Legacy Dependencies
Perhaps the single greatest risk is to business processes that are entangled with external links to now-blocked file types. For some organizations—especially those in regulated sectors running specialized reporting or data ingestion workflows—this could break operational capability overnight unless surveyed and remediated in advance.Anecdotal evidence from IT forums and Microsoft’s own support channels suggests that “shadow IT” (unofficial or user-built workbooks and macros) frequently make use of interlinked files. In many cases, these dependencies are poorly documented, raising the risk of silent breakage come enforcement day.
User Confusion and Error Recovery
The introduction of the #BLOCKED error in affected workbooks, while clear in its intent, may not be self-explanatory for non-technical users. Organizations will need to prepare support documentation and user training resources to head off a surge of helpdesk tickets when links start to fail. Microsoft’s own documentation currently covers the high-level mechanism, but tailored guidance for edge cases may be necessary.Administrative Overhead
The recommended remediation—registry edits or custom group policy—raises its own risks if used incautiously. Allowing such controls can become an unwelcome loophole if over-applied or left unmonitored. For enterprises running mixed-version environments or supporting bring-your-own-device policies, ensuring consistent enforcement could prove burdensome.Incomplete Coverage of All Threats
While the new policy closes important pathways, no single control is a panacea. Attackers will continue to probe for unpatched vulnerabilities, exploit user trust, or leverage social engineering tactics beyond the scope of the newly blocked links. Microsoft’s policy must therefore be part of a layered security strategy, rather than a substitute for broader protections such as antimalware, endpoint detection and response (EDR), and zero-trust network segmentation.Guidance for Enterprises and Power Users
Business and IT leaders should take immediate, proactive steps to get ahead of the coming enforcement window. Microsoft’s own guidance, now being broadcast via the Microsoft 365 admin center and support documentation, is clear:- Conduct an Audit: Inventory all critical Excel workbooks, noting any and all external references—especially those connecting to file types now classified as blocked.
- Communicate Early: Educate users and key stakeholders about the policy change, the appearance of the #BLOCKED error, and the rationale behind it.
- Remediate Where Necessary: For essential workflows, consider migrating referenced data to supported file types or restructuring integrations.
- Limit Override to Documented Needs: Only re-enable blocked links via registry keys or group policy in tightly controlled circumstances, with capital accountability for exceptions.
- Monitor for Unexpected Impacts: After initial enforcement, track workbook errors and user reports to identify overlooked dependencies or emergent problems.
The Road Ahead: Microsoft’s Security Stance and Industry Trends
Microsoft’s posture in recent years reflects a broader movement across enterprise computing: features that were once prized for flexibility now undergo rigorous vetting for latent risk. As cloud adoption, “work from anywhere” policies, and regulatory scrutiny all climb, the calculus for what constitutes “safe by default” has shifted decisively. Productivity without protection is increasingly seen as a false economy.By blocking Excel workbook links to risky file types, Microsoft both closes an exploited attack vector and stakes out a philosophical position: Legacy convenience must yield to clear, present security imperatives. Beta feedback and evolving threat data will likely inform further refinement of these controls, but the general principle—building secure defaults and allowing local override sparingly—appears firmly entrenched.
Conclusion
The decision to block Excel external links to blocked file types by default is more than a technical tune-up; it signifies a profound change in the culture and strategy of secure collaboration. While disruptions are inevitable—especially for organizations slow to audit and refactor legacy workflows—the net impact will be a dramatic reduction in successful exploit attempts against unsuspecting users. By providing administrators with both warning and fine control, Microsoft treads a careful path between stringent protection and long-term compatibility.Such measures, layered alongside broader hardening of the Office suite, point to a future in which user safety trumps all but the most foundational compatibility requirements. For end users, the message is clear: Expect the guardrails to get higher with each passing year, and plan your workflows with both convenience and cyber resilience in mind. For IT and security professionals, the imperative is unmistakable—audit now, educate continuously, and embrace the era of secure-by-default productivity.
Source: BleepingComputer Microsoft to disable Excel workbook links to blocked file types
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