Security Policy Errors

Fix Office Apps Cannot Share Files Due to Defender ASR Rules

Muhammad Talha Oct 1, 2026 869 views

Question details

Users are unable to use the 'Share > Attach a Copy' feature in Word and Excel to send files via Outlook.

How to Fix Office Apps Failing to Share Files Because of Defender ASR Rules

Product: Microsoft Office, Microsoft Defender
Device & OS: not provided
Scenario: Attempting to share a document as an email attachment directly from within Word or Excel.
Observed behavior: The action fails because Microsoft Defender Attack Surface Reduction (ASR) rules block the creation of Office child processes, though files can still be attached manually within Outlook.

Before you start

Verify whether your system is using the classic Outlook desktop application or the New Outlook, and ensure you have administrative access to your Microsoft Defender security policies.

Solution 1

Set Classic Outlook as the Default Mail Client

Desktop Office applications rely on MAPI support to share files, which is supported by classic Outlook but not by the New Outlook.

The New Outlook is not MAPI-compliant, which prevents traditional desktop Office applications like Word and Excel from successfully handing off file attachments.

Open Windows Default Apps

Press the Windows key, type 'Default apps' in the search bar, and hit Enter to open your system settings.

Change the default email application

Scroll down to the 'Email' section and switch the default application from New Outlook to the classic Outlook.exe client.

Restart and test

Restart Word or Excel and try using the 'Share > Attach a Copy' feature again to confirm if the email initiates.

Solution 2Recommended

Configure ASR Exclusions via Microsoft Defender

Identify the specific Attack Surface Reduction rule blocking Office applications and create a minimal exception without disabling your overall security posture.

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Frequently Asked Questions

Why does attaching a file directly in Outlook work, but sharing from Word fails?

Attaching a file manually from within Outlook does not trigger the creation of a child process. Conversely, using 'Share > Attach a Copy' in Word attempts to launch Outlook as a child process, which Defender ASR rules frequently block to prevent macro-based malware attacks.

Why isn't the 'New Outlook' compatible with Word and Excel sharing features?

Desktop Office applications require MAPI (Messaging Application Programming Interface) to send emails directly from the app interface. The 'New Outlook' is currently not MAPI-compliant, preventing these built-in sharing features from functioning properly.

Should I just disable the ASR rules completely to resolve this sharing error?

No. Broadly disabling ASR rules or leaving them 'Not Configured' creates a significant security vulnerability, as the blocked behavior (Office child processes) is a very common vector for malicious attacks. It is highly recommended to use minimal, well-documented exclusions instead.

Microsoft Office Security Issues