CVE-2025-27749: Microsoft 365 Apps Use After Free Flaw

CVE-2025-27749 Overview

CVE-2025-27749 is a use-after-free vulnerability [CWE-416] in Microsoft Office that allows an unauthorized attacker to execute arbitrary code locally. The flaw affects Microsoft 365 Apps, Microsoft Office 2016 and 2019, and Microsoft Office Long Term Servicing Channel 2021 and 2024 on both Windows and macOS. Exploitation requires the target user to open a crafted Office document, which triggers memory corruption and enables code execution within the user context. Microsoft published the advisory on April 8, 2025 as part of its security update guide.

Critical Impact
Successful exploitation grants attackers the ability to execute code with the privileges of the current user, leading to full compromise of confidentiality, integrity, and availability on the affected host.

Affected Products

Microsoft 365 Apps (Enterprise)
Microsoft Office 2016 and Microsoft Office 2019
Microsoft Office Long Term Servicing Channel 2021 and 2024 (Windows and macOS)

Discovery Timeline

2025-04-08 - CVE-2025-27749 published to NVD and Microsoft releases security update
2025-07-09 - Last updated in NVD database

Technical Details for CVE-2025-27749

Vulnerability Analysis

The vulnerability is a use-after-free condition in Microsoft Office. Use-after-free flaws occur when an application continues to reference memory after it has been freed, allowing an attacker to manipulate the freed allocation and influence subsequent program behavior. In this case, the dangling pointer can be steered to attacker-controlled data, enabling arbitrary code execution within the Office process.

The attack vector is local and requires user interaction. An attacker delivers a malicious Office document through email, file share, or web download. When the victim opens the document, Office parses embedded structures that trigger the unsafe object reuse condition. Execution proceeds in the security context of the user opening the file, so a non-administrative target still yields meaningful access for follow-on activity such as credential theft, lateral movement, or ransomware staging.

Root Cause

The root cause is improper lifetime management of an internal Office object, classified as [CWE-416] Use After Free. After the object is released, a stale pointer remains in use, allowing controlled reallocation of the freed region and corruption of program state during document parsing or rendering.

Attack Vector

Delivery typically relies on social engineering. A weaponized .docx, .xlsx, .rtf, or related Office file is sent to the target. Opening the file, or in some configurations previewing it, is sufficient to trigger the vulnerable code path. No network authentication is required, and the Preview Pane has historically expanded the exposure of similar Office memory-corruption issues.

No verified public proof-of-concept code is available for CVE-2025-27749. Refer to the Microsoft Security Update Guide for CVE-2025-27749 for vendor technical details.

Detection Methods for CVE-2025-27749

Indicators of Compromise

Unexpected child processes spawned by winword.exe, excel.exe, or powerpnt.exe, particularly cmd.exe, powershell.exe, wscript.exe, mshta.exe, or rundll32.exe.
Office applications writing executable content (.exe, .dll, .ps1, .hta) to user-writable locations such as %TEMP%, %APPDATA%, or %PUBLIC%.
Office processes initiating outbound network connections to uncategorized or newly registered domains shortly after a document open event.
Crash events or Watson reports referencing Office modules with access-violation exceptions consistent with use-after-free corruption.

Detection Strategies

Hunt for Office-to-shell process lineage across endpoint telemetry, correlating parent-child relationships with command-line arguments containing encoded payloads.
Inspect inbound email attachments and SharePoint uploads for Office documents containing unusual embedded objects, OLE streams, or external relationships.
Apply behavioral rules that flag Office applications loading non-standard DLLs or invoking COM components outside of expected baselines.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction rules that block Office from creating child processes and from injecting code into other processes.
Forward Office, Sysmon (Event IDs 1, 7, 11), and Windows Defender telemetry to a centralized SIEM for cross-host correlation.
Track patch deployment status of all CPE-listed Office builds and alert on hosts that remain unpatched beyond the maintenance window.

How to Mitigate CVE-2025-27749

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory for CVE-2025-27749 to all affected Office and Microsoft 365 Apps installations.
Prioritize patching on hosts that routinely handle external Office documents, including executive assistants, finance, HR, and helpdesk endpoints.
Verify update channels for Microsoft 365 Apps so client devices receive the fixed build automatically.

Patch Information

Microsoft released a security update for CVE-2025-27749 on April 8, 2025. The fix is distributed through Microsoft Update, the Microsoft 365 Apps update channels, and the Microsoft Update Catalog. Detailed build numbers per product and channel are listed in the Microsoft Security Update Guide for CVE-2025-27749.

Workarounds

Enable Protected View and Office Application Guard so untrusted documents open in an isolated container.
Block macros in files originating from the internet via Group Policy and disable the Outlook Reading Pane for untrusted mailboxes.
Restrict delivery of high-risk Office file types at the email gateway and apply allow-listing for known business senders where feasible.

bash

# Configuration example: enforce Protected View and block internet macros via Group Policy registry keys
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security" /v BlockContentExecutionFromInternet /t REG_DWORD /d 1 /f