CVE-2025-53735: Microsoft 365 Apps Use-After-Free Flaw

CVE-2025-53735 Overview

CVE-2025-53735 is a use-after-free vulnerability [CWE-416] in Microsoft Office Excel that allows an unauthorized attacker to execute arbitrary code locally. The flaw exists across multiple Microsoft Office product lines, including Microsoft 365 Apps, Excel 2016, Office 2019, Office LTSC 2021/2024, and Office Online Server. Exploitation requires user interaction, typically convincing a target to open a crafted Excel document. Successful exploitation grants the attacker the same privileges as the current user and can lead to full compromise of confidentiality, integrity, and availability on the affected system.

Critical Impact
A crafted Excel file can trigger code execution in the context of the user, enabling malware installation, credential theft, and lateral movement from a single document open.

Affected Products

Microsoft 365 Apps (Enterprise, x86 and x64)
Microsoft Excel 2016, Microsoft Office 2019
Microsoft Office LTSC 2021 and 2024 (Windows and macOS), Microsoft Office Online Server

Discovery Timeline

2025-08-12 - CVE-2025-53735 published to NVD
2025-08-12 - Microsoft releases security update via MSRC advisory
2025-08-15 - Last updated in NVD database

Technical Details for CVE-2025-53735

Vulnerability Analysis

The vulnerability is a use-after-free condition in Microsoft Office Excel. Excel parses complex binary and XML-based spreadsheet structures, and certain object lifetimes are not correctly tracked during document processing. An attacker crafts a malicious workbook that releases a memory object while a pointer to that object remains in use. When Excel later dereferences the dangling pointer, the attacker can redirect execution to controlled memory.

Because the affected component runs in the user's session, code executes with the privileges of the user opening the document. Office macros are not required, which broadens the attack surface to Protected View bypass chains and preview-pane interactions in some configurations.

Root Cause

The root cause is improper management of object lifetime in Excel's document parsing logic, classified as [CWE-416] Use After Free. Excel frees an internal object but continues to reference it through a stale pointer. A crafted file shapes the freed memory region so that subsequent access invokes attacker-supplied data as a virtual function pointer or control structure.

Attack Vector

The attack vector is local and requires user interaction. An attacker delivers a malicious .xlsx, .xls, or related spreadsheet through email, file share, or web download. Opening the file in a vulnerable Excel build triggers the use-after-free and executes attacker code. No authentication is required on the target. See the Microsoft Security Update CVE-2025-53735 for vendor technical details.

// No verified public proof-of-concept code is available.
// Exploitation flow (conceptual):
//   1. Attacker crafts an Excel workbook containing a malformed object record.
//   2. Excel parses the record and frees an internal object.
//   3. A second record references the freed object via a dangling pointer.
//   4. Heap grooming places attacker-controlled data at the freed address.
//   5. Dereference of the stale pointer transfers control to attacker code.

Detection Methods for CVE-2025-53735

Indicators of Compromise

Excel processes (EXCEL.EXE) spawning unusual child processes such as cmd.exe, powershell.exe, rundll32.exe, or regsvr32.exe.
Unexpected outbound network connections initiated by EXCEL.EXE shortly after a document is opened.
Crash dumps or Windows Error Reporting events referencing access violations in Excel modules.
Spreadsheet files arriving from untrusted senders with embedded objects or unusual binary records.

Detection Strategies

Hunt for parent-child process anomalies where Office applications launch scripting or living-off-the-land binaries.
Inspect email and web gateways for Excel attachments and apply sandbox detonation prior to delivery.
Use YARA and static analysis to flag Excel files containing malformed records that target known UAF sinks.
Correlate Excel crash telemetry with subsequent suspicious process or persistence activity on the same host.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction rules that block Office apps from creating child processes.
Forward Sysmon process creation, image load, and network connection events to a centralized analytics platform.
Monitor file creation in user profile directories by Office processes for dropped executables or scripts.
Track Office version inventory to identify endpoints that remain unpatched against CVE-2025-53735.

How to Mitigate CVE-2025-53735

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory for CVE-2025-53735 to all affected Office installations.
Prioritize patching endpoints used by high-risk users such as finance, executives, and developers who routinely open external spreadsheets.
Verify Microsoft 365 Apps update channels are configured to receive current security builds without delay.

Patch Information

Microsoft has released security updates for all affected products through the August 2025 Patch Tuesday cycle. Administrators should review the Microsoft Security Update CVE-2025-53735 for product-specific build numbers covering Microsoft 365 Apps, Excel 2016, Office 2019, Office LTSC 2021, Office LTSC 2024, and Office Online Server.

Workarounds

Enforce Protected View and Block Macros from the Internet through Group Policy for all Office applications.
Disable the Outlook and File Explorer preview pane for Excel attachments to reduce automatic parsing of untrusted files.
Configure Attack Surface Reduction rules to prevent Office applications from spawning child processes and injecting into other processes.
Restrict opening of Excel files from untrusted network locations using File Block Settings in the Trust Center.

bash

# Example: enable ASR rule to block Office apps from creating child processes (PowerShell, run as admin)
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled

# Verify the rule is active
Get-MpPreference | Select-Object -ExpandProperty AttackSurfaceReductionRules_Ids