CVE-2025-53737: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2025-53737 Overview

CVE-2025-53737 is a heap-based buffer overflow in Microsoft Office Excel that allows an unauthorized attacker to execute code locally. The flaw affects multiple Office product lines, including Microsoft 365 Apps, Excel 2016, Office 2019, Office LTSC 2021/2024, and Office Online Server. Exploitation requires a user to open a crafted spreadsheet, which makes targeted phishing and document-delivery campaigns the most likely attack path. The vulnerability is tracked under [CWE-122] (heap-based buffer overflow) and [CWE-787] (out-of-bounds write), and Microsoft has issued an advisory through the Microsoft Security Response Center.

Critical Impact
A successful exploit grants the attacker code execution in the context of the current user, enabling malware installation, credential theft, and lateral movement from the compromised endpoint.

Affected Products

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

Discovery Timeline

2025-08-12 - CVE-2025-53737 published to the National Vulnerability Database
2025-08-15 - Last updated in NVD database

Technical Details for CVE-2025-53737

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in Excel's parsing logic. When Excel processes a malformed spreadsheet, it writes data past the bounds of a heap-allocated buffer, corrupting adjacent memory structures. An attacker who controls the overflow contents can overwrite function pointers, vtables, or object headers used by later Excel operations.

Because the bug combines a heap overflow [CWE-122] with an out-of-bounds write [CWE-787], it provides the primitives needed for reliable exploitation. Modern Office mitigations such as Address Space Layout Randomization (ASLR) and Control Flow Guard (CFG) raise the bar, but skilled attackers can chain information leaks within the same document to bypass them.

Execution occurs in the user-mode context of the Excel process, so the attacker inherits the privileges of the signed-in user. The EPSS score of 0.902 percent places this issue in the upper quartile of CVEs for likelihood of near-term exploitation.

Root Cause

The defect originates in Excel's handling of structured spreadsheet content, where a length or count field embedded in the file is trusted without bounds validation. The parser allocates a buffer sized for benign data and then copies attacker-controlled bytes into it, producing the overflow.

Attack Vector

Exploitation requires local user interaction. The attacker delivers a crafted .xls, .xlsx, or .xlsm file via email, cloud share, or web download, and convinces the target to open it. The Preview Pane is also a recognized trigger surface for similar Office parsing flaws. No network authentication is required, and no elevated privileges are needed on the target system.

The vulnerability mechanism is not accompanied by a public proof-of-concept. Detailed parsing internals are described in Microsoft's CVE-2025-53737 Security Update Guide entry.

Detection Methods for CVE-2025-53737

Indicators of Compromise

Excel spreadsheets arriving from external senders with unusually large or malformed embedded objects, drawings, or OLE streams
EXCEL.EXE spawning child processes such as cmd.exe, powershell.exe, rundll32.exe, regsvr32.exe, or mshta.exe
Unexpected outbound network connections initiated by EXCEL.EXE shortly after a document is opened
New persistence artifacts (Run keys, scheduled tasks, startup folder entries) created by an Excel-spawned process

Detection Strategies

Hunt for process-tree anomalies where Office applications launch scripting or living-off-the-land binaries
Apply YARA and Attack Surface Reduction (ASR) rules that flag suspicious OLE, drawing, and formula structures in Office files
Correlate document open events with subsequent file writes to %APPDATA%, %TEMP%, or %PROGRAMDATA%
SentinelOne Singularity Endpoint applies behavioral AI to identify Office-borne exploits by detecting anomalous child processes and memory activity originating from EXCEL.EXE without relying on file signatures

Monitoring Recommendations

Forward Office telemetry, Sysmon process and image-load events, and EDR alerts into a centralized data lake for cross-host correlation
Track Excel crash events (Application Error 1000 with faulting module EXCEL.EXE) as potential exploitation attempts
Monitor email gateways and cloud storage for inbound spreadsheets with macros or external links from low-reputation sources

How to Mitigate CVE-2025-53737

Immediate Actions Required

Apply the Microsoft security update for CVE-2025-53737 across all affected Office channels as the primary remediation
Inventory endpoints to confirm patch deployment on Microsoft 365 Apps, Excel 2016, Office 2019, and Office LTSC 2021/2024 installations
Block inbound spreadsheets from untrusted senders at the email gateway until patching is verified
Enable Microsoft Defender Attack Surface Reduction rules that block Office applications from creating child processes

Patch Information

Microsoft has released fixes through the standard Office and Microsoft 365 update channels. Administrators should consult the Microsoft CVE-2025-53737 Update guidance for build numbers specific to each affected SKU, and deploy through Microsoft Update, Configuration Manager, or Intune.

Workarounds

Disable the Outlook and File Explorer Preview Pane to prevent automatic rendering of untrusted spreadsheets
Enforce Protected View and Office File Block policies for files originating from the internet or untrusted locations
Restrict execution of macros from documents sourced from the internet using Group Policy
Run Office under standard (non-administrator) user accounts to limit the impact of successful exploitation

bash

# Example Group Policy registry settings to harden Excel against malicious documents
# Force Protected View for files originating from the internet
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f

# Block macros in files from the internet
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security" /v BlockContentExecutionFromInternet /t REG_DWORD /d 1 /f

# Disable the Preview Pane handler for Excel files
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\FileBlock" /v OpenInProtectedView /t REG_DWORD /d 1 /f