CVE-2025-21383: Microsoft Excel Information Disclosure

CVE-2025-21383 Overview

CVE-2025-21383 is an information disclosure vulnerability in Microsoft Excel that allows a local attacker to read sensitive memory contents when a user opens a crafted spreadsheet. The flaw is tracked as an out-of-bounds read [CWE-125] and affects multiple Microsoft Office and Microsoft 365 Apps deployments across Windows and macOS. Exploitation requires user interaction, typically opening an attacker-supplied .xlsx or related Excel file. Microsoft addressed the issue in the February 2025 Patch Tuesday release.

Critical Impact
Successful exploitation exposes uninitialized memory contents from the Excel process, which may include document data, credentials, or address-space information usable to bypass memory protections.

Affected Products

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

Discovery Timeline

2025-02-11 - CVE-2025-21383 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-21383

Vulnerability Analysis

CVE-2025-21383 is classified as an out-of-bounds read [CWE-125] within Microsoft Excel file parsing. When Excel processes a malformed spreadsheet, the application reads memory beyond the intended buffer boundary. The disclosed bytes originate from adjacent heap or stack regions belonging to the Excel process.

The vulnerability impacts confidentiality only. Integrity and availability of the host remain intact, and no privilege escalation is possible through this bug alone. Attackers typically chain information disclosure flaws with a separate memory corruption vulnerability to defeat Address Space Layout Randomization (ASLR) and achieve reliable code execution.

Because exploitation requires the victim to open a file locally, delivery relies on phishing, malicious attachments, or file-sharing services. No network-facing attack surface exists.

Root Cause

The root cause is missing bounds validation on a length or offset field inside an Excel document structure. When the parser trusts the attacker-controlled value, it copies or references memory outside the allocated object, returning residual data into a location the attacker can observe. Microsoft has not publicly disclosed the specific file format record responsible.

Attack Vector

An attacker crafts a malicious Excel workbook and delivers it to a target through email, cloud storage, or a web download. When the user opens the file in a vulnerable Excel client, the parser triggers the out-of-bounds read. The leaked bytes can be exfiltrated through embedded objects, formula evaluation output, or by rendering the data into cells visible to the attacker in a returned document.

The vulnerability requires local file interaction and user interaction. Preview handlers and automated processing pipelines that render untrusted Office documents expand the exposure surface. See the Microsoft CVE-2025-21383 Update Guide for vendor-specific technical detail.

Detection Methods for CVE-2025-21383

Indicators of Compromise

Excel spreadsheets received from untrusted senders that contain malformed or non-standard OOXML records
EXCEL.EXE spawning unusual child processes or reading unexpected memory regions during document open
Outbound network connections from Excel to attacker-controlled infrastructure shortly after a file open event
Office telemetry events indicating parser exceptions or Protected View bypass attempts

Detection Strategies

Inspect email gateways and file-sharing platforms for Excel attachments with anomalous binary structures or oversized record fields
Correlate Microsoft Office ETW and Defender telemetry with process behavior to identify parser crashes tied to specific document opens
Deploy Attack Surface Reduction (ASR) rules that block Office applications from creating child processes or injecting into other processes

Monitoring Recommendations

Track Office application versions across the fleet and flag hosts running Excel builds released before the February 2025 patch
Enable audit logging for document opens from untrusted network zones and internet-downloaded files marked with Mark-of-the-Web
Alert on Excel processes reading from or writing to unusual file paths, especially temporary or user profile directories used for staging

How to Mitigate CVE-2025-21383

Immediate Actions Required

Apply the February 2025 security update for Microsoft 365 Apps, Excel 2016, Office 2019, and Office LTSC 2021 and 2024 on both Windows and macOS
Enforce Protected View for documents originating from the internet, email attachments, and unsafe locations
Block or quarantine inbound Excel attachments from external senders pending inspection at the mail gateway

Patch Information

Microsoft released fixes as part of the February 2025 Patch Tuesday cycle. Administrators should consult the Microsoft CVE-2025-21383 Update Guide for channel-specific build numbers covering Current Channel, Monthly Enterprise Channel, Semi-Annual Enterprise Channel, and the LTSC releases. macOS builds of Office LTSC 2021 and 2024 require the corresponding Microsoft AutoUpdate package.

Workarounds

Disable Excel file preview handlers in Windows Explorer and Outlook to prevent automatic parsing of untrusted files
Restrict opening of legacy Excel formats using File Block Settings via Group Policy to limit exposure to malformed content
Require users to open Excel files only after validating the sender through out-of-band communication

bash

# Configuration example: enforce Protected View via Group Policy registry keys
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\excel\security\protectedview" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\excel\security\protectedview" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\excel\security\protectedview" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f