CVE-2025-27752: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2025-27752 Overview

CVE-2025-27752 is a heap-based buffer overflow vulnerability in Microsoft Office Excel. The flaw, classified as [CWE-122], allows an unauthorized attacker to execute arbitrary code locally on a vulnerable system. Exploitation requires user interaction, typically opening a crafted Excel document. The vulnerability affects Microsoft 365 Apps, Microsoft Office 2016 and 2019, and the Microsoft Office Long Term Servicing Channel (LTSC) 2021 and 2024 on both Windows and macOS. Microsoft published the advisory on April 8, 2025. The vulnerability carries a CVSS 3.1 score of 7.8 and an EPSS probability of 0.903% (75.95th percentile).

Critical Impact
Successful exploitation grants the attacker code execution in the context of the current user, enabling further compromise of confidentiality, integrity, and availability.

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-2025-27752 published to NVD
2025-04-08 - Microsoft releases security advisory and patch guidance
2025-07-09 - Last updated in NVD database

Technical Details for CVE-2025-27752

Vulnerability Analysis

CVE-2025-27752 is a heap-based buffer overflow [CWE-122] in the Microsoft Office Excel parsing logic. Excel allocates a heap buffer to process structured spreadsheet content, but a length or boundary check is missing or incorrect. When a crafted document supplies oversized or malformed data, Excel writes past the end of the allocated heap region. The overflow corrupts adjacent heap metadata or function pointers, which an attacker can leverage to redirect execution.

The attack vector is local and requires user interaction, consistent with file-format exploitation through email attachments, shared drives, or web downloads. An attacker who convinces a user to open a weaponized .xls, .xlsx, or related Excel file can run code in the user's security context. The flaw impacts confidentiality, integrity, and availability.

Root Cause

The root cause is improper validation of input size before writing to a heap-allocated buffer during Excel's document parsing. Specific technical details have not been disclosed by Microsoft in the public advisory. See the Microsoft CVE-2025-27752 Advisory for vendor guidance.

Attack Vector

An attacker crafts a malicious Excel document containing the trigger data and delivers it through phishing, watering-hole sites, or shared storage. When the victim opens the file, Excel parses the malformed structure and triggers the heap overflow, leading to local code execution. No prior authentication is required, but the user must open the file.

No verified proof-of-concept code is publicly available. The vulnerability mechanism is described in prose without synthetic code per disclosure constraints.

Detection Methods for CVE-2025-27752

Indicators of Compromise

Excel processes (EXCEL.EXE) spawning unusual child processes such as cmd.exe, powershell.exe, rundll32.exe, or wscript.exe.
Office applications writing executable files or scripts to user-writable directories such as %TEMP%, %APPDATA%, or Downloads.
Excel making outbound network connections to unfamiliar domains immediately after opening a document.
Crash dumps or Windows Error Reporting entries for EXCEL.EXE referencing heap corruption.

Detection Strategies

Hunt for Excel child-process anomalies using EDR telemetry, focusing on script interpreters and LOLBins launched from Office binaries.
Inspect inbound email attachments and shared documents for Excel files with embedded objects, unusual streams, or anomalous record structures.
Correlate document-open events with subsequent process creation and network activity to surface exploitation chains.

Monitoring Recommendations

Enable and forward Microsoft-Windows-Sysmon and Office telemetry to a centralized SIEM for behavioral analysis.
Apply Microsoft Defender Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and child executable content.
Monitor patch compliance for Microsoft 365 Apps, Office 2016/2019, and Office LTSC 2021/2024 across the fleet.

How to Mitigate CVE-2025-27752

Immediate Actions Required

Apply Microsoft's April 2025 security updates for all affected Office versions as referenced in the Microsoft CVE-2025-27752 Advisory.
Prioritize patching endpoints used by high-risk users such as executives, finance, and HR who frequently receive external spreadsheets.
Validate that Microsoft 365 Apps clients have automatic update channels enabled and are running a fixed build.

Patch Information

Microsoft released fixes for CVE-2025-27752 on April 8, 2025. Administrators should consult the Microsoft Security Response Center advisory to identify the exact build numbers for Microsoft 365 Apps, Office 2016, Office 2019, and Office LTSC 2021/2024 (Windows and macOS), then deploy through Microsoft Update, WSUS, Intune, or Configuration Manager.

Workarounds

Open untrusted Excel files only in Protected View and block macros from files originating from the internet via Group Policy.
Enable Microsoft Defender ASR rules Block all Office applications from creating child processes and Block Office applications from injecting code into other processes.
Strip or quarantine Excel attachments at the email gateway when senders are unknown or files originate from external sources.
Restrict standard users from running with administrative privileges to limit the impact of post-exploitation activity.

bash

# Enable Microsoft Defender ASR rules to reduce Office exploitation risk
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled
Set-MpPreference -AttackSurfaceReductionRules_Ids 75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84 -AttackSurfaceReductionRules_Actions Enabled