CVE-2026-55947: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-55947 Overview

CVE-2026-55947 is a heap-based buffer overflow in Microsoft Office Excel that allows an unauthorized attacker to execute code locally. The flaw is tracked under CWE-122 and affects multiple Excel and Microsoft 365 releases across Windows and macOS. Exploitation requires the target user to open a specially crafted spreadsheet, after which the attacker gains code execution in the context of the current user. Microsoft published the advisory on the Microsoft Security Update Guide.

Critical Impact
Successful exploitation grants arbitrary code execution in the victim's user context, enabling malware installation, data theft, and lateral movement from a single malicious Excel file.

Affected Products

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

Discovery Timeline

2026-07-14 - CVE-2026-55947 published to the National Vulnerability Database
2026-07-15 - Last updated in NVD database

Technical Details for CVE-2026-55947

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow inside Microsoft Excel's file parsing logic. When Excel processes a malformed spreadsheet, it writes past the bounds of a heap-allocated buffer. The overflow corrupts adjacent heap metadata or object pointers, which an attacker can shape to hijack control flow. Because the corruption occurs during document parsing, no additional privileges are required beyond a user opening the file. The resulting code execution runs with the privileges of the Excel process, typically the interactive user.

Root Cause

The issue is classified under [CWE-122: Heap-based Buffer Overflow]. It stems from missing or insufficient bounds checking when Excel copies attacker-controlled data from a crafted workbook into a fixed-size heap allocation. Undersized allocations combined with attacker-influenced length fields allow the copy operation to exceed the buffer boundary.

Attack Vector

The attack vector is local and requires user interaction. An attacker delivers a weaponized .xlsx, .xls, or related Excel file through phishing email, a malicious download, a network share, or a collaboration platform. When the victim opens the document in a vulnerable Excel build, the parser triggers the overflow. Preview handlers and Office Online Server rendering pipelines expand the attack surface where documents are auto-processed.

No public proof-of-concept exploit is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. See the Microsoft Security Update Guide CVE-2026-55947 advisory for vendor technical details.

Detection Methods for CVE-2026-55947

Indicators of Compromise

Excel spreadsheets received from untrusted senders that contain unusually large embedded objects, malformed records, or oversized string tables
EXCEL.EXE spawning unexpected child processes such as cmd.exe, powershell.exe, rundll32.exe, or mshta.exe
Unexpected outbound network connections initiated by EXCEL.EXE shortly after a document is opened
Crash events or Windows Error Reporting entries referencing heap corruption in Excel modules

Detection Strategies

Monitor process lineage for Office applications launching script interpreters or LOLBins, which is a common post-exploitation pattern
Alert on EXCEL.EXE writing executable content (.exe, .dll, .js, .hta) to disk or to %AppData%, %Temp%, or startup locations
Correlate Excel crash telemetry with subsequent process creation to catch failed or successful exploitation attempts

Monitoring Recommendations

Ingest Microsoft Defender, EDR process, and Sysmon Event ID 1 and 11 telemetry into a central analytics platform for Office child-process hunting
Track email gateway detections for spreadsheets carrying obfuscated OLE, VBA, or external data connections targeting Excel users
Review Office Online Server logs for parsing errors on uploaded workbooks that may indicate exploitation attempts against server-side rendering

How to Mitigate CVE-2026-55947

Immediate Actions Required

Apply the Microsoft security updates referenced in the MSRC advisory for CVE-2026-55947 to all affected Excel, Microsoft 365 Apps, and Office LTSC installations
Prioritize patching endpoints used by high-risk roles such as finance, executives, and helpdesk staff who routinely open external spreadsheets
Patch Office Online Server instances that render user-submitted workbooks, since server-side processing removes the user-interaction barrier

Patch Information

Microsoft has issued cumulative security updates covering Microsoft 365 Apps, Excel 2016, Office 2019, Office 2021 LTSC, Office 2024 LTSC (including macOS builds), and Office Online Server. Refer to the Microsoft Security Update Guide entry for CVE-2026-55947 for exact build numbers and KB identifiers per channel.

Workarounds

Enable Protected View and Office Application Guard so untrusted spreadsheets open in an isolated container
Block macros in files from the internet and enforce the Attack Surface Reduction rule preventing Office applications from creating child processes
Route inbound Excel attachments through a sandboxing gateway and quarantine files that fail structural validation

bash

# Example: enforce ASR rule blocking Office child processes via PowerShell
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled