CVE-2026-55137: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-55137 Overview

CVE-2026-55137 is a heap-based buffer overflow vulnerability in Microsoft Office Excel that allows an unauthorized attacker to execute code locally. The flaw affects multiple Microsoft Office and Microsoft 365 releases across Windows and macOS platforms. Exploitation requires user interaction, typically by opening a specially crafted Excel document. The weakness is classified as [CWE-122] Heap-based Buffer Overflow.

Critical Impact
A successful attack grants the attacker the ability to execute arbitrary code in the context of the current user, resulting in full compromise of confidentiality, integrity, and availability on the affected host.

Affected Products

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

Discovery Timeline

2026-07-14 - CVE-2026-55137 published to NVD
2026-07-15 - Last updated in NVD database

Technical Details for CVE-2026-55137

Vulnerability Analysis

The vulnerability resides in Microsoft Excel's parsing logic for spreadsheet file formats. When Excel processes a malformed record within a crafted workbook, it writes beyond the bounds of an allocated heap buffer. This out-of-bounds write corrupts adjacent heap metadata and application objects. An attacker who controls the overflowing data can influence execution flow and execute arbitrary code in the process context of the user opening the file.

The attack vector is local and requires the victim to open a weaponized document. Common delivery mechanisms include phishing emails carrying malicious .xlsx, .xls, or .xlsm attachments, as well as documents hosted on attacker-controlled file shares or web pages. No authentication is required from the attacker's side, and the exploit runs with the privileges of the interactive user.

Root Cause

The root cause is improper validation of length or size fields within Excel document structures during heap buffer allocation and copy operations. Excel allocates a fixed-size heap chunk based on assumptions about record boundaries, then copies attacker-controlled content that exceeds those boundaries. This condition matches the [CWE-122] pattern of heap-based buffer overflow through improper bounds checking.

Attack Vector

An attacker crafts a malicious Excel file containing malformed records that trigger the overflow during parsing. The file is delivered through email, chat, or web download. When the victim opens the document, Excel processes the record, corrupts the heap, and hands execution to attacker-controlled data. The resulting code runs under the user's account and can be used to install malware, harvest credentials, or pivot within the environment.

No verified proof-of-concept code is publicly available at this time. Refer to the Microsoft Security Update CVE-2026-55137 advisory for vendor technical detail.

Detection Methods for CVE-2026-55137

Indicators of Compromise

Excel documents delivered by email or download that contain unusually large or malformed BIFF/OOXML records.
excel.exe spawning unexpected child processes such as cmd.exe, powershell.exe, rundll32.exe, or wscript.exe.
Unexpected outbound network connections initiated by excel.exe shortly after a document is opened.
New persistence entries or scheduled tasks created following the opening of an untrusted spreadsheet.

Detection Strategies

Hunt for process lineage where excel.exe is the parent of scripting or LOLBin processes.
Flag Excel processes that write executable content to %APPDATA%, %TEMP%, or user profile directories.
Inspect email gateways and web proxies for Excel attachments from untrusted senders, and detonate suspicious files in a sandbox.

Monitoring Recommendations

Enable command-line and module-load logging on endpoints running Microsoft Office.
Forward Office telemetry, Sysmon events, and EDR alerts to a central data lake for correlation.
Alert on crash events involving excel.exe with heap corruption signatures reported by Windows Error Reporting.

How to Mitigate CVE-2026-55137

Immediate Actions Required

Apply the Microsoft security update referenced in the vendor advisory to all affected Office installations without delay.
Inventory endpoints running Microsoft 365 Apps, Office 2019, Office 2021 LTSC, Office 2024 LTSC, Excel 2016, and Office Online Server, and prioritize patching.
Restrict opening of Excel files sourced from email or the internet through Protected View and Mark-of-the-Web enforcement.

Patch Information

Microsoft has published guidance and updates through the Microsoft Security Response Center. Administrators should review the Microsoft Security Update CVE-2026-55137 advisory for the specific build numbers and update channels applicable to each product edition.

Workarounds

Enforce Protected View for files originating from the internet and other untrusted locations.
Deploy Attack Surface Reduction rules that block Office applications from creating child processes and from injecting into other processes.
Block macro execution in Office documents downloaded from the internet through group policy.
Educate users to avoid opening unsolicited Excel attachments and to report suspicious documents to the security team.

bash

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