CVE-2026-68798: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-68798 Overview

CVE-2026-68798 is a heap-based buffer overflow [CWE-122] in Microsoft Office Excel. An unauthorized attacker can execute arbitrary code locally on a targeted system by convincing a user to open a crafted Excel file. The flaw affects Microsoft 365 Apps, Microsoft 365, Office 2021, and Office 2024 across Windows and macOS platforms. Exploitation requires user interaction but no prior privileges, and successful compromise yields high impact to confidentiality, integrity, and availability. Microsoft has published guidance through the Microsoft Security Response Center (MSRC) update guide.

Critical Impact
A single crafted spreadsheet can trigger code execution in the context of the user running Excel, enabling malware deployment, credential theft, and lateral movement within enterprise environments.

Affected Products

Microsoft 365 Apps (Enterprise x64 and x86)
Microsoft 365 for macOS
Microsoft Office 2021 LTSC (x64, x86, and macOS)
Microsoft Office 2024 LTSC (x64, x86, and macOS)

Discovery Timeline

2026-08-11 - CVE-2026-68798 published to NVD
2026-08-12 - Last updated in NVD database

Technical Details for CVE-2026-68798

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow within Microsoft Office Excel's file parsing logic. When Excel processes a specially crafted workbook, an internal routine writes more data than the allocated heap buffer can hold. The overflow corrupts adjacent heap metadata or object pointers, giving an attacker control over program flow.

Because Excel documents contain complex object structures such as formulas, styles, embedded objects, and record streams, heap layout manipulation is feasible for a determined attacker. Chained with a suitable heap grooming primitive, the overflow enables arbitrary code execution in the current user's context. The vulnerability carries the CWE-122 classification, indicating the memory corruption occurs on the heap rather than the stack.

Root Cause

The root cause is insufficient bounds validation during deserialization of untrusted data within an Excel file. A size field controlled by the attacker is trusted without proper comparison against the destination buffer size, resulting in an out-of-bounds write. Heap allocator metadata or in-object function pointers become viable targets for hijacking control flow.

Attack Vector

Exploitation is local and requires user interaction. An attacker delivers the malicious .xlsx, .xlsm, or legacy .xls file through phishing, watering-hole sites, or removable media. When the recipient opens the file in a vulnerable Excel build, the parser triggers the overflow. The Preview Pane in Windows Explorer or Outlook can also serve as a rendering surface, reducing the interaction required from the user.

The vulnerability description does not indicate authenticated exploitation, and no proof-of-concept code is publicly available at this time.

Detection Methods for CVE-2026-68798

Indicators of Compromise

Excel processes (excel.exe) spawning unusual child processes such as cmd.exe, powershell.exe, wscript.exe, or rundll32.exe.
Unexpected outbound network connections initiated by Office processes shortly after a document is opened.
Creation of executables, scripts, or scheduled tasks in user-writable directories following spreadsheet activity.
Crash dumps or Windows Error Reporting entries referencing heap corruption in Excel modules.

Detection Strategies

Hunt for Office-to-shell process lineage across endpoint telemetry, correlating parent-child relationships with recent document opens.
Alert on Office applications loading unusual DLLs from %TEMP%, %APPDATA%, or other user-writable locations.
Inspect inbound email attachments and shared file uploads for spreadsheets containing malformed records or oversized structures.

Monitoring Recommendations

Enable and forward Microsoft Defender AntiVirus, Windows Event Log, and Sysmon telemetry to a central SIEM for behavioral analytics.
Baseline normal Excel child-process behavior per business unit and flag deviations for review.
Monitor MSRC advisories and vendor patch feeds for updates to CVE-2026-68798 and related Office vulnerabilities.

How to Mitigate CVE-2026-68798

Immediate Actions Required

Apply the security updates referenced in the Microsoft Security Update Guide for CVE-2026-68798 across all affected Office installations.
Prioritize patching for users who routinely handle externally sourced spreadsheets, including finance, procurement, and executive assistants.
Enforce Protected View and Application Guard for Office to open documents from the internet and email in an isolated container.
Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and injecting code.

Patch Information

Microsoft has issued fixed builds for Microsoft 365 Apps, Microsoft 365 for macOS, Office 2021 LTSC, and Office 2024 LTSC. Refer to the Microsoft Security Update Guide for the specific build numbers, channel-specific release notes, and deployment guidance. Confirm that click-to-run channels have received the update by checking File > Account > About Excel.

Workarounds

Block or quarantine spreadsheet attachments from external senders at the mail gateway until patches are deployed.
Disable the Windows Explorer and Outlook preview handlers for Excel file types on unpatched systems.
Restrict execution of Office macros by policy and require signed macros only from trusted publishers.
Deploy AppLocker or Windows Defender Application Control policies to prevent Office from launching interpreters and LOLBins.

bash

# Example Group Policy registry keys to disable Excel preview handlers
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security" /v DisableAttachmentsInPV /t REG_DWORD /d 1 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f