CVE-2026-78505: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-78505 Overview

CVE-2026-78505 is a heap-based buffer overflow vulnerability in Microsoft Office that allows an unauthorized attacker to execute arbitrary code over a network. The flaw affects multiple Office product lines across Windows and macOS, including Microsoft 365 Apps, Microsoft 365, and standalone Office 2016, 2019, 2021, and 2024 installations. Exploitation requires user interaction, typically opening a crafted document, but no prior authentication is needed. Microsoft has published the vulnerability in its Security Update Guide and classified it as [CWE-122] Heap-based Buffer Overflow.

Critical Impact
Successful exploitation grants an attacker code execution in the context of the current user, enabling data theft, ransomware deployment, or lateral movement within the enterprise.

Affected Products

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

Discovery Timeline

2026-09-08 - CVE-2026-78505 published to the National Vulnerability Database
2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-78505

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow [CWE-122] in Microsoft Office file-parsing routines. When Office processes a specially crafted document, it writes data past the boundaries of a heap-allocated buffer. The overwrite can corrupt adjacent heap metadata or object pointers, which the attacker can then leverage to redirect execution flow.

Because the flaw resides in Office document parsing, execution occurs in the context of the user opening the file. On workstations where users hold local administrator rights, attackers can achieve full system compromise. The EPSS model currently estimates a low near-term exploitation probability, but Office document flaws are historically weaponized in phishing campaigns and commodity malware loaders.

Root Cause

Heap-based buffer overflows in Office typically arise from missing or incorrect bounds checks when the application copies attacker-controlled fields from a document into a fixed-size heap allocation. Length values embedded in the file are trusted without validation against the destination buffer, allowing an attacker to write beyond the allocated region and corrupt adjacent heap structures.

Attack Vector

The attack vector is network-based with required user interaction. An attacker delivers a malicious Office document through email, a web download, a shared drive, or a collaboration platform such as SharePoint or Teams. When the target opens the file (or, in some Office configurations, previews it), the vulnerable parser processes the crafted content and the overflow triggers. Preview Pane rendering in Outlook and Windows Explorer can broaden the exposure for similar Office bugs and should be treated as an active risk surface until patches are verified.

No public proof-of-concept exploit or in-the-wild activity has been reported. Refer to the Microsoft Security Update Guide CVE-2026-78505 for authoritative technical detail.

Detection Methods for CVE-2026-78505

Indicators of Compromise

Office applications (WINWORD.EXE, EXCEL.EXE, POWERPNT.EXE) spawning unusual child processes such as cmd.exe, powershell.exe, wscript.exe, mshta.exe, or rundll32.exe.
Unexpected outbound network connections initiated by Office processes shortly after a document is opened.
New executables, DLLs, or scripts written to %APPDATA%, %TEMP%, or %PUBLIC% by an Office process.
Office process crashes with heap corruption exceptions in Windows Error Reporting or crash dumps.

Detection Strategies

Hunt for process-lineage anomalies where Office parents spawn scripting or living-off-the-land binaries.
Correlate document open events with subsequent DNS lookups, HTTP(S) beacons, and file write telemetry.
Apply YARA and content inspection at the mail gateway to flag Office files containing malformed embedded objects or oversized structure fields.

Monitoring Recommendations

Enable and forward Microsoft Defender for Office 365 and Attack Surface Reduction (ASR) telemetry to a central analytics platform.
Alert on Office child processes and Office-initiated network egress across the endpoint fleet.
Retain endpoint and email logs for at least 90 days to support retroactive hunting once indicators emerge.

How to Mitigate CVE-2026-78505

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide CVE-2026-78505 across all Windows and macOS Office installations.
Prioritize patching endpoints belonging to users who routinely handle external documents, such as finance, HR, and executive assistants.
Enforce Microsoft Office Protected View and block macros originating from the internet through Group Policy.
Restrict local administrator rights to limit the blast radius of a successful exploit.

Patch Information

Microsoft has released updates for all affected Office channels. Administrators should verify Click-to-Run build numbers for Microsoft 365 Apps and confirm MSI-based Office 2016, 2019, 2021, and 2024 installations receive the corresponding cumulative security update. macOS Office builds should be updated through Microsoft AutoUpdate. Consult the vendor advisory for build-specific fixed versions.

Workarounds

Enable Attack Surface Reduction rules that block Office applications from creating child processes and from injecting into other processes.
Disable the Outlook Preview Pane and Windows Explorer preview handlers for Office file types until patches are applied.
Route inbound Office documents through a sandboxing or Content Disarm and Reconstruction (CDR) solution.
Block or quarantine legacy Office binary formats (.doc, .xls, .ppt) at the mail gateway when they are not business-critical.

bash

# Example: Enable ASR rules that constrain Office exploitation via PowerShell
Set-MpPreference -AttackSurfaceReductionRules_Ids `
  D4F940AB-401B-4EFC-AADC-AD5F3C50688A, `
  3B576869-A4EC-4529-8536-B80A7769E899, `
  75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84 `
  -AttackSurfaceReductionRules_Actions Enabled, Enabled, Enabled