CVE-2026-69778: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-69778 Overview

CVE-2026-69778 is a heap-based buffer overflow [CWE-122] in Microsoft Office Access that allows an unauthorized attacker to execute code over a network. The flaw affects Microsoft 365 Apps, Microsoft Access 2016, and Microsoft Office 2016, 2019, 2021 LTSC, and 2024 LTSC across both x86 and x64 builds. Exploitation requires the target user to open a crafted Access file, giving attackers arbitrary code execution in the context of the current user.

Critical Impact
A successful exploit yields remote code execution with the user's privileges, enabling malware deployment, data theft, and lateral movement across enterprise networks.

Affected Products

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

Discovery Timeline

2026-09-08 - CVE-2026-69778 published to the National Vulnerability Database (NVD)
2026-09-10 - Last updated in NVD database

Technical Details for CVE-2026-69778

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow [CWE-122] in Microsoft Office Access. Access fails to validate the size of attacker-controlled data before writing it into a heap-allocated buffer during file parsing. The out-of-bounds write corrupts adjacent heap structures, allowing an attacker to influence program control flow.

Because the flaw resides in file-format processing, exploitation is delivered through a malicious Access database (for example, .accdb or .mdb) sent by email, hosted on a website, or shared via collaboration platforms. User interaction is required, but simply opening the crafted file is sufficient.

Root Cause

Insufficient bounds checking within Microsoft Access's parsing routines allows a length field or record structure inside the database file to drive a write past the end of a heap buffer. This memory corruption can be shaped by the attacker to overwrite function pointers or object metadata reachable from the parser.

Attack Vector

The attack vector is network-based with low complexity and no prior authentication. An attacker crafts a malicious Access file and delivers it to the victim through phishing, watering-hole distribution, or file-sharing services. When the user opens the file, the corrupted parsing path triggers arbitrary code execution in the Access process. The compromise inherits the user's rights, including access to local files, network shares, and cached credentials.

No verified public proof-of-concept code is available for CVE-2026-69778. See the Microsoft CVE-2026-69778 Update Guide for vendor technical details.

Detection Methods for CVE-2026-69778

Indicators of Compromise

Unexpected child processes spawned by MSACCESS.EXE, such as cmd.exe, powershell.exe, rundll32.exe, or mshta.exe.
Access opening database files (.accdb, .mdb, .accde) delivered from email attachments, browser downloads, or removable media.
Outbound network connections initiated by MSACCESS.EXE to previously unseen or low-reputation domains.
Creation of persistence artifacts (Run keys, Scheduled Tasks, Startup folder entries) following an Access session.

Detection Strategies

Hunt for anomalous process lineage where MSACCESS.EXE is the parent of scripting or living-off-the-land binaries.
Correlate Office telemetry with file-write events in user-writable directories such as %APPDATA%, %TEMP%, and %PUBLIC%.
Alert on Office applications loading unsigned DLLs or performing suspicious memory allocations consistent with heap grooming.
Map observed behavior to MITRE ATT&CK techniques T1203 (Exploitation for Client Execution) and T1204.002 (User Execution: Malicious File).

Monitoring Recommendations

Centralize Windows Sysmon, EDR, and Microsoft Defender telemetry for Office processes into a SIEM or data lake for cross-host analytics.
Monitor email gateways for inbound Access database attachments and unusual archive formats containing them.
Track patch compliance across all Microsoft 365 Apps and Office LTSC installations to identify unpatched endpoints.

How to Mitigate CVE-2026-69778

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-69778 Update Guide to all affected Office and Microsoft 365 Apps installations.
Prioritize patching for users who routinely open Access databases from external sources.
Block inbound .accdb, .mdb, and .accde attachments at the email gateway when they are not required for business workflows.
Enable Protected View and Office File Block policies for legacy Access file formats.

Patch Information

Microsoft has released fixes for all affected products through its monthly security update process. Refer to the Microsoft CVE-2026-69778 Update Guide for the specific build numbers that remediate the heap-based buffer overflow across Microsoft 365 Apps, Access 2016, and Office 2016, 2019, 2021 LTSC, and 2024 LTSC.

Workarounds

Deploy Microsoft Attack Surface Reduction rules to block Office applications from creating child processes and from injecting into other processes.
Restrict use of Microsoft Access to users with a documented business need through application control policies such as AppLocker or Windows Defender Application Control.
Deliver end-user awareness reminders that discourage opening unsolicited database files, even from apparently trusted senders.

bash

# Configuration example: enable ASR rule blocking Office child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled