CVE-2026-64908: MS Office Access Buffer Overflow Flaw

CVE-2026-64908 Overview

CVE-2026-64908 is a heap-based buffer overflow [CWE-122] in Microsoft Office Access. An unauthorized attacker can execute code locally on a targeted system by convincing a user to open a crafted Access file. The vulnerability requires user interaction and executes in the security context of the current user.

Microsoft published the advisory through the Microsoft Security Response Center (MSRC). The issue affects the integrity, confidentiality, and availability of the host once exploited.

Critical Impact
Successful exploitation grants local code execution in the user's context, enabling malware deployment, credential theft, and lateral movement from a single crafted Access document.

Affected Products

Microsoft Office Access (refer to the Microsoft Security Update Guide CVE-2026-64908 for exact build ranges)

Discovery Timeline

2026-08-11 - CVE CVE-2026-64908 published to NVD
2026-08-12 - Last updated in NVD database

Technical Details for CVE-2026-64908

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow inside Microsoft Office Access. Access writes attacker-controlled data past the bounds of a heap-allocated buffer while parsing a specially crafted database file. The overflow corrupts adjacent heap metadata or object pointers, which an attacker leverages to redirect execution flow.

The attack vector is local and requires user interaction. A victim must open a malicious .accdb, .mdb, or related Access file delivered through email, a network share, or a web download. No prior authentication to the target system is required, so unprivileged users are sufficient carriers.

Once triggered, the exploit runs code with the privileges of the interactive user. Combined with a privilege escalation flaw, it becomes a full endpoint compromise chain. The vulnerability does not cross a trust boundary on its own; the process runs in the caller's session.

Root Cause

The root cause is improper validation of a size or length field during deserialization of Access file structures. Access allocates a fixed-size heap buffer based on one field and copies data using a different, larger field controlled by the file. This mismatch produces a classic heap overflow governed by [CWE-122].

Attack Vector

Delivery relies on social engineering. An attacker sends the crafted Access file to a victim, hosts it on a lure page, or drops it onto a compromised share. When the user opens the file in Access, the parser processes the malformed structure and triggers the overflow. Protected View and Mark-of-the-Web can reduce, but do not always eliminate, the attack surface for legacy Access formats.

No verified public proof-of-concept is available. See the Microsoft Security Update Guide CVE-2026-64908 for authoritative technical detail.

Detection Methods for CVE-2026-64908

Indicators of Compromise

Unexpected MSACCESS.EXE child processes such as cmd.exe, powershell.exe, rundll32.exe, or regsvr32.exe.
Access files with Mark-of-the-Web arriving through email or browser downloads and opened shortly after delivery.
Crash events or Windows Error Reporting entries referencing heap corruption in MSACCESS.EXE or associated Office DLLs.
Outbound network connections initiated by MSACCESS.EXE to previously unseen domains or IP addresses.

Detection Strategies

Alert on Office process lineage where MSACCESS.EXE spawns scripting or living-off-the-land binaries.
Correlate file-open telemetry for .accdb, .mdb, .accde, and .mde files against subsequent process creation and network activity.
Hunt for anomalous DLL loads or shellcode-like memory allocations inside MSACCESS.EXE using behavioral endpoint telemetry.

Monitoring Recommendations

Ingest Microsoft Defender, Sysmon, and EDR process, file, and network events into a central data lake for cross-host correlation.
Track Office application crashes at scale to surface exploitation attempts against unpatched hosts.
Monitor email gateways and web proxies for Access file attachments and downloads, and quarantine unsigned files from external senders.

How to Mitigate CVE-2026-64908

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide CVE-2026-64908 to every host running Microsoft Office Access.
Block inbound Access file types (.accdb, .mdb, .accde, .mde) at email and web gateways where business use does not require them.
Enforce Protected View and Office macro-blocking policies for files originating from the internet.

Patch Information

Microsoft distributes fixes through the standard Office and Microsoft 365 update channels. Administrators should confirm patch deployment through Microsoft Update, Windows Server Update Services (WSUS), Microsoft Configuration Manager, or Intune. Verify build numbers against the vendor advisory before closing remediation tickets.

Workarounds

Restrict opening of Access files to trusted locations only through Group Policy Trust Center settings.
Disable or uninstall Microsoft Access on endpoints that do not require it, particularly on high-value systems and privileged workstations.
Enforce Attack Surface Reduction (ASR) rules that block Office applications from creating child processes.

bash

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