CVE-2026-64911: Microsoft Office RCE Vulnerability

CVE-2026-64911 Overview

CVE-2026-64911 is an integer overflow or wraparound vulnerability in Microsoft Office that enables local code execution. The flaw is classified under [CWE-122] (Heap-based Buffer Overflow) and stems from arithmetic miscalculations during memory allocation. An unauthorized attacker can execute arbitrary code on a target system after a user opens a crafted Office document. Exploitation requires user interaction but no prior authentication to the target. Microsoft published the advisory on 2026-08-11, and the entry was last modified on 2026-08-12.

Critical Impact
Successful exploitation grants attackers arbitrary code execution in the context of the current user, enabling data theft, persistence, and lateral movement from a single opened document.

Affected Products

Microsoft Office (specific versions listed in the Microsoft Security Response Center advisory)
Refer to the Microsoft CVE-2026-64911 Advisory for the authoritative product and build matrix

Discovery Timeline

2026-08-11 - CVE-2026-64911 published to NVD
2026-08-12 - Last updated in NVD database

Technical Details for CVE-2026-64911

Vulnerability Analysis

CVE-2026-64911 is an integer overflow condition within Microsoft Office document parsing routines. When Office processes attacker-controlled structures inside a document, a size or length calculation wraps around the maximum representable value. The resulting undersized allocation is followed by writes that exceed the true buffer bounds, producing a heap-based buffer overflow as tracked by [CWE-122]. Attackers weaponize this primitive to corrupt adjacent heap metadata or function pointers and redirect execution.

Because the attack vector is local and requires user interaction, exploitation typically arrives through phishing emails, shared network locations, or malicious downloads. The target user must open the crafted document for the vulnerable parser to trigger. Code executes with the privileges of the user running Office, which on many workstations includes broad access to user data and network resources.

Root Cause

The root cause is unchecked arithmetic on a length or count field parsed from the document. When the computed value exceeds the integer type's range, it wraps to a small value used for buffer allocation. Subsequent copy operations write past the allocated region on the heap, corrupting adjacent objects.

Attack Vector

An attacker crafts an Office document containing manipulated structures that trigger the overflow during parsing. Delivery mechanisms include email attachments, embedded links, and files hosted on collaboration platforms. Once opened, the document forces the vulnerable code path and pivots into shellcode or a return-oriented programming chain. See the Microsoft CVE-2026-64911 Advisory for vendor-supplied technical details.

No verified public proof-of-concept code is available at the time of publication. The vulnerability is described in prose because no validated exploit artifacts have been released.

Detection Methods for CVE-2026-64911

Indicators of Compromise

Unexpected child processes spawned by winword.exe, excel.exe, or powerpnt.exe, such as cmd.exe, powershell.exe, rundll32.exe, or mshta.exe
Office applications writing executable files, scripts, or DLLs into user-writable directories like %TEMP%, %APPDATA%, or %LOCALAPPDATA%
Outbound network connections initiated directly by Office processes to uncategorized or newly registered domains
Crash reports or Windows Error Reporting entries referencing heap corruption in Office binaries

Detection Strategies

Deploy behavioral detections for Office process lineage anomalies, focusing on script interpreter or LOLBin execution as children of Office
Enable and forward Microsoft Defender ASR (Attack Surface Reduction) audit logs for rules that block Office child processes and executable content creation
Correlate document open events with subsequent process creation, file write, and network telemetry across a 60-second window
Hunt for anomalous macro-free document delivery patterns in email gateways, including unusual file extensions and archive-embedded Office files

Monitoring Recommendations

Ingest Sysmon Event IDs 1 (process create), 3 (network), 7 (image load), and 11 (file create) from endpoints running Office
Monitor Windows Defender Exploit Guard events for exploitation attempts against Office processes
Track patch deployment status via endpoint management tooling to confirm coverage against CVE-2026-64911

How to Mitigate CVE-2026-64911

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-64911 Advisory across all Office installations
Prioritize patching for users who routinely process external documents, including finance, HR, and executive assistants
Enable Microsoft Defender Attack Surface Reduction rules that block Office applications from creating child processes and executable content
Enforce Protected View for files originating from the internet and email attachments

Patch Information

Microsoft has released updates addressing CVE-2026-64911. Consult the Microsoft CVE-2026-64911 Advisory for the specific KB articles, build numbers, and channels that resolve the integer overflow. Distribute updates through Microsoft Update, WSUS, Intune, or your standard patch management pipeline.

Workarounds

Block or quarantine Office documents received from untrusted external senders at the email gateway until patches are deployed
Disable file types that are not required for business operations using Office File Block policy
Restrict local administrator privileges to limit post-exploitation impact if a user opens a malicious document
Deliver targeted user awareness reminders about opening unsolicited attachments during the patch window

bash

# Enable ASR rule: Block Office applications from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled

# Enable ASR rule: Block Office applications from creating executable content
Set-MpPreference -AttackSurfaceReductionRules_Ids 3B576869-A4EC-4529-8536-B80A7769E899 -AttackSurfaceReductionRules_Actions Enabled