CVE-2026-64899 Overview
CVE-2026-64899 is an out-of-bounds read vulnerability in Microsoft Office that allows an unauthorized attacker to disclose information locally. The flaw is classified under [CWE-125] (Out-of-bounds Read) and requires user interaction to trigger. An attacker who successfully exploits the flaw can read memory contents outside the intended buffer boundary, potentially exposing sensitive process data. The vulnerability affects confidentiality only and does not impact integrity or availability. Microsoft has published a security advisory addressing the issue.
Critical Impact
Successful exploitation discloses in-memory Office application data to a local attacker after the victim opens a crafted document.
Affected Products
- Microsoft Office (specific versions listed in the Microsoft Security Response Center advisory)
Discovery Timeline
- 2026-08-11 - CVE-2026-64899 published to NVD
- 2026-08-11 - Last updated in NVD database
Technical Details for CVE-2026-64899
Vulnerability Analysis
The vulnerability is an out-of-bounds read in Microsoft Office. The application reads memory beyond the bounds of an allocated buffer when parsing a maliciously crafted document. The read operation returns adjacent memory contents to the attacker-controlled parsing context, enabling information disclosure. The attack vector is local, meaning an attacker must deliver the crafted file to the victim through email, download, or another channel. User interaction is required — the victim must open the document in Microsoft Office. The scope is unchanged and only confidentiality is impacted.
Root Cause
The underlying weakness is [CWE-125]: Out-of-bounds Read. Office fails to validate a length or index value while processing structured content in a document. The parser then reads past the end of an allocated buffer. The disclosed memory may contain heap metadata, pointers, or fragments of previously processed documents.
Attack Vector
An attacker crafts a malicious Office document that triggers the out-of-bounds read during parsing. The attacker delivers the file through phishing, a file share, or a web download. When the victim opens the document, Office reads memory outside the allocated buffer and the attacker recovers disclosed data through document rendering artifacts or a companion exploitation primitive. No privileges are required on the target system, but user interaction is mandatory.
No public proof-of-concept code is available for this vulnerability. Refer to the Microsoft Security Update Guide for authoritative technical details.
Detection Methods for CVE-2026-64899
Indicators of Compromise
- Unexpected Microsoft Office process crashes or hangs immediately after opening a document from an external source
- Office documents delivered from untrusted email senders that request macros, external content, or protected view bypass
- Anomalous child processes spawned by winword.exe, excel.exe, or powerpnt.exe following document open events
Detection Strategies
- Monitor Windows Event Logs and endpoint telemetry for Office application faults with exception codes consistent with access violations during file parsing
- Correlate email gateway telemetry with endpoint document-open events to identify suspicious document delivery patterns
- Use YARA rules keyed to malformed structures in Office file formats once vendor or research signatures are published
Monitoring Recommendations
- Track Office application patch levels across the fleet and alert on hosts running unpatched builds
- Log document provenance using Mark-of-the-Web enforcement and audit Protected View bypass events
- Monitor for outbound network connections initiated by Office processes shortly after document open
How to Mitigate CVE-2026-64899
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft Security Update Guide as soon as it is available for your Office channel
- Enforce Protected View for documents originating from the internet and email attachments
- Restrict opening of Office documents from untrusted sources through email gateway policies and user training
Patch Information
Microsoft has published a security update for CVE-2026-64899. Administrators should consult the Microsoft Security Update Guide to identify the applicable Office builds and deployment channels. Apply the update through Microsoft Update, Windows Server Update Services, or Microsoft Intune according to organizational patch management policy.
Workarounds
- Enable and enforce Protected View for files originating from the internet, unsafe locations, and Outlook attachments
- Configure Attack Surface Reduction rules to block Office applications from creating child processes and injecting into other processes
- Disable preview handlers for Office file types in Windows Explorer and Outlook until the patch is deployed
# Enforce Protected View via Group Policy registry keys (example, PowerShell)
Set-ItemProperty -Path 'HKCU:\Software\Microsoft\Office\16.0\Word\Security\ProtectedView' -Name 'DisableInternetFilesInPV' -Value 0
Set-ItemProperty -Path 'HKCU:\Software\Microsoft\Office\16.0\Word\Security\ProtectedView' -Name 'DisableAttachmentsInPV' -Value 0
Set-ItemProperty -Path 'HKCU:\Software\Microsoft\Office\16.0\Word\Security\ProtectedView' -Name 'DisableUnsafeLocationsInPV' -Value 0
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
