CVE-2026-44821: Microsoft Office Information Disclosure

CVE-2026-44821 Overview

CVE-2026-44821 is an out-of-bounds read vulnerability [CWE-125] in Microsoft Office that enables local information disclosure. An unauthorized attacker can read memory contents outside the intended buffer boundary by tricking a user into opening a crafted Office document. Exploitation requires user interaction but no prior authentication or elevated privileges.

The flaw exposes sensitive process memory that may contain credentials, document fragments, or pointers useful for bypassing memory protections such as Address Space Layout Randomization (ASLR). Microsoft documented the issue in the Microsoft Security Update CVE-2026-44821 advisory.

Critical Impact
Successful exploitation discloses confidential in-process memory from Microsoft Office, which attackers can chain with other vulnerabilities to weaken exploit mitigations and stage follow-on attacks.

Affected Products

Microsoft Office (refer to the Microsoft Security Response Center advisory for the full version matrix)

Discovery Timeline

2026-06-09 - CVE-2026-44821 published to the National Vulnerability Database (NVD)
2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-44821

Vulnerability Analysis

The vulnerability is an out-of-bounds read [CWE-125] in Microsoft Office's document parsing logic. The application reads memory past the end of an allocated buffer when processing a malformed document structure. The read operation returns adjacent heap or stack contents to the parser, which can then be exfiltrated through legitimate document features or surfaced to the attacker through embedded objects.

The scope is unchanged and the impact is limited to confidentiality. Integrity and availability are not affected, meaning the attacker cannot modify data or crash the application reliably through this issue alone. However, the disclosed memory frequently contains material that supports broader attack chains.

Root Cause

Out-of-bounds reads in Office file parsers typically originate from insufficient validation of length fields, offsets, or record counts inside container formats such as Office Open XML (OOXML) or legacy compound binary formats. The parser trusts an attacker-controlled size or index value and accesses memory beyond the allocated buffer. Without runtime bounds checks, the read proceeds and returns whatever resides in adjacent memory.

Attack Vector

The attack vector is local with required user interaction. An attacker delivers a crafted Office document through email, a shared drive, a malicious website, or removable media. When the victim opens the file in a vulnerable Office application, the parser triggers the out-of-bounds read. The leaked memory contents can be returned to the attacker through document fields, embedded scripts, or external references that exfiltrate the data.

No verified exploit code is publicly available at the time of publication. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The vulnerability mechanism is described in prose because no verified proof-of-concept code has been released. Refer to the Microsoft Security Update CVE-2026-44821 advisory for vendor-supplied technical details.

Detection Methods for CVE-2026-44821

Indicators of Compromise

Office documents from untrusted sources containing malformed OOXML parts, abnormally large record headers, or unexpected embedded objects.
Office processes (WINWORD.EXE, EXCEL.EXE, POWERPNT.EXE) generating outbound network connections immediately after opening a document.
Crash dumps or Windows Error Reporting entries referencing access violations during Office document parsing.

Detection Strategies

Inspect inbound email and file shares for Office documents using sandbox detonation that flags out-of-bounds memory access during rendering.
Hunt for Office child processes spawning unexpected utilities such as cmd.exe, powershell.exe, or mshta.exe shortly after document open events.
Correlate document-open events with anomalous outbound HTTP, DNS, or SMB connections that could carry exfiltrated memory contents.

Monitoring Recommendations

Enable Microsoft Office telemetry and audit logging through Microsoft 365 Defender and forward to a centralized analytics platform.
Monitor endpoint Exploit Protection events for Office-hosted processes, focusing on memory access violations and Data Execution Prevention (DEP) triggers.
Track patch deployment status for Office across the fleet and alert on hosts that remain on vulnerable builds beyond the remediation window.

How to Mitigate CVE-2026-44821

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-44821 to all Office installations.
Enforce Protected View for documents originating from the internet, email attachments, and unsafe locations.
Block or quarantine Office documents from untrusted senders at the email gateway pending patch rollout.

Patch Information

Microsoft has issued a security update through the Microsoft Security Response Center. Administrators should consult the Microsoft Security Update CVE-2026-44821 advisory for the exact KB identifiers and product builds. Deploy the update through Windows Update, Microsoft Update, or Microsoft Configuration Manager according to the organization's patch management policy.

Workarounds

Configure Office File Block policy to prevent opening of legacy or untrusted file formats until the patch is deployed.
Open documents from untrusted sources only inside Office Protected View or an isolated environment such as Application Guard for Office.
Disable preview handlers in Windows Explorer and Outlook to prevent automatic parsing of attached documents.

bash

# Example Group Policy registry settings to enforce Protected View for Microsoft Word
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f