CVE-2026-55042: Microsoft 365 Apps Disclosure Vulnerability

CVE-2026-55042 Overview

CVE-2026-55042 is an information disclosure vulnerability affecting multiple versions of Microsoft Office. The flaw stems from the use of an uninitialized resource [CWE-908] within the Office application. An attacker can exploit this weakness locally to read memory contents that should not be accessible to the current user context. Exploitation requires user interaction, typically through opening a specially crafted Office document. Microsoft published the advisory on July 14, 2026, and the vulnerability affects Microsoft 365 Apps, Microsoft 365 for macOS, and Office 2016, 2019, 2021, and 2024 editions across x86, x64, and macOS platforms.

Critical Impact
Successful exploitation exposes confidential in-process memory, which may include sensitive document content, authentication material, or other data residing in the Office process address space.

Affected Products

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

Discovery Timeline

2026-07-14 - CVE-2026-55042 published to NVD
2026-07-14 - Microsoft releases security update for CVE-2026-55042
2026-07-16 - Last updated in NVD database

Technical Details for CVE-2026-55042

Vulnerability Analysis

The vulnerability is classified under [CWE-908]: Use of Uninitialized Resource. Microsoft Office allocates a resource, such as a memory buffer or structure, and then reads from it before assigning a defined value. The uninitialized contents reflect whatever data previously occupied that memory region within the Office process. When Office serializes, renders, or otherwise exposes this data through document output, an attacker who controls the triggering document can capture the leaked bytes. The scope is limited to information confidentiality, with no direct impact on integrity or availability of the system.

Root Cause

The root cause is a missing initialization step in an Office code path that consumes a resource before it is written. Uninitialized memory disclosure bugs typically arise when a structure is allocated on the heap or stack and a subset of fields is populated, while padding bytes or unused fields are later read as part of a larger operation. The current EPSS probability is 0.382% at the 30.5 percentile, reflecting low observed exploitation likelihood at publication.

Attack Vector

Exploitation is local and requires user interaction. An attacker crafts a malicious Office document, such as a Word, Excel, or PowerPoint file, and delivers it through email, a file share, or a web download. When the target opens the document in a vulnerable Office version, the parsing logic triggers the uninitialized read. The leaked contents can then be exfiltrated back to the attacker through document fields, embedded objects, or outbound requests the document initiates.

No public proof-of-concept exploit or verified code sample is available. See the Microsoft Security Update CVE-2026-55042 advisory for vendor technical detail.

Detection Methods for CVE-2026-55042

Indicators of Compromise

Office documents from untrusted sources containing unusual embedded objects, fields, or external references designed to capture and transmit rendered output.
Outbound network connections from winword.exe, excel.exe, or powerpnt.exe to previously unseen domains shortly after a document is opened.
Anomalous child processes or scripting engines spawned by Office applications following document open events.

Detection Strategies

Hunt for Office processes making unexpected HTTP or DNS requests immediately after file open, which may indicate exfiltration of leaked memory contents.
Correlate email gateway telemetry with endpoint document-open events to flag documents delivered from low-reputation senders and opened by high-value users.
Inspect Office documents at the mail gateway for suspicious external image references, IncludePicture fields, or DDE constructs that can smuggle out data.

Monitoring Recommendations

Enable Microsoft Office telemetry and forward Office process events to a centralized log store for retention and analysis.
Monitor Office application versions across the estate to identify unpatched hosts and prioritize remediation.
Alert on Office processes reading credential stores, browser data, or files outside the user's document directories.

How to Mitigate CVE-2026-55042

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-55042 advisory across all affected Office installations.
Confirm patch deployment on both Windows and macOS Office builds, including Microsoft 365 Apps, Office 2016, 2019, 2021 LTSC, and 2024 LTSC.
Prioritize workstations belonging to users who routinely receive external documents, such as finance, legal, and executive staff.

Patch Information

Microsoft has issued security updates for all listed Office SKUs. Refer to the Microsoft Security Update CVE-2026-55042 advisory for the exact build numbers and update channels applicable to your deployment. Microsoft 365 Apps receive updates automatically through the configured update channel, while perpetual Office 2016, 2019, 2021, and 2024 installations receive updates through Microsoft Update or the Click-to-Run service.

Workarounds

Open documents from untrusted sources in Protected View, which restricts document capabilities and reduces attack surface.
Block or quarantine Office documents from external senders at the mail gateway pending validation.
Configure Office to disable automatic content activation, external content download, and legacy features such as DDE where operationally acceptable.

bash

# Example: verify Microsoft 365 Apps build and force an update check
"C:\Program Files\Common Files\microsoft shared\ClickToRun\OfficeC2RClient.exe" /update user

# Query installed Office build on Windows
reg query "HKLM\SOFTWARE\Microsoft\Office\ClickToRun\Configuration" /v VersionToReport