CVE-2026-88192: Microsoft 365 Apps Information Disclosure

CVE-2026-88192 Overview

CVE-2026-88192 is an out-of-bounds read vulnerability in Microsoft Office Excel that allows an unauthorized remote attacker to disclose sensitive information. The flaw affects multiple Excel-bearing Microsoft Office products across Windows and macOS. Exploitation requires user interaction, typically opening a crafted Excel document delivered through email, a shared drive, or a web download. Successful exploitation leaks data from process memory, which may include document contents, cached credentials, or memory addresses useful for bypassing exploit mitigations. The weakness is tracked under [CWE-125: Out-of-bounds Read].

Critical Impact
A crafted Excel file can leak memory contents from the Excel process to a remote attacker, enabling information disclosure and reconnaissance that supports follow-on attacks.

Affected Products

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

Discovery Timeline

2026-10-13 - CVE-2026-88192 published to NVD
2026-10-14 - Last updated in NVD database

Technical Details for CVE-2026-88192

Vulnerability Analysis

The vulnerability resides in Excel's parsing logic for structured spreadsheet content. When Excel processes a malformed record, it reads past the bounds of an allocated buffer. The read operation returns memory that was never intended to be exposed to document rendering code. That memory is then surfaced through document behavior accessible to the attacker, enabling information disclosure.

Because the attack vector is network-based with no privileges required, delivery is straightforward through phishing emails, SharePoint links, or OneDrive previews. User interaction is still required: a user must open or preview the crafted workbook. The scope is unchanged and only confidentiality is affected, with no direct integrity or availability impact.

Leaked bytes can include fragments of other open documents, authentication tokens held in process memory, or heap pointers useful for defeating address space layout randomization (ASLR) in a chained exploit.

Root Cause

The root cause is missing or incorrect bounds validation on a length or offset field inside an Excel file structure. The parser trusts an attacker-controlled size value and reads memory beyond the end of a buffer, consistent with [CWE-125]. Microsoft has not published granular technical details beyond the advisory.

Attack Vector

An attacker crafts a malicious .xlsx, .xls, or related Excel file and delivers it to a target. Opening or previewing the file triggers the vulnerable parser path. Disclosed memory contents can be returned to the attacker through document-embedded channels such as external references, remote image loads, or user-visible fields the attacker can later retrieve.

No verified public exploit code or proof-of-concept is available at the time of writing. See the Microsoft Security Update CVE-2026-88192 advisory for vendor-authoritative technical details.

Detection Methods for CVE-2026-88192

Indicators of Compromise

Excel files arriving from untrusted senders that contain malformed BIFF, OOXML, or embedded object records
Excel processes (excel.exe) initiating outbound network connections shortly after document open, especially to attacker-controlled hosts
Office telemetry showing repeated Excel crashes or exceptions tied to a specific document hash

Detection Strategies

Hunt for excel.exe spawning child processes or performing unusual outbound HTTP/SMB connections after a document open event
Inspect email gateways and shared storage for Excel attachments that fail structural validation or contain anomalous record sizes
Correlate Microsoft Defender ASR and Office telemetry with EDR process events to identify suspicious document execution chains

Monitoring Recommendations

Alert on Excel accessing credential stores, LSASS, or protected process memory
Monitor for Excel loading unexpected DLLs or writing executables to %TEMP% or %APPDATA%
Track outbound connections from Office applications to newly observed or low-reputation domains

How to Mitigate CVE-2026-88192

Immediate Actions Required

Apply the October 2026 Microsoft security updates covering Microsoft 365 Apps, Microsoft 365 for macOS, Office LTSC 2021, and Office LTSC 2024
Enable Protected View and Block Macros From Running In Office Files From The Internet policies for all Excel users
Instruct users not to open unexpected spreadsheet attachments and route suspicious files to the security team for analysis

Patch Information

Microsoft published the fix in its October 2026 Patch Tuesday release. Refer to the Microsoft Security Update CVE-2026-88192 advisory for build numbers and channel-specific guidance. Microsoft 365 Apps customers should confirm the Current, Monthly Enterprise, and Semi-Annual channels have received the update. LTSC customers must deploy the standalone security update for their specific Office version.

Workarounds

Enforce Protected View for files originating from the internet, Outlook attachments, and unsafe locations
Use Attack Surface Reduction (ASR) rules to block Office applications from creating child processes and injecting into other processes
Strip or quarantine Excel attachments at the mail gateway until patch deployment is verified across the fleet

bash

# Example: enforce Excel Protected View via Group Policy registry keys
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f