CVE-2026-54988: Microsoft 365 Apps Info Disclosure Flaw

CVE-2026-54988 Overview

CVE-2026-54988 is an out-of-bounds read vulnerability in Microsoft Office Excel that allows an unauthorized attacker to disclose information locally. The flaw is tracked under CWE-125 and affects multiple Microsoft Office and Microsoft 365 product lines on both Windows and macOS. Exploitation requires user interaction, typically by opening a crafted spreadsheet file. Successful exploitation can leak memory contents from the Excel process, which may include sensitive data or information useful for chaining further attacks.

Critical Impact
A local attacker can trigger an out-of-bounds read in Excel to disclose in-process memory and potentially cause the application to terminate.

Affected Products

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

Discovery Timeline

2026-07-14 - CVE-2026-54988 published to NVD
2026-07-16 - Last updated in NVD database

Technical Details for CVE-2026-54988

Vulnerability Analysis

The vulnerability is an out-of-bounds read [CWE-125] in the Microsoft Office Excel file parser. When Excel processes a crafted spreadsheet, the code reads memory outside the intended buffer boundary. The read returns data from adjacent memory regions inside the Excel process. An attacker who controls the malformed structure can influence which bytes get read and returned. This class of flaw supports information disclosure and can also crash the application, producing a local denial of service.

Root Cause

The root cause is missing or insufficient bounds validation on a field parsed from an untrusted Excel document. The parser relies on a length or offset value taken from the file without confirming it stays within the allocated buffer. Microsoft's advisory does not publish the specific record type or function, but out-of-bounds reads in Excel typically originate in record or stream parsing logic that trusts attacker-controlled size fields.

Attack Vector

Exploitation requires local access and user interaction. The attacker delivers a weaponized .xlsx, .xls, or related Excel file through email, a shared drive, a chat platform, or a web download. When the victim opens the file in a vulnerable Excel client, or when Office Online Server renders it, the parser triggers the out-of-bounds read. The Preview Pane can also render Office content, expanding the attack surface. The disclosed memory may include document contents, tokens, or heap metadata that supports follow-on exploitation. The vulnerability is not known to be exploited in the wild and no public proof of concept is available.

Detection Methods for CVE-2026-54988

Indicators of Compromise

Unexpected Excel process crashes (EXCEL.EXE) or Windows Error Reporting entries referencing Excel following the opening of an external document.
Excel or Office Online Server rendering documents received from untrusted external senders or newly registered domains.
Office child processes spawning after opening a spreadsheet, which may indicate chained exploitation.

Detection Strategies

Hunt for Excel opening files from user download, temp, or email attachment directories followed by abnormal termination or module faults.
Correlate mail gateway telemetry on inbound spreadsheets with endpoint file-open events to identify suspicious delivery patterns.
Inspect Office telemetry and DLP logs for spreadsheets that contain malformed records or unusually structured OLE streams.

Monitoring Recommendations

Monitor Office application crash events and Windows Error Reporting for repeated faults in Excel across multiple users.
Track Office Online Server rendering errors that could indicate probing with malformed documents.
Alert on Excel process memory anomalies and unexpected outbound network connections from EXCEL.EXE after document open.

How to Mitigate CVE-2026-54988

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-54988 advisory across all affected Office channels.
Prioritize patching endpoints and Office Online Server instances that process externally sourced spreadsheets.
Instruct users not to open Excel attachments from untrusted senders until updates are deployed.

Patch Information

Microsoft has published fixed builds for Microsoft 365 Apps, Excel 2016, Office 2019, Office 2021 LTSC, Office 2024 LTSC, Microsoft 365 for macOS, and Office Online Server. Refer to the Microsoft CVE-2026-54988 Update for build numbers and deployment channels. Confirm patch deployment by verifying installed Click-to-Run or MSI build versions across managed endpoints.

Workarounds

Enable Protected View and Application Guard for Office to isolate documents originating from the internet or email.
Block or quarantine Excel attachments at the mail gateway when patch rollout is incomplete.
Disable the Windows Explorer Preview Pane for Office files to reduce inadvertent parsing of malicious spreadsheets.

bash

# Configuration example: disable Preview Pane handlers for Excel files via registry
reg add "HKCU\Software\Microsoft\Windows\CurrentVersion\PreviewHandlers" /v "{00020827-0000-0000-C000-000000000046}" /d "" /f