CVE-2026-50408: Microsoft 365 Apps Info Disclosure Flaw

CVE-2026-50408 Overview

CVE-2026-50408 is an out-of-bounds read vulnerability [CWE-125] in Microsoft Office Excel. The flaw allows an unauthorized attacker to disclose information locally when a victim opens a crafted spreadsheet file. Exploitation requires user interaction, typically opening a malicious .xlsx or .xls document delivered through phishing or an untrusted source. The vulnerability affects multiple Microsoft Office editions including Microsoft 365 Apps, Office 2019, Office 2021, Office 2024, and Office Online Server. Microsoft published the advisory on July 14, 2026, tracking the issue through the Microsoft Security Response Center (MSRC).

Critical Impact
An attacker who successfully exploits CVE-2026-50408 can read memory contents outside of allocated buffers, potentially leaking sensitive process data such as document contents, memory pointers, or other in-memory secrets.

Affected Products

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

Discovery Timeline

2026-07-14 - CVE-2026-50408 published to NVD
2026-07-16 - Last updated in NVD database

Technical Details for CVE-2026-50408

Vulnerability Analysis

CVE-2026-50408 is classified as an out-of-bounds read weakness [CWE-125] within Microsoft Excel's file parsing logic. When Excel processes a malformed spreadsheet, the parser reads memory beyond the intended buffer boundary. This condition results in disclosure of adjacent memory contents to the attacker-controlled document context. The scope remains unchanged, meaning the disclosed information is limited to the Excel process address space. Confidentiality impact is high because leaked memory can contain document data, credentials cached in memory, or address information useful for chaining additional exploits. No integrity or availability impact is associated with this specific flaw.

Root Cause

The root cause is missing or insufficient bounds validation when Excel deserializes structured data from a spreadsheet file. The parser trusts a length or offset field embedded in the document without confirming that the referenced range lies within the allocated buffer. When the field points beyond the buffer, Excel returns memory that was never intended to be exposed to the file's content stream.

Attack Vector

Exploitation is local and requires user interaction. An attacker crafts a malicious Excel file and delivers it through email, chat, or a compromised web download. When the victim opens the file, Excel parses the malformed structure and triggers the out-of-bounds read. The attacker does not need prior authentication on the target system. Combined with a data exfiltration primitive such as an external cell reference or macro, the leaked memory can be transmitted back to the attacker.

No verified public exploit code is available. See the Microsoft CVE-2026-50408 Advisory for vendor technical details.

Detection Methods for CVE-2026-50408

Indicators of Compromise

Unexpected Excel process crashes or Windows Error Reporting entries referencing EXCEL.EXE following the opening of an untrusted spreadsheet.
Excel documents originating from external senders that contain unusual embedded objects, malformed BIFF or OOXML structures, or unexpected external data connections.
Outbound network connections initiated by EXCEL.EXE shortly after a document open event, particularly to uncategorized domains.

Detection Strategies

Inspect email attachments and file share uploads for Office documents with malformed record structures using content disarm and reconstruction (CDR) tooling.
Monitor endpoint telemetry for anomalous child processes spawned by EXCEL.EXE, or memory access violations logged by the Office process.
Correlate Office application telemetry with user open events to identify documents that trigger parser faults across multiple hosts.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and from injecting into other processes.
Forward Office telemetry, EXCEL.EXE crash events, and Windows Error Reporting data to a centralized SIEM for correlation.
Track document provenance using Mark-of-the-Web (MOTW) attributes and alert when Protected View is bypassed on files from untrusted zones.

How to Mitigate CVE-2026-50408

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-50408 Advisory across all Office deployments.
Verify that Microsoft 365 Apps clients are receiving updates from the Current Channel and confirm build numbers post-patch.
Prioritize patching for endpoints belonging to users who routinely process externally sourced spreadsheets, such as finance and procurement teams.

Patch Information

Microsoft has released updates addressing CVE-2026-50408 through its standard update channels for Microsoft 365 Apps, Excel 2016, Office 2019, Office 2021 LTSC, Office 2024 LTSC, Microsoft 365 for macOS, and Office Online Server. Consult the Microsoft CVE-2026-50408 Advisory for the specific build numbers and KB identifiers applicable to each edition.

Workarounds

Enforce Protected View for files originating from the internet, email attachments, and unsafe locations to contain parsing of untrusted spreadsheets.
Block or restrict inbound Office file types at the email gateway when the sender is external and the recipient does not require such attachments.
Configure Group Policy to disable external data connections and Dynamic Data Exchange (DDE) in Excel to reduce the ability to exfiltrate leaked memory.

bash

# Configuration example: enforce 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