CVE-2026-108446 Overview
CVE-2026-108446 is a use-after-free vulnerability [CWE-416] in Microsoft Office Excel that allows an unauthorized attacker to execute code over a network. The flaw affects multiple Office versions, including Microsoft 365 Apps, Microsoft 365 for macOS, Office LTSC 2021, Office LTSC 2024, and Office Online Server. Exploitation requires user interaction, typically the opening of a crafted Excel document. Successful exploitation can lead to arbitrary code execution in the context of the logged-in user, impacting confidentiality, integrity, and availability.
Critical Impact
Attackers can execute arbitrary code on target systems when a user opens a malicious Excel file delivered through phishing, file shares, or web downloads.
Affected Products
- Microsoft 365 Apps (enterprise, x64 and x86)
- Microsoft 365 for macOS, Office LTSC 2021, Office LTSC 2024
- Microsoft Office Online Server
Discovery Timeline
- 2026-10-15 - CVE-2026-108446 published to the National Vulnerability Database (NVD)
- 2026-10-19 - Last updated in NVD database
Technical Details for CVE-2026-108446
Vulnerability Analysis
The vulnerability is a use-after-free condition [CWE-416] in the Excel component of Microsoft Office. A use-after-free occurs when an application continues to reference memory after that memory has been freed, allowing an attacker to control the contents of the freed region and influence program execution flow.
In this case, parsing a crafted Excel workbook triggers premature deallocation of an object while another code path retains a reference to it. When Excel subsequently dereferences the stale pointer, attacker-controlled data in the reclaimed allocation can redirect execution. The result is arbitrary code execution within the Excel process, running with the privileges of the user who opened the file.
The attack is network-reachable because the malicious document can be delivered via email, SharePoint, OneDrive, or web download. Office Online Server exposure extends the attack surface to server-side rendering of untrusted spreadsheets.
Root Cause
The root cause is improper management of object lifetimes during Excel file parsing. Specific object references are not invalidated after the underlying memory is freed, leaving a dangling pointer that is later dereferenced during continued processing of the document.
Attack Vector
An attacker crafts a malicious .xlsx, .xlsm, or related Excel file and delivers it to a victim. When the user opens the document in a vulnerable Office client, or when Office Online Server processes it, the use-after-free is triggered. No prior authentication to the target is required, but user interaction is necessary. Refer to the Microsoft Security Update for CVE-2026-108446 for parsing specifics.
No verified proof-of-concept code is publicly available for CVE-2026-108446.
See the Microsoft Security Response Center advisory for official details.
Detection Methods for CVE-2026-108446
Indicators of Compromise
- Excel (EXCEL.EXE) spawning unexpected child processes such as cmd.exe, powershell.exe, wscript.exe, rundll32.exe, or mshta.exe.
- Excel process crashes or unexpected exceptions correlated with opening attachments from untrusted senders.
- Outbound network connections initiated by EXCEL.EXE to unknown or newly registered domains shortly after document open.
- Creation of persistence artifacts (scheduled tasks, Run keys, startup folder entries) within minutes of a user opening a spreadsheet.
Detection Strategies
- Monitor process-ancestry trees for Office applications executing scripting interpreters or living-off-the-land binaries.
- Alert on anomalous memory access violations and crash dumps generated by Office processes.
- Hunt for suspicious Excel-delivered payloads using behavioral analytics focused on child process creation and script execution.
Monitoring Recommendations
- Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and injecting code.
- Collect Office telemetry, Sysmon event IDs 1 (process create), 7 (image load), and 11 (file create) into your SIEM for correlation.
- Track email gateway logs for inbound attachments with Excel macros or external data connections from unverified senders.
How to Mitigate CVE-2026-108446
Immediate Actions Required
- Apply the Microsoft security updates referenced in the MSRC advisory for CVE-2026-108446 across all affected Office installations.
- Prioritize patching Office Online Server instances, which expose server-side parsing of untrusted workbooks.
- Verify macOS Microsoft 365 clients receive the corresponding update through AutoUpdate.
Patch Information
Microsoft published fixes through its October 2026 Patch Tuesday release cycle. Administrators should deploy updates to Microsoft 365 Apps, Office LTSC 2021, Office LTSC 2024, Microsoft 365 for macOS, and Office Online Server. Consult the Microsoft Security Update Guide entry for CVE-2026-108446 for the exact build numbers per channel.
Workarounds
- Enable Protected View and Office Application Guard to isolate documents from untrusted sources until patches are applied.
- Block or quarantine Excel attachments from external senders at the email gateway where business processes allow.
- Disable or restrict macros and external data connections through Group Policy for high-risk user populations.
# Enable ASR rule: Block all Office applications from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
-AttackSurfaceReductionRules_Actions Enabled
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
