CVE-2026-108448: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-108448 Overview

CVE-2026-108448 is a heap-based buffer overflow vulnerability in Microsoft Office Outlook that allows an unauthorized attacker to execute code over a network. The flaw affects multiple Microsoft Office product lines, including Microsoft 365 Apps, Microsoft 365, Office 2021 LTSC, and Office 2024 LTSC, across both Windows and macOS platforms. The vulnerability is tracked under [CWE-122] (Heap-based Buffer Overflow). Successful exploitation requires user interaction, such as opening a crafted email or attachment, but no authentication or prior access to the target system.

Critical Impact
An attacker who successfully exploits this vulnerability can execute arbitrary code on the targeted system over a network, compromising confidentiality, integrity, and availability of the affected Outlook host.

Affected Products

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

Discovery Timeline

2026-10-15 - CVE-2026-108448 published to the National Vulnerability Database
2026-10-19 - Last updated in NVD database

Technical Details for CVE-2026-108448

Vulnerability Analysis

The vulnerability resides in Microsoft Office Outlook's parsing logic for network-delivered content. When Outlook processes a specially crafted message or attachment, it writes data past the bounds of a heap-allocated buffer. This out-of-bounds write corrupts adjacent heap memory, including object pointers, function references, or control structures used by the Outlook process.

An attacker who controls the overflow contents can corrupt critical structures and redirect execution flow. Because Outlook runs in the user's security context, successful exploitation yields code execution with the privileges of the signed-in user. The flaw is network-reachable through standard mail delivery paths, which expands the attack surface beyond local file handling.

Exploitation requires user interaction, consistent with phishing-style delivery through email messages, preview pane rendering, or attachment handling. High attack complexity indicates the attacker must satisfy specific memory layout or timing conditions to achieve reliable code execution. The affected scope spans both Windows and macOS Office installations, broadening the pool of exposed endpoints.

Root Cause

The root cause is improper validation of input length or structure before copying attacker-controlled data into a fixed-size heap allocation. This classic [CWE-122] pattern allows the write operation to exceed the destination buffer, overwriting adjacent heap metadata or object fields.

Attack Vector

The attack vector is network-based. An attacker delivers a crafted email or embedded content that Outlook parses when the message is received, previewed, or opened by the recipient. No credentials or prior session on the target system are required.

Microsoft has not released public exploitation details or proof-of-concept code. Refer to the Microsoft CVE-2026-108448 Advisory for vendor-authoritative technical references.

Detection Methods for CVE-2026-108448

Indicators of Compromise

Unexpected child processes spawned by OUTLOOK.EXE or the macOS Outlook binary, particularly scripting hosts such as powershell.exe, cmd.exe, wscript.exe, or osascript.
Outlook process crashes with heap corruption signatures logged in Windows Error Reporting or macOS crash reports immediately after message receipt or preview.
Outbound network connections initiated by the Outlook process to non-Microsoft infrastructure following inbound mail activity.

Detection Strategies

Hunt for anomalous process lineage where Outlook spawns command interpreters, LOLBins, or writes executables to user-writable directories such as %APPDATA% or ~/Library.
Correlate inbound email delivery events with endpoint telemetry showing Outlook memory faults, DLL side-loading, or unexpected module loads.
Deploy behavioral identification rules that flag Office applications performing network connections or persistence operations inconsistent with normal mail client behavior.

Monitoring Recommendations

Enable detailed process creation logging (Windows Event ID 4688 with command-line auditing, or equivalent macOS endpoint telemetry) for Outlook and child processes.
Monitor mail gateway telemetry for messages with malformed MIME structures, unusual attachment types, or oversized header fields that could carry an exploit payload.
Centralize Office crash dumps and endpoint alerts in a SIEM for correlation with mail flow events and user activity.

How to Mitigate CVE-2026-108448

Immediate Actions Required

Apply the Microsoft security updates referenced in the Microsoft CVE-2026-108448 Advisory to all affected Microsoft 365 Apps, Office 2021 LTSC, and Office 2024 LTSC installations on Windows and macOS.
Prioritize patching for users whose mailboxes are exposed to external senders, including executives, finance, HR, and IT administrators.
Verify Click-to-Run and Microsoft 365 update channels are enabled and reporting successful deployment for the current patch cycle.

Patch Information

Microsoft has published a vendor advisory with update guidance at the Microsoft Security Response Center. Administrators should consult the advisory to identify the specific build numbers and KB articles that remediate CVE-2026-108448 for each supported channel and platform. Deploy updates through Microsoft Configuration Manager, Intune, WSUS, Jamf, or the Microsoft AutoUpdate service for macOS, depending on environment.

Workarounds

Disable the Outlook reading pane for untrusted mailboxes to reduce automatic parsing of inbound messages until patches are applied.
Enforce attachment filtering and content disarm and reconstruction (CDR) at the mail gateway for high-risk MIME types.
Restrict Outlook child-process creation through application control policies, such as Windows Defender Application Control, AppLocker, or macOS endpoint controls, to blunt post-exploitation activity.

bash

# Configuration example: block common Office child processes via Attack Surface Reduction (Windows)
Set-MpPreference -AttackSurfaceReductionRules_Ids 26190899-1602-49e8-8b27-eb1d0a1ce869 -AttackSurfaceReductionRules_Actions Enabled
Set-MpPreference -AttackSurfaceReductionRules_Ids d4f940ab-401b-4efc-aadc-ad5f3c50688a -AttackSurfaceReductionRules_Actions Enabled