CVE-2026-55049: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-55049 Overview

CVE-2026-55049 is a heap-based buffer overflow vulnerability [CWE-122] in Microsoft Office that allows an unauthorized attacker to execute arbitrary code locally on affected systems. The flaw affects multiple supported Office releases, including Microsoft 365 Apps and Office 2016 through 2024. Exploitation requires user interaction, typically through opening a crafted Office document. Successful exploitation grants the attacker code execution in the context of the current user, impacting confidentiality, integrity, and availability.

Critical Impact
A crafted Office document can trigger a heap overflow that leads to arbitrary code execution in the user's context, enabling malware deployment, data theft, and lateral movement.

Affected Products

Microsoft 365 Apps (Enterprise, x86 and x64)
Microsoft Office 2016 and Office 2019 (x86 and x64)
Microsoft Office 2021 and Office 2024 LTSC (Windows x86/x64 and macOS)

Discovery Timeline

2026-07-14 - CVE-2026-55049 published to the National Vulnerability Database (NVD)
2026-07-16 - Last updated in NVD database

Technical Details for CVE-2026-55049

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow [CWE-122] within Microsoft Office document parsing routines. When Office processes a specially crafted file, it writes data beyond the bounds of a heap-allocated buffer. This corruption can overwrite adjacent heap metadata or object pointers used by Office components. An attacker who controls the overflow contents can steer execution flow to attacker-supplied code. The result is arbitrary code execution running with the privileges of the user who opened the document.

Root Cause

The root cause is improper validation of size or length values during parsing of structured document content. Office allocates a heap buffer based on assumed input constraints, then copies attacker-controlled data without enforcing bounds. Because the overflow occurs on the heap rather than the stack, exploitation typically relies on manipulating adjacent heap objects or function pointers to achieve control-flow hijack.

Attack Vector

The attack vector is local and requires user interaction. An attacker delivers a malicious Office document through email, a web download, a file share, or a collaboration platform. When the victim opens the file with a vulnerable Office version, the parser triggers the overflow. No authentication or elevated privileges are required on the target, but the attacker must convince the user to open the file. Code executes at the current user's privilege level and can be chained with local privilege escalation flaws for full system compromise.

No verified public proof-of-concept code is available at the time of publication. See the Microsoft Security Update CVE-2026-55049 advisory for technical details.

Detection Methods for CVE-2026-55049

Indicators of Compromise

Unexpected child processes spawned from winword.exe, excel.exe, powerpnt.exe, or outlook.exe, such as cmd.exe, powershell.exe, rundll32.exe, or mshta.exe.
Office application crashes with heap corruption exceptions in the Windows Event Log immediately after opening a document.
Newly created executables, scripts, or scheduled tasks appearing in user-writable directories after document open events.
Outbound network connections initiated directly by Office processes to previously unseen domains or IP addresses.

Detection Strategies

Monitor process ancestry for Office binaries spawning interpreters, shells, or living-off-the-land binaries.
Alert on Office processes performing suspicious file writes to %APPDATA%, %TEMP%, or startup folders.
Inspect email gateways and web proxies for Office file types containing anomalous embedded objects or oversized structured storage streams.
Correlate Windows Error Reporting entries showing Office crashes with subsequent process creation or network activity.

Monitoring Recommendations

Enable Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and injecting into other processes.
Forward Sysmon process, file, and network telemetry to a centralized analytics platform for behavioral correlation.
Track document open events and correlate them with subsequent user-context process launches over short time windows.

How to Mitigate CVE-2026-55049

Immediate Actions Required

Apply the Microsoft security update for CVE-2026-55049 to all affected Office installations as soon as testing permits.
Inventory endpoints running Microsoft 365 Apps and Office 2016, 2019, 2021, and 2024 to confirm patch coverage.
Block or quarantine inbound Office documents from untrusted external senders at the email gateway pending patch deployment.
Enforce Protected View and Office macro restrictions through Group Policy for documents originating from the internet.

Patch Information

Microsoft published a security update addressing CVE-2026-55049. Refer to the Microsoft Security Update CVE-2026-55049 guide for the specific build numbers and channels applicable to each Office SKU. Microsoft 365 Apps receive the fix through the standard update channels, while Office 2016, 2019, 2021, and 2024 LTSC require the corresponding monthly security update package.

Workarounds

Open untrusted Office documents in Protected View and prevent users from exiting Protected View without justification.
Configure Microsoft Defender ASR rules to block child process creation from Office applications.
Restrict opening of Office file types delivered from external sources by enforcing Mark-of-the-Web propagation and blocking risky file extensions at the mail gateway.

bash

# Enable Attack Surface Reduction rule: Block Office applications from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled

# Enforce Protected View for files originating from the internet (registry)
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f