CVE-2026-55018: Microsoft 365 Apps Use-After-Free Flaw

CVE-2026-55018 Overview

CVE-2026-55018 is a use-after-free vulnerability [CWE-416] in Microsoft Office that allows an unauthorized attacker to execute code locally. The flaw affects multiple supported Office releases across Windows and macOS, including Microsoft 365 Apps, Microsoft 365 for Mac, and Office 2016, 2019, 2021, and 2024. Exploitation requires user interaction, typically by opening a specially crafted document. Successful exploitation results in arbitrary code execution in the context of the current user, compromising confidentiality, integrity, and availability of the host.

Critical Impact
A crafted Office document can trigger use-after-free memory corruption and execute attacker-controlled code in the user's session, enabling initial access and further compromise.

Affected Products

Microsoft 365 Apps (x86 and x64 Enterprise)
Microsoft 365 for macOS
Microsoft Office 2016, 2019, 2021 LTSC, and 2024 LTSC (Windows and macOS builds where applicable)

Discovery Timeline

2026-07-14 - CVE-2026-55018 published to NVD
2026-07-16 - Last updated in NVD database

Technical Details for CVE-2026-55018

Vulnerability Analysis

The vulnerability is a use-after-free condition [CWE-416] in Microsoft Office document parsing and rendering code. Office continues to reference a memory object after the underlying allocation has been freed. When the attacker controls the freed region through subsequent allocations, the dangling pointer dereference redirects execution to attacker-influenced data. This yields arbitrary code execution within the security context of the user opening the document. The attack is local and requires user interaction, but does not require prior authentication to the target system.

Root Cause

The root cause is improper lifetime management of an internal object within Office. Code paths release an allocation while other references remain live, violating the invariant that a pointer must not be used after free(). An attacker can shape the heap so that the released chunk is reclaimed with controlled content, converting the stale reference into a controlled read or write primitive.

Attack Vector

The attack vector is local with user interaction (AV:L/UI:R). A typical exploitation chain involves delivering a weaponized Office file over email, a file share, or a web download. When the user opens the document, embedded content or an object type triggers the vulnerable code path. Because delivery relies on social engineering, phishing campaigns and malicious attachments are the most likely operational scenarios. No verified public proof-of-concept code was available at the time of publication, and this vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

See the Microsoft Security Response Center advisory for CVE-2026-55018 for vendor technical details.

Detection Methods for CVE-2026-55018

Indicators of Compromise

Office processes (WINWORD.EXE, EXCEL.EXE, POWERPNT.EXE) spawning unexpected child processes such as cmd.exe, powershell.exe, wscript.exe, mshta.exe, or rundll32.exe.
Unexpected file writes by Office processes into user-writable locations such as %APPDATA%, %TEMP%, or startup folders.
Outbound network connections initiated directly by an Office process to previously unseen domains or IPs shortly after a document open event.
Crash artifacts (WerFault.exe) referencing Office binaries after opening a specific document.

Detection Strategies

Alert on Office application process trees that deviate from baseline, particularly parent-child relationships involving script interpreters or LOLBins.
Inspect inbound email attachments and downloaded files for Office documents containing embedded objects, ActiveX controls, or unusual OLE structures.
Correlate document-open telemetry with subsequent process, file, and network events to identify exploitation chains originating from a single file.

Monitoring Recommendations

Enable and forward Microsoft Office telemetry, Sysmon process and image-load events, and EDR document-open events to a centralized data lake.
Monitor for repeated Office crashes on the same user or host, which can indicate exploitation attempts or unstable exploit development.
Track execution of Office add-ins and macros loaded from untrusted paths, and review Protected View bypasses.

How to Mitigate CVE-2026-55018

Immediate Actions Required

Apply Microsoft's security update for CVE-2026-55018 across all Windows and macOS Office installations as soon as testing permits.
Confirm Microsoft 365 Apps update channels are current and that offline or LTSC installations receive the corresponding patch.
Enforce Protected View and Office Application Guard for documents originating from the internet or email.
Block or quarantine Office documents from untrusted external senders at the mail gateway pending patch deployment.

Patch Information

Microsoft has published guidance and updates through the Microsoft Security Response Center. Refer to the Microsoft CVE-2026-55018 Update advisory for the specific build numbers and KB identifiers applicable to each affected product and channel.

Workarounds

Configure Attack Surface Reduction (ASR) rules to block Office applications from creating child processes and from injecting code into other processes.
Disable ActiveX controls and set macros to disabled by default via Group Policy or the Office Trust Center.
Deliver high-risk documents through a rendered preview or sandboxed viewer instead of full Office clients until patches are applied.

bash

# Example: enable ASR rule blocking Office child process creation (PowerShell)
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled