CVE-2025-29823: Microsoft 365 Apps Use After Free Flaw

CVE-2025-29823 Overview

CVE-2025-29823 is a use-after-free vulnerability in Microsoft Office Excel that allows an unauthorized attacker to execute code locally. The flaw affects Microsoft 365 Apps and is tracked under [CWE-416: Use After Free]. Exploitation requires user interaction, meaning a victim must open a crafted Excel document. Successful exploitation grants the attacker the ability to run arbitrary code in the context of the current user. Microsoft published the advisory on April 8, 2025, and the entry was last updated on July 8, 2025.

Critical Impact
An attacker who convinces a user to open a malicious Excel file can execute arbitrary code locally, compromising confidentiality, integrity, and availability of the host.

Affected Products

Microsoft 365 Apps Enterprise (x64)
Microsoft 365 Apps Enterprise (x86)
Microsoft Office Excel component within Microsoft 365 Apps

Discovery Timeline

2025-04-08 - CVE-2025-29823 published to NVD
2025-04-08 - Microsoft releases security update via the Microsoft CVE-2025-29823 Advisory
2025-07-08 - Last updated in NVD database

Technical Details for CVE-2025-29823

Vulnerability Analysis

The vulnerability is a use-after-free condition [CWE-416] within Microsoft Office Excel. Excel parses complex spreadsheet structures, including objects, styles, formulas, and embedded content. A use-after-free occurs when the application continues to reference memory after it has been freed, allowing an attacker to manipulate the contents of that freed region. When Excel later dereferences the dangling pointer, attacker-controlled data is treated as a valid object, leading to arbitrary code execution under the user's privileges.

The attack vector is local and requires user interaction. An attacker delivers a crafted .xlsx, .xlsm, or related Excel file via email, web download, or shared storage. Opening the file triggers the vulnerable parsing path. The Microsoft advisory categorizes this issue under remote code execution despite the local attack vector, because the payload arrives through a user-opened document.

Root Cause

The root cause is improper management of object lifetime within an Excel parsing routine. A pointer to a heap-allocated object is retained and later dereferenced after the object has been released. Attacker-supplied data in the crafted document controls the reallocation, enabling code flow hijacking.

Attack Vector

Attackers weaponize a malicious Excel document and rely on social engineering to trigger user interaction. Once the file is opened in a vulnerable build of Microsoft 365 Apps, the use-after-free triggers, and shellcode runs with the privileges of the logged-in user. Refer to the Microsoft CVE-2025-29823 Advisory for vendor-supplied technical context.

// No verified proof-of-concept is publicly available for CVE-2025-29823.
// Refer to the Microsoft Security Response Center advisory for technical details.

Detection Methods for CVE-2025-29823

Indicators of Compromise

Excel documents arriving from untrusted senders that contain unusual embedded objects, ActiveX controls, or malformed XML parts
EXCEL.EXE spawning child processes such as cmd.exe, powershell.exe, rundll32.exe, or regsvr32.exe
Unexpected outbound network connections from EXCEL.EXE shortly after a document is opened
New persistence artifacts (scheduled tasks, Run keys, startup folder entries) created in the user context following Excel activity

Detection Strategies

Hunt for process lineage where EXCEL.EXE is the parent of script interpreters or living-off-the-land binaries (LOLBins)
Inspect Office telemetry for crashes in EXCEL.EXE correlated with document opens, which can indicate failed exploitation attempts
Apply YARA rules to email gateways and file shares to flag Excel files with anomalous structures consistent with public exploit research on Office UAF issues
Correlate document open events with subsequent file writes to %APPDATA%, %TEMP%, or %PROGRAMDATA%

Monitoring Recommendations

Forward Microsoft Defender for Office 365 and endpoint telemetry to a centralized SIEM or data lake for retrospective hunting
Monitor Office Click-to-Run version numbers across the fleet to identify hosts that have not received the April 2025 update
Alert on Mark-of-the-Web (MOTW) bypass attempts, since attackers commonly strip MOTW to defeat Protected View

How to Mitigate CVE-2025-29823

Immediate Actions Required

Apply the April 2025 Microsoft 365 Apps security update referenced in the Microsoft CVE-2025-29823 Advisory
Verify the Office Click-to-Run channel and confirm the deployed build matches or exceeds the patched version for your update channel
Enforce Protected View and Application Guard for documents originating from the internet or email
Block or quarantine Excel attachments from untrusted external senders at the mail gateway

Patch Information

Microsoft addressed CVE-2025-29823 in the April 8, 2025 security release for Microsoft 365 Apps. Administrators should consult the Microsoft CVE-2025-29823 Advisory for the specific build numbers corresponding to each update channel (Current, Monthly Enterprise, and Semi-Annual Enterprise) and force update deployment through Microsoft Configuration Manager, Intune, or the Office Deployment Tool.

Workarounds

Enable Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and from creating executable content
Configure Group Policy to open files from the internet only in Protected View and disable macro execution for files from the internet
Restrict opening of Excel files from untrusted sources until patching is verified across the environment

bash

# Force a Microsoft 365 Apps update via Click-to-Run on Windows
"C:\Program Files\Common Files\Microsoft Shared\ClickToRun\OfficeC2RClient.exe" /update user displaylevel=true

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