CVE-2025-21402: Microsoft Office OneNote RCE Vulnerability

CVE-2025-21402 Overview

CVE-2025-21402 is a remote code execution vulnerability affecting Microsoft Office OneNote on macOS platforms. This vulnerability allows attackers to execute arbitrary code on a target system when a user opens a specially crafted file. The attack requires user interaction, making social engineering an important component of potential exploitation scenarios.

Critical Impact

Successful exploitation could allow an attacker to execute arbitrary code with the privileges of the current user, potentially leading to complete system compromise, data theft, or lateral movement within an organization.

Affected Products

• Microsoft Office 2021 LTSC for macOS
• Microsoft Office 2024 LTSC for macOS
• Microsoft OneNote for macOS

Discovery Timeline

• 2025-01-14 - CVE CVE-2025-21402 published to NVD
• 2025-01-27 - Last updated in NVD database

Technical Details for CVE-2025-21402

Vulnerability Analysis

This vulnerability is classified under CWE-641 (Improper Restriction of Names for Files and Other Resources), indicating that the flaw relates to how OneNote processes or handles file names or resource identifiers. The local attack vector requires an attacker to deliver a malicious file to the victim, typically through email attachments, malicious downloads, or shared storage locations.

The vulnerability requires no special privileges to exploit, but does require user interaction—specifically, the victim must open a maliciously crafted file. Once exploited, the attacker gains the ability to compromise the confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of this vulnerability stems from improper restriction of names for files and other resources within Microsoft OneNote. This weakness allows specially crafted content to bypass normal security controls and execute arbitrary code. The improper handling of resource names can lead to unexpected behavior when processing malicious input.

Attack Vector

The attack vector is local, meaning an attacker cannot directly exploit this vulnerability over the network. Instead, attackers must deliver a specially crafted OneNote file (.one) to the target user. Common delivery mechanisms include:

• Phishing emails with malicious OneNote attachments
• Shared network drives or cloud storage containing malicious files
• Website downloads disguised as legitimate documents
• USB drives or other removable media

When the victim opens the malicious file, the vulnerability is triggered, allowing code execution in the context of the current user.

The exploitation mechanism leverages the improper resource name handling in OneNote to execute arbitrary code. Technical details are available in the Microsoft Security Update Guide.

Detection Methods for CVE-2025-21402

Indicators of Compromise

• Unexpected OneNote (.one) files appearing in download folders or email attachments from unknown sources
• Abnormal process spawning from the OneNote application process
• Suspicious network connections originating from OneNote processes
• Unusual file system activity following the opening of OneNote documents

Detection Strategies

• Monitor for unusual child processes spawned by Microsoft OneNote
• Implement email filtering rules to quarantine suspicious OneNote attachments
• Deploy endpoint detection rules to identify anomalous behavior in Office applications
• Review application logs for signs of exploitation attempts

Monitoring Recommendations

• Enable enhanced logging for Microsoft Office applications
• Configure SIEM rules to alert on suspicious OneNote file access patterns
• Monitor user workstations for unexpected code execution following document opens
• Track network traffic anomalies associated with OneNote processes

How to Mitigate CVE-2025-21402

Immediate Actions Required

• Apply the latest security updates from Microsoft for affected Office products
• Educate users about the risks of opening unexpected OneNote files from untrusted sources
• Consider blocking OneNote file attachments at the email gateway until patches are applied
• Enable application sandboxing features where available

Patch Information

Microsoft has released security updates to address this vulnerability. Administrators should apply the appropriate patches for their environment as soon as possible. Detailed patching guidance is available in the Microsoft Security Update Guide.

For macOS environments, ensure Microsoft AutoUpdate is configured to retrieve and install the latest Office updates, or manually download and apply patches from Microsoft's official channels.

Workarounds

• Block or quarantine OneNote (.one) files at email gateways until patching is complete
• Restrict OneNote file execution to verified trusted sources only
• Implement application whitelisting to control executable behavior
• Train users to verify the source of any OneNote documents before opening

# Example: Block OneNote files using mail server rules (conceptual)
# Consult your email gateway documentation for specific syntax
# Add .one extension to blocked attachment list
# Enable content scanning for Office document attachments