CVE-2026-21260: Microsoft 365 Apps Info Disclosure Flaw

CVE-2026-21260 Overview

CVE-2026-21260 is a high-severity information disclosure vulnerability affecting Microsoft Office Outlook and related Microsoft products. This vulnerability allows an unauthorized attacker to expose sensitive information to unauthorized actors, enabling network-based spoofing attacks without requiring user interaction or authentication.

The vulnerability stems from improper handling of sensitive information (CWE-200) in the affected Microsoft Office components, potentially allowing attackers to intercept or access confidential data transmitted over the network.

Critical Impact

Unauthorized attackers can exploit this vulnerability remotely over a network to access sensitive information and perform spoofing attacks, potentially compromising email communications and user credentials across enterprise environments.

Affected Products

• Microsoft 365 Apps for Enterprise (x64 and x86)
• Microsoft Office 2019 (x64 and x86)
• Microsoft Office Long Term Servicing Channel 2021 and 2024 (x64 and x86)
• Microsoft Outlook 2016 (x64 and x86)
• Microsoft SharePoint Server 2016, 2019, and Subscription Edition

Discovery Timeline

• February 10, 2026 - CVE-2026-21260 published to NVD
• February 11, 2026 - Last updated in NVD database

Technical Details for CVE-2026-21260

Vulnerability Analysis

This vulnerability is classified as an Information Exposure issue (CWE-200) that enables spoofing attacks over the network. The vulnerability can be exploited remotely without requiring any privileges or user interaction, making it particularly dangerous in enterprise environments where Microsoft Office products are widely deployed.

The attack targets the confidentiality of data processed by Microsoft Outlook and related Office applications. Once exploited, an attacker gains access to sensitive information that can be leveraged for subsequent spoofing attacks, potentially impersonating legitimate users or services within the organization.

Root Cause

The root cause of CVE-2026-21260 lies in the improper handling of sensitive information within Microsoft Office Outlook's network communication components. The vulnerability allows sensitive data to be exposed to unauthorized actors due to insufficient protection mechanisms when processing or transmitting information over network connections.

This information disclosure flaw enables attackers to gather sensitive details that facilitate network-based spoofing, potentially allowing them to impersonate trusted entities or intercept communications.

Attack Vector

The attack is network-based and requires no authentication or user interaction, making it exploitable by any remote attacker who can reach the vulnerable system. The exploitation flow involves:

1. An attacker positions themselves to intercept network traffic or initiate connections to vulnerable Microsoft Office installations
2. The attacker exploits the information disclosure flaw to extract sensitive data
3. Using the obtained information, the attacker can perform spoofing attacks to impersonate legitimate users or services
4. The attack compromises the confidentiality of communications without impacting integrity or availability

The vulnerability does not require social engineering or phishing - it can be exploited purely through network-level access to systems running affected Microsoft Office products.

Detection Methods for CVE-2026-21260

Indicators of Compromise

• Unusual network traffic patterns from Microsoft Outlook processes, particularly unexpected outbound connections
• Anomalous authentication attempts using credentials associated with Outlook users
• Evidence of spoofed email messages appearing to originate from legitimate internal users
• Network logs showing suspicious data extraction patterns from Office application traffic

Detection Strategies

• Deploy network monitoring to detect unusual communication patterns from Microsoft Office applications
• Implement email security solutions to identify spoofed messages exploiting disclosed information
• Monitor for anomalous Outlook process behavior using endpoint detection and response (EDR) tools
• Review authentication logs for signs of credential abuse following potential information disclosure

Monitoring Recommendations

• Enable enhanced logging for Microsoft Office applications and email servers
• Configure SIEM rules to alert on suspicious Outlook network activity patterns
• Implement network traffic analysis to identify potential information exfiltration attempts
• Monitor for signs of spoofing attacks such as unusual sender patterns or authentication anomalies

How to Mitigate CVE-2026-21260

Immediate Actions Required

• Apply Microsoft security updates immediately to all affected Office installations
• Review and restrict network access to systems running vulnerable Microsoft Office versions
• Implement network segmentation to limit potential attack surface
• Enable multi-factor authentication to reduce the impact of credential disclosure

Patch Information

Microsoft has released security updates to address this vulnerability. Administrators should apply the patches available through Microsoft Update or Windows Server Update Services (WSUS). Refer to the Microsoft Security Response Center advisory for specific patch details and download links.

For enterprise deployments, utilize Microsoft Endpoint Configuration Manager or Intune to deploy updates across all affected systems. Prioritize patching for internet-facing systems and those handling sensitive communications.

Workarounds

• Restrict outbound network access from Microsoft Office applications using firewall rules where feasible
• Implement enhanced email authentication protocols (SPF, DKIM, DMARC) to mitigate spoofing impact
• Consider temporary isolation of critical systems running vulnerable Office versions until patches can be applied
• Enable additional email security filtering to detect potential spoofing attempts

Organizations should prioritize patching over workarounds, as workarounds may not fully address the vulnerability and could impact legitimate functionality.