CVE-2023-36766: Microsoft Excel Information Disclosure

CVE-2023-36766 Overview

CVE-2023-36766 is an information disclosure vulnerability affecting Microsoft Excel and related Microsoft Office products. This vulnerability allows attackers to potentially access sensitive information from affected systems through specially crafted Excel documents. The vulnerability requires user interaction, as a victim must open a malicious file for exploitation to occur.

Critical Impact

Successful exploitation of this vulnerability could result in unauthorized disclosure of sensitive information stored in memory, potentially exposing confidential data to attackers.

Affected Products

• Microsoft 365 Apps (Enterprise)
• Microsoft Excel 2013 SP1, Excel 2016
• Microsoft Office 2019 (Windows and macOS)
• Microsoft Office Long Term Servicing Channel 2021
• Microsoft Office Online Server

Discovery Timeline

• September 12, 2023 - CVE-2023-36766 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-36766

Vulnerability Analysis

This information disclosure vulnerability is classified under CWE-125 (Out-of-Bounds Read), indicating that the underlying issue involves improper memory access operations within Microsoft Excel's file parsing routines. When processing certain malformed Excel documents, the application may read data from memory locations outside the intended buffer boundaries, potentially exposing sensitive information that resides in adjacent memory regions.

The vulnerability requires local access and user interaction, meaning an attacker must convince a user to open a specially crafted Excel file. Once opened, the malicious document can trigger the out-of-bounds read condition, causing the application to leak confidential data that may include process memory contents, credentials, or other sensitive information stored in memory at the time of exploitation.

Root Cause

The root cause of CVE-2023-36766 stems from insufficient bounds checking during the parsing of Excel file structures. When Excel processes certain elements within a document, it fails to properly validate the size or offset parameters, leading to an out-of-bounds read condition. This allows the application to access memory beyond the allocated buffer, resulting in information disclosure.

Attack Vector

The attack vector for this vulnerability is local, requiring an attacker to deliver a malicious Excel file to the victim through social engineering techniques such as phishing emails, compromised websites, or file-sharing platforms. The victim must then open the file using a vulnerable version of Microsoft Excel. No privileges are required by the attacker, but user interaction is necessary to trigger the vulnerability.

The exploitation scenario typically involves:

1. Attacker creates a specially crafted Excel document containing malformed structures
2. Document is delivered to the target via email attachment, download link, or other means
3. Victim opens the document in a vulnerable Microsoft Excel version
4. The out-of-bounds read is triggered during file parsing
5. Sensitive memory contents are exposed to the attacker, potentially through embedded macros or other data exfiltration mechanisms

Detection Methods for CVE-2023-36766

Indicators of Compromise

• Unusual Excel files with abnormal internal structure or malformed embedded objects
• Excel processes exhibiting unexpected memory access patterns or crashes
• Outbound network connections from Excel processes to unknown destinations following document opening
• Suspicious email attachments containing Excel files from untrusted sources

Detection Strategies

• Implement file integrity monitoring to detect malformed Excel documents entering the environment
• Deploy endpoint detection and response (EDR) solutions to monitor Excel process behavior for anomalous memory access patterns
• Configure email security gateways to scan and sandbox Excel attachments before delivery
• Enable Microsoft Defender for Office 365 to detect malicious document behavior

Monitoring Recommendations

• Monitor for unusual Excel process memory consumption or access violations
• Track Excel file opens from untrusted sources such as email attachments or internet downloads
• Review Windows event logs for application crashes related to EXCEL.EXE
• Implement user behavior analytics to identify users opening suspicious attachments

How to Mitigate CVE-2023-36766

Immediate Actions Required

• Apply the security updates provided by Microsoft immediately to all affected systems
• Enable Protected View for files originating from the internet in Microsoft Office Trust Center settings
• Block Excel files from untrusted sources at the email gateway level
• Educate users about the risks of opening Excel attachments from unknown senders

Patch Information

Microsoft has released security updates to address CVE-2023-36766 as part of their September 2023 security update cycle. Organizations should apply the appropriate patches for their installed Microsoft Office and Microsoft 365 Apps versions. Detailed patch information is available in the Microsoft Security Update Advisory.

For Microsoft 365 Apps for Enterprise, ensure automatic updates are enabled to receive the latest security fixes. For standalone Office installations, download and apply the relevant cumulative updates from the Microsoft Update Catalog.

Workarounds

• Enable Protected View for all files by default in Office Trust Center settings to prevent automatic execution of malicious content
• Implement Application Guard for Office to isolate potentially malicious documents in a containerized environment
• Configure Microsoft Defender Application Guard policies to open untrusted Excel files in an isolated container
• Restrict macro execution by enabling "Disable all macros without notification" in Trust Center settings

# Registry configuration to enable Protected View for all locations
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableAttachementsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f