CVE-2023-21741: Microsoft 365 Apps Info Disclosure Flaw

CVE-2023-21741 Overview

CVE-2023-21741 is an information disclosure vulnerability affecting Microsoft Office Visio. This vulnerability allows an authenticated attacker to exploit an out-of-bounds read condition to access sensitive information from memory. The flaw exists in how Visio processes certain file structures, enabling unauthorized disclosure of potentially sensitive data that could be leveraged in further attacks against affected systems.

Critical Impact

Authenticated attackers can exploit this vulnerability remotely to access sensitive information from memory, potentially exposing confidential data and enabling reconnaissance for subsequent attacks against the affected system.

Affected Products

• Microsoft 365 Apps for Enterprise
• Microsoft Office 2019
• Microsoft Office Long Term Servicing Channel 2021
• Microsoft Visio 2013 SP1
• Microsoft Visio 2016

Discovery Timeline

• January 10, 2023 - CVE-2023-21741 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-21741

Vulnerability Analysis

This vulnerability is classified under CWE-125 (Out-of-Bounds Read), a memory corruption class weakness where the software reads data past the end or before the beginning of the intended buffer. In Microsoft Visio, this manifests when the application processes malformed or specially crafted input, causing it to read memory beyond allocated boundaries.

The out-of-bounds read condition allows attackers to access portions of memory that should not be accessible, potentially revealing sensitive information such as memory addresses, cryptographic keys, or other confidential data stored in adjacent memory locations. This type of vulnerability is particularly concerning as the disclosed information can be used to bypass security mechanisms like Address Space Layout Randomization (ASLR) or to extract confidential business data.

The network-based attack vector with low complexity makes this vulnerability relatively easy to exploit once an attacker has low-privilege authentication to the target system.

Root Cause

The root cause stems from insufficient bounds checking when Visio processes certain data structures. When parsing input, the application fails to properly validate buffer boundaries before performing read operations, allowing memory contents beyond the intended buffer to be accessed. This is a common programming error where array indices or pointer arithmetic is not properly constrained to the allocated memory region.

Attack Vector

The vulnerability is exploitable over the network by an authenticated attacker. The attack requires a user with low privileges to interact with the Visio application in a way that triggers the vulnerable code path. This could involve opening a specially crafted Visio file or processing malformed data through the application.

The attacker does not require user interaction beyond their own authentication, and the attack can be initiated remotely. Once exploited, the attacker gains access to potentially sensitive information from the application's memory space, which could include:

• Memory layout information useful for bypassing ASLR
• Sensitive data from other documents or processes
• Internal application state information

For detailed technical analysis and remediation guidance, refer to the Microsoft Security Update for CVE-2023-21741.

Detection Methods for CVE-2023-21741

Indicators of Compromise

• Unusual memory access patterns in Microsoft Visio processes
• Visio application crashes or unexpected termination events
• Network connections from Visio to unexpected external destinations
• Presence of suspicious or malformed .vsd, .vsdx, or .vstx files

Detection Strategies

• Monitor for anomalous behavior in VISIO.EXE process, including unusual memory read operations
• Implement file integrity monitoring for Visio document directories to detect malformed files
• Deploy endpoint detection rules to identify out-of-bounds memory access attempts
• Use SentinelOne's behavioral AI to detect exploitation attempts targeting Microsoft Office applications

Monitoring Recommendations

• Enable Windows Event Logging for application crashes and exceptions in Office applications
• Configure alerts for unusual network activity originating from Visio processes
• Implement document scanning solutions to inspect Visio files before opening
• Monitor for repeated authentication attempts from unusual sources accessing Office applications

How to Mitigate CVE-2023-21741

Immediate Actions Required

• Apply the Microsoft security update addressing CVE-2023-21741 immediately
• Verify all Microsoft Office and Visio installations are updated to patched versions
• Review network access policies to limit exposure of vulnerable Visio installations
• Implement the principle of least privilege to minimize authenticated attacker access

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should apply the appropriate updates for their installed versions:

• Microsoft 365 Apps for Enterprise - Apply the latest security update through Microsoft Update
• Microsoft Office 2019 - Install the January 2023 security update
• Microsoft Office LTSC 2021 - Install the January 2023 security update
• Microsoft Visio 2013 SP1 - Install the January 2023 security update
• Microsoft Visio 2016 - Install the January 2023 security update

For complete patch details and download links, refer to the Microsoft Security Update Guide.

Workarounds

• Restrict network access to systems running unpatched Visio installations until updates can be applied
• Implement application whitelisting to prevent unauthorized Visio file types from executing
• Configure Office Protected View to open files from untrusted locations in read-only mode
• Train users to avoid opening Visio files from untrusted or unexpected sources

# Example: Enforce Protected View via Group Policy
# Path: User Configuration > Administrative Templates > Microsoft Visio > Visio Options > Security > Trust Center > Protected View
# Enable: "Always open files from the Internet in Protected View"
# Enable: "Always open files in Unsafe Locations in Protected View"