CVE-2021-31174: Microsoft Excel Information Disclosure

CVE-2021-31174 Overview

CVE-2021-31174 is an information disclosure vulnerability affecting Microsoft Excel and related Microsoft Office products. This vulnerability stems from an out-of-bounds read condition (CWE-125) that allows attackers with local access to potentially extract sensitive information from memory. When successfully exploited, attackers can read data beyond the intended boundaries of a buffer, potentially exposing confidential information stored in memory.

Critical Impact

Successful exploitation allows attackers to read sensitive information from memory through out-of-bounds read operations, potentially compromising confidentiality of data processed by Microsoft Excel.

Affected Products

• Microsoft 365 Apps (Enterprise)
• Microsoft Excel 2013 SP1 and Excel 2016
• Microsoft Office 2013 SP1, Office 2016, and Office 2019
• Microsoft Office Online Server
• Microsoft Office Web Apps Server 2013 SP1

Discovery Timeline

• May 11, 2021 - CVE-2021-31174 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-31174

Vulnerability Analysis

This vulnerability is classified as an out-of-bounds read (CWE-125), a memory corruption issue where the application reads data from a memory location that is outside the bounds of the intended buffer. In the context of Microsoft Excel, this type of vulnerability typically occurs during file parsing or data processing operations.

The local attack vector requires an attacker to either have direct access to the target system or convince a user to open a specially crafted Excel file. Once triggered, the vulnerability enables unauthorized reading of memory contents that should not be accessible, potentially exposing sensitive information such as credentials, encryption keys, or other confidential data stored in adjacent memory locations.

Root Cause

The root cause of CVE-2021-31174 is an out-of-bounds read condition within Microsoft Excel's data processing routines. This occurs when the application fails to properly validate array indices or buffer boundaries before accessing memory, allowing read operations to extend beyond the allocated buffer space. The vulnerability likely exists in code paths responsible for parsing complex Excel file structures or handling specific data types.

Attack Vector

The attack requires local access to the target system, meaning an attacker must either:

1. Have direct access to a machine running vulnerable Microsoft Office products
2. Convince a legitimate user to open a malicious Excel document crafted to trigger the out-of-bounds read condition

Once the malicious file is opened or the vulnerable code path is reached, the out-of-bounds read occurs, potentially leaking memory contents to the attacker. The vulnerability does not require elevated privileges, as it can be triggered by a standard user with low privileges.

The vulnerability exploits insufficient boundary checking in Excel's memory access operations. When processing malformed or specially crafted data, the application reads beyond the intended memory buffer boundaries, exposing adjacent memory contents. This information disclosure can serve as a stepping stone for more sophisticated attacks by revealing memory layout information or sensitive data.

Detection Methods for CVE-2021-31174

Indicators of Compromise

• Unusual Microsoft Excel crash events or application stability issues when opening specific files
• Presence of suspicious Excel files with abnormal internal structures in email attachments or downloads
• Memory access violations logged in Windows Event Viewer associated with Excel processes
• Unexpected network connections from Excel processes attempting to exfiltrate data

Detection Strategies

• Monitor for abnormal Excel process behavior including unexpected memory access patterns
• Implement file integrity monitoring on incoming Excel documents before user access
• Deploy endpoint detection solutions capable of identifying out-of-bounds memory read attempts
• Configure security tools to flag Excel files from untrusted sources

Monitoring Recommendations

• Enable detailed Windows Event logging for Office application crashes and errors
• Monitor for suspicious file access patterns in user download and email attachment directories
• Implement behavioral analysis to detect anomalous Excel process memory operations
• Review security logs for patterns indicating information exfiltration attempts

How to Mitigate CVE-2021-31174

Immediate Actions Required

• Apply Microsoft security updates released in May 2021 Patch Tuesday for all affected Office products
• Restrict users from opening Excel files from untrusted or unknown sources
• Enable Protected View for files originating from the internet or email attachments
• Deploy application whitelisting to prevent execution of unauthorized Office add-ins

Patch Information

Microsoft has released security patches addressing this vulnerability as part of their May 2021 security updates. Organizations should apply the appropriate patches based on their installed Office versions:

• Microsoft 365 Apps for Enterprise: Apply latest updates through Microsoft Update
• Microsoft Excel 2013 SP1 / Excel 2016: Install the corresponding security updates
• Microsoft Office 2013 SP1 / Office 2016 / Office 2019: Apply cumulative security updates
• Office Online Server / Office Web Apps Server 2013 SP1: Deploy server-side patches

For detailed patch information and download links, refer to the Microsoft Security Advisory for CVE-2021-31174.

Workarounds

• Enable Protected View for all Office documents from external sources to provide sandboxed viewing
• Configure Microsoft Office File Block settings to prevent opening of suspicious file formats
• Implement email filtering rules to quarantine Excel attachments from untrusted senders for analysis
• Disable ActiveX controls and macros in Excel through Group Policy to reduce attack surface

# PowerShell: Check installed Office version for patch verification
Get-ItemProperty HKLM:\Software\Microsoft\Office\ClickToRun\Configuration | Select-Object -Property ProductReleaseIds, VersionToReport

# Registry: Enable Protected View for files from the internet
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f