Excel Under Siege: Unpacking CVE-2025-29977's Use-After-Free Vulnerability

Introduction

Imagine opening a seemingly harmless Excel spreadsheet, only to inadvertently grant an attacker complete control over your system. CVE-2025-29977 brings this alarming scenario to reality through a critical use-after-free vulnerability in Microsoft Excel. This flaw, disclosed in Microsoft's May 2025 Patch Tuesday, underscores the persistent threat posed by memory corruption vulnerabilities.

Affected Systems and Versions

This vulnerability specifically affects the following Microsoft Excel versions:

Microsoft Excel 2016
Microsoft Excel 2019
Microsoft Excel 2021
Microsoft Excel 2024
Microsoft 365 Apps (all updates prior to May 2025)

Systems configured with Excel's Preview Pane enabled in Outlook are particularly vulnerable, as exploitation can occur without direct file execution.

Technical Information

CVE-2025-29977 is a use-after-free vulnerability (CWE-416) arising from improper memory management in Excel. When Excel frees memory but continues to reference it, attackers can exploit this condition by crafting malicious Excel files. Opening or even previewing these files triggers memory corruption, allowing attackers to execute arbitrary code with the user's privileges.

Attack Vectors

Malicious Excel Documents: Users opening a maliciously crafted Excel file.
Preview Pane Exploitation: Simply previewing the malicious file in Outlook's Preview Pane can trigger exploitation.

Patch Information

Microsoft has released a critical security update addressing this vulnerability:

KB5002695 for Microsoft Office 2016 and corresponding updates for newer versions.

Organizations should apply these patches immediately. Additionally, disabling the Preview Pane in Outlook and restricting macro execution can further mitigate risks.

Detection Methods

Organizations can detect potential exploitation by monitoring Excel processes for unusual behavior, such as unexpected child processes or abnormal memory usage patterns. Endpoint detection tools configured to flag anomalous Excel activity can provide early warnings.

Vendor Security History

Microsoft consistently addresses memory corruption vulnerabilities, including use-after-free flaws. The May 2025 Patch Tuesday alone resolved multiple critical vulnerabilities, reflecting Microsoft's ongoing effort to secure its widely used software products.

References

Security teams should prioritize patching and monitoring to mitigate the risks associated with CVE-2025-29977 effectively.