CVE-2021-28452: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2021-28452 Overview

CVE-2021-28452 is a memory corruption vulnerability affecting Microsoft Outlook and related Microsoft Office products. This vulnerability is classified as CWE-787 (Out-of-Bounds Write), which occurs when a program writes data past the boundary of allocated memory. If successfully exploited, an attacker could potentially execute arbitrary code on the affected system with the same privileges as the user running Microsoft Outlook.

Critical Impact

This out-of-bounds write vulnerability in Microsoft Outlook can allow attackers to execute arbitrary code on the victim's system, potentially leading to complete system compromise, data theft, or further network intrusion when a user opens a maliciously crafted file.

Affected Products

• Microsoft 365 Apps
• Microsoft Office 2019
• Microsoft Outlook 2010 SP2
• Microsoft Outlook 2013 SP1
• Microsoft Outlook 2013 SP1 RT
• Microsoft Outlook 2016

Discovery Timeline

• April 13, 2021 - CVE-2021-28452 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-28452

Vulnerability Analysis

This memory corruption vulnerability stems from improper handling of objects in memory by Microsoft Outlook. The flaw is categorized as an out-of-bounds write vulnerability (CWE-787), which allows an attacker to write data beyond the allocated memory buffer boundaries. The attack requires local access and user interaction, meaning an attacker would need to convince a user to open a specially crafted file or interact with malicious content within Outlook.

Successful exploitation requires that the user performs a specific action, such as opening a maliciously crafted email attachment or email message. Once triggered, the vulnerability could allow an attacker to execute arbitrary code in the context of the current user. If the user has administrative privileges, the attacker could take complete control of the affected system, including installing programs, viewing or modifying data, or creating new accounts with full user rights.

Root Cause

The root cause of CVE-2021-28452 lies in how Microsoft Outlook handles certain memory operations when processing objects. When Outlook parses specific malformed data, it fails to properly validate buffer boundaries before writing data to memory. This out-of-bounds write condition occurs due to insufficient bounds checking, allowing memory adjacent to the intended buffer to be overwritten with attacker-controlled data.

Attack Vector

The attack vector for this vulnerability is local, requiring user interaction. An attacker must craft a malicious file or email content specifically designed to exploit the memory corruption flaw. The attack scenario typically involves:

1. The attacker creates a specially crafted file that exploits the memory corruption vulnerability
2. The attacker delivers this file to the victim via email, file share, or other means
3. The victim opens the malicious file or email content in Microsoft Outlook
4. The vulnerability is triggered, corrupting memory and potentially allowing code execution
5. Arbitrary code runs with the privileges of the logged-in user

The vulnerability does not require authentication and has low attack complexity once the malicious file reaches the target user.

Detection Methods for CVE-2021-28452

Indicators of Compromise

• Unexpected crashes or abnormal behavior in Microsoft Outlook when opening emails or attachments
• Memory access violations or exception errors logged in Windows Event Viewer related to OUTLOOK.EXE
• Suspicious child processes spawned from OUTLOOK.EXE that are not typical for email client operations
• Unusual memory allocation patterns or heap corruption indicators in Outlook process dumps

Detection Strategies

• Deploy endpoint detection and response (EDR) solutions configured to monitor for memory corruption exploitation techniques
• Implement application whitelisting to detect and block unauthorized code execution spawned from Microsoft Outlook
• Enable Windows Defender Exploit Guard memory protection features including Data Execution Prevention (DEP) and Address Space Layout Randomization (ASLR)
• Configure security information and event management (SIEM) rules to alert on abnormal Outlook process behavior

Monitoring Recommendations

• Monitor Windows Event Logs for Application Crash events (Event ID 1000) involving OUTLOOK.EXE
• Track process creation events for suspicious child processes launched by Microsoft Outlook
• Implement network monitoring to detect potential command and control communications following exploitation
• Review endpoint telemetry for memory protection violations or exploit mitigation triggers

How to Mitigate CVE-2021-28452

Immediate Actions Required

• Apply the Microsoft security update for CVE-2021-28452 immediately to all affected systems
• Enable automatic updates for Microsoft Office and Microsoft 365 Apps to receive future security patches
• Restrict user privileges to limit the impact of potential exploitation
• Educate users about the risks of opening unsolicited email attachments or suspicious files

Patch Information

Microsoft has released security updates to address this vulnerability as part of their April 2021 security updates. Organizations should apply the appropriate patches for their installed versions of Microsoft Outlook and Office products. Detailed patch information and download links are available in the Microsoft Security Advisory for CVE-2021-28452.

The security update resolves the vulnerability by correcting how Microsoft Outlook handles objects in memory, ensuring proper boundary validation during write operations.

Workarounds

• Configure Microsoft Outlook to read emails in plain text format to reduce attack surface from HTML-based exploits
• Block suspicious file types at the email gateway before they reach end users
• Implement attachment sandboxing to analyze potentially malicious files before delivery
• Consider using Microsoft's Attack Surface Reduction rules to block Office applications from creating executable content

# Enable reading email in plain text via Group Policy
# Path: User Configuration > Administrative Templates > Microsoft Outlook > Outlook Options > Preferences > E-mail Options
# Setting: Read all standard mail in plain text - Enabled

# PowerShell command to check installed Office version for patch verification
Get-ItemProperty HKLM:\Software\Microsoft\Windows\CurrentVersion\Uninstall\* | Where-Object {$_.DisplayName -like "*Microsoft Office*" -or $_.DisplayName -like "*Microsoft 365*"} | Select-Object DisplayName, DisplayVersion