Microsoft Office Access Connectivity Engine Remote Code Execution Vulnerability (CVE-2021-38646) Security Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTMicrosoft Office, Microsoft 365 Apps, and Access Database Engine Redistributable packages
Successful exploitation allows an attacker to execute arbitrary remote code (RCE) with the privileges of the active user by tricking them into opening a crafted database file
Apply the official Microsoft security updates immediately and restrict execution of untrusted database files
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Apply the official security updates provided by Microsoft. In enterprise environments, you can force the Office update client to run using the following PowerShell command:
# Force Microsoft Office Click-to-Run client to check and apply updates immediately
Start-Process -FilePath 'C:\Program Files\Common Files\microsoft shared\ClickToRun\OfficeC2RClient.exe' -ArgumentList '/update user displaylevel=true forceappshutdown=true' -Wait2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, restrict the execution of database files downloaded from untrusted sources using Group Policy (GPO) or registry modifications.
Windows Registry Editor Version 5.00
; Block untrusted database content execution and restrict VBA macros in Access
[HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\access\security]
'BlockContentExecutionFromInternet'=dword:00000001
'VBAWarnings'=dword:000000043. Log Audit & IoC Verification Commands
Use the following PowerShell script to locate and verify the version of the Access Connectivity Engine DLL (acecore.dll) to determine if the system is vulnerable.
# Search for ACECORE.DLL and verify its file version
$TargetDll = Get-ChildItem -Path 'C:\Program Files\Common Files\Microsoft Shared\Office*' -Filter 'acecore.dll' -Recurse -ErrorAction SilentlyContinue
if ($TargetDll) {
foreach ($file in $TargetDll) {
$version = $file.VersionInfo.FileVersion
Write-Host 'Found Acecore.dll at:' $file.FullName
Write-Host 'Current Version:' $version
# Compare against known patched baseline (example threshold)
if ([version]$version -lt [version]'16.0.14326.20238') {
Write-Warning 'Vulnerable version detected! Immediate patch required.'
} else {
Write-Host 'Patch status: SECURE' -ForegroundColor Green
}
}
} else {
Write-Host 'Access Connectivity Engine DLL not found.' -ForegroundColor Gray
}Block Kernel Privilege Escalation & Ransomware Payloads Before You Can Reboot
Protect unpatched Windows, Linux, and macOS workloads against in-memory exploitation and lateral movement with autonomous EDR.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability is a memory corruption issue within the Microsoft Office Access Connectivity Engine when parsing specially crafted database file formats. Due to insufficient input validation, processing specific records or index structures within a database file can trigger a buffer overflow or invalid pointer dereference, allowing attackers to hijack the process control flow.
2. Exploitation Mechanics & Attack Scenario
An attacker crafts a malicious Access database file (.accdb or .mdb) containing corrupted table structures, malicious macros, or manipulated index metadata. The attacker then distributes this file via phishing emails, malicious websites, or shared network drives, requiring user interaction (UI:R) to open the file. Once the victim opens the file, the Access Connectivity Engine loads the vulnerable parsing routine, triggering memory corruption and executing the attacker's payload within the context of the logged-in user.
3. Infrastructure & Business Impact
This vulnerability is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Successful exploitation grants the attacker full control over the victim's workstation, which can serve as an entry point for lateral movement, sensitive data exfiltration, and ransomware deployment across the enterprise network.
Official Vendor Advisories & References
Microsoft Office Access Connectivity Engine Remote Code Execution Vulnerability
Frequently Asked SecOps Questions (CVE-2021-38646)
FAQQ1. How critical is CVE-2021-38646 and is it actively exploited in the wild?
CVE-2021-38646 is rated CVSS 7.8 (HIGH) with a 30-day EPSS exploitation probability of 8.0%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-38646?
Primary impacted vendors include microsoft across office, access_connectivity_engine. Scope summary: Microsoft Office 2013, 2016, 2019, Microsoft 365 Apps, and Microsoft Access Database Engine 2010/2016 Redistributable
Q3. What temporary mitigations or workarounds exist for CVE-2021-38646 before patching?
[Mitigation] Apply the official Microsoft security updates immediately and restrict execution of untrusted database files Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2021-38646
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