Microsoft Internet Explorer Use-After-Free Remote Code Execution Vulnerability (CVE-2021-26411) Technical Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTWindows systems running Internet Explorer 11 or Microsoft Edge in Internet Explorer mode
Remote attackers can execute arbitrary code and compromise the host system by tricking a user into visiting a specially crafted malicious website
Apply the official Microsoft cumulative security updates immediately, disable Internet Explorer 11, and transition to modern browsers
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
The primary remediation is to apply the official cumulative security updates provided by Microsoft. You can force the installation of the required security updates using PowerShell.
# Install and import Windows Update module to apply the latest cumulative updates
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck
Get-WindowsUpdate -AcceptAll -Install -AutoReboot2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, enforce the restriction of Internet Explorer 11 execution via the registry to mitigate the attack vector.
# Disable Internet Explorer 11 via Registry Policy
$Path = 'HKLM:\SOFTWARE\Policies\Microsoft\Internet Explorer\Main'
if (!(Test-Path $Path)) { New-Item -Path $Path -Force }
New-ItemProperty -Path $Path -Name 'DisableIE11' -Value 'Always' -PropertyType 'String' -Force3. Log Audit & IoC Verification Commands
Verify the version of the MSHTML rendering engine library (mshtml.dll) to ensure the system is running a patched version.
# Check the file version of mshtml.dll to verify patch status
$DllPath = Join-Path $env:SystemRoot 'System32\mshtml.dll'
if (Test-Path $DllPath) {
$Version = (Get-Item $DllPath).VersionInfo.FileVersion
Write-Output "Active MSHTML.dll Version: $Version"
} else {
Write-Warning 'mshtml.dll was not detected in the standard system directory.'
}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 (CVE-2021-26411) is a Use-After-Free (UAF) memory corruption flaw within the MSHTML rendering engine of Internet Explorer. The root cause lies in the improper handling of specific HTML objects (such as Attributes or Nodes) in memory. When the engine fails to properly track object lifecycles, an attacker can trigger premature memory deallocation while retaining a reference to the freed memory space.
2. Exploitation Mechanics & Attack Scenario
In a real-world exploitation scenario, threat actors host a malicious website containing obfuscated JavaScript. When a target user visits the site, the script manipulates DOM elements to free a specific object and immediately fills the vacant memory slot with controlled shellcode via heap spraying. When the browser attempts to access the freed object's virtual function table (vtable), control flow is hijacked, leading to arbitrary code execution under the context of the current user.
3. Infrastructure & Business Impact
As a CISA KEV-listed vulnerability with a high EPSS score, this flaw has been actively exploited in targeted campaigns by advanced persistent threat (APT) groups. Successful exploitation allows attackers to gain initial access to enterprise endpoints, bypass security controls, and perform lateral movement, potentially leading to ransomware deployment or sensitive data exfiltration.
Official Vendor Advisories & References
Internet Explorer Memory Corruption Vulnerability
Frequently Asked SecOps Questions (CVE-2021-26411)
FAQQ1. How critical is CVE-2021-26411 and is it actively exploited in the wild?
CVE-2021-26411 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 81.7%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-26411?
Primary impacted vendors include microsoft across internet explorer, edge. Scope summary: All Windows operating systems (7, 8.1, 10, Server 2008/2012/2016/2019/2022) utilizing Internet Explorer 11 and the MSHTML rendering engine
Q3. What temporary mitigations or workarounds exist for CVE-2021-26411 before patching?
[Mitigation] Apply the official Microsoft cumulative security updates immediately, disable Internet Explorer 11, and transition to modern browsers 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-2021-26411
Instantly format this advisory into an executive CISO summary or engineering patch ticket.
🔗 Related Advisories in Similar Stack (microsoft)
View Category →Windows AFD.sys Use-After-Free Local Privilege Escalation Vulnerability (CVE-2026-68820) Security Advisory
[Affected Scope] Systems running Microsoft Windows operating systems utilizing the Ancillary Function Driver for WinSock (afd.sys)
Microsoft Windows Shell Command Injection and Security Feature Bypass (CVE-2026-96951) Advisory
[Affected Scope] All supported versions of Windows OS including Windows 10, 11, and Windows Server.
Windows ALPC Heap-Based Buffer Overflow Privilege Escalation Vulnerability (CVE-2026-85880) Security Advisory
[Affected Scope] Locally authenticated users across Microsoft Windows operating systems
Windows Update Stack Privilege Escalation Vulnerability via Link Following (CVE-2026-81963) Technical Advisory
[Affected Scope] Microsoft Windows operating systems utilizing the Windows Update Stack
Subscribe to Daily Vulnerability Briefings & KEV Alerts
Concise morning briefings for SecOps, DevSecOps, and SRE teams covering newly exploited CVEs and mitigation priorities.