Microsoft Internet Explorer Remote Code Execution Vulnerability (CVE-2021-27085) Analysis and Mitigation Guide
Executive & SecOps Summary
SECOPS ASSESSMENTWindows client and server environments utilizing Internet Explorer 11
Attackers can execute arbitrary remote code in the context of the current user by tricking them into visiting a specially crafted malicious web page
Immediately apply the official Microsoft cumulative security updates, disable legacy Internet Explorer, and transition to Microsoft Edge with IE mode
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
The primary remediation is to apply the latest Microsoft cumulative security updates. You can force Windows Update to search and install the required patches using PowerShell.
# Install Windows Update module and trigger updates
Install-Module PSWindowsUpdate -Force -Verbose
Get-WindowsUpdate -AcceptAll -Install -AutoReboot2. Temporary Workaround & Hardening Configuration
If patching cannot be applied immediately, disable Internet Explorer 11 as a standalone browser and redirect users to Microsoft Edge using Group Policy or Registry modifications.
# Disable Internet Explorer 11 via Registry
$RegistryPath = 'HKLM:\SOFTWARE\Policies\Microsoft\Internet Explorer\Main'
if (-not (Test-Path $RegistryPath)) {
New-Item -Path $RegistryPath -Force
}
New-ItemProperty -Path $RegistryPath -Name 'DisableInternetExplorer' -Value '1' -PropertyType 'String' -Force3. Log Audit & IoC Verification Commands
You can verify the patch level of your system by checking the file version of the core rendering engine library (mshtml.dll).
# Check the file version of mshtml.dll to verify patch status
$mshtmlPath = '${env:SystemRoot}\System32\mshtml.dll'
if (Test-Path $mshtmlPath) {
$fileVersion = (Get-Item $mshtmlPath).VersionInfo.FileVersion
Write-Host 'Current mshtml.dll Version:' $fileVersion
# Compare this version against the latest Microsoft security bulletin release for your OS build.
} else {
Write-Host 'mshtml.dll not found.'
}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-2021-27085) is a critical Remote Code Execution (RCE) flaw residing in the memory management of the Microsoft Internet Explorer (IE) browser engine. It is triggered by a Use-After-Free (UAF) condition when the browser parses and renders specific Document Object Model (DOM) objects or stylesheet elements within an HTML document. By manipulating the memory layout, an attacker can reuse freed memory addresses to hijack the execution flow.
2. Exploitation Mechanics & Attack Scenario
An attacker hosts a malicious website containing specially crafted HTML, JavaScript, or CSS. The attacker then lures target users to the site via phishing emails, social engineering, or watering hole attacks. When the victim visits the page using Internet Explorer, the browser engine fails to handle memory allocation properly, leading to heap corruption. This allows the attacker's shellcode to execute with the privileges of the logged-in user, potentially granting full administrative control if the victim has elevated privileges.
3. Infrastructure & Business Impact
As this vulnerability is listed in the CISA KEV (Known Exploited Vulnerabilities) catalog, it has been actively exploited in the wild. In enterprise environments where legacy Internet Explorer is still enabled for backward compatibility, this flaw represents a significant entry point for threat actors. Successful exploitation can lead to initial access, lateral movement, ransomware deployment, and sensitive data exfiltration.
Official Vendor Advisories & References
Internet Explorer Remote Code Execution Vulnerability
Frequently Asked SecOps Questions (CVE-2021-27085)
FAQQ1. How critical is CVE-2021-27085 and is it actively exploited in the wild?
CVE-2021-27085 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 5.5%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-27085?
Primary impacted vendors include microsoft across internet explorer. Scope summary: Internet Explorer 11 on supported versions of Windows Client and Windows Server
Q3. What temporary mitigations or workarounds exist for CVE-2021-27085 before patching?
[Mitigation] Immediately apply the official Microsoft cumulative security updates, disable legacy Internet Explorer, and transition to Microsoft Edge with IE mode 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-2021-27085
Instantly format this advisory into an executive CISO summary or engineering patch ticket.
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