CVE-2021-26411: Microsoft Edge Use-After-Free Vulnerability

CVE-2021-26411 Overview

CVE-2021-26411 is a memory corruption vulnerability affecting Internet Explorer and Microsoft Edge (EdgeHTML-based) on supported Windows operating systems. The flaw is a use-after-free condition [CWE-416] in the browser engine that allows remote code execution when a user views a specially crafted webpage. Microsoft published the advisory on March 11, 2021, and CISA added the CVE to its Known Exploited Vulnerabilities catalog after observing in-the-wild abuse. The vulnerability requires user interaction, but successful exploitation yields execution at the privilege level of the current user.

Critical Impact

Attackers can achieve remote code execution in the browser process simply by luring a user to a malicious or compromised website, and active exploitation has been confirmed by CISA.

Affected Products

• Microsoft Internet Explorer 9 and 11
• Microsoft Edge (EdgeHTML)
• Windows 7 SP1, Windows 8.1, Windows RT 8.1, Windows 10 (1507, 1607, 1803, 1809, 1909, 2004, 20H2)
• Windows Server 2008 SP2, Server 2008 R2 SP1, Server 2012, Server 2012 R2, Server 2016, Server 2019

Discovery Timeline

• 2021-03-11 - CVE-2021-26411 published by Microsoft and added to NVD
• 2025-10-30 - Last updated in NVD database

Technical Details for CVE-2021-26411

Vulnerability Analysis

The vulnerability is a use-after-free condition in the rendering engine shared by Internet Explorer and the legacy Edge browser. When the engine processes specific HTML, CSS, or scripting constructs, it frees an object while leaving a dangling reference that subsequent code dereferences. An attacker who controls the page layout and JavaScript on the target site can shape the freed memory to contain attacker-chosen data. Dereferencing the stale pointer then transfers control into attacker-controlled memory.

Microsoft classifies the flaw as remote code execution because successful exploitation runs arbitrary native code in the context of the browser process. The bug requires user interaction, typically navigation to a malicious URL or opening a crafted document that loads the IE rendering engine. Threat actors weaponized this vulnerability against security researchers and other targets shortly after disclosure, prompting its inclusion in CISA KEV.

Root Cause

The root cause is improper object lifetime management within the legacy browser engine [CWE-416]. A reference to a heap object is retained after the object is released, and the engine later operates on the freed memory. Heap grooming and JavaScript-driven allocations let an attacker reliably reclaim the freed slot with controlled content.

Attack Vector

Exploitation occurs over the network with low complexity and no privileges, but requires user interaction. A target user must visit an attacker-controlled webpage, click a malicious link, or open a document that instantiates the Internet Explorer rendering component, such as an Office file with embedded HTML. Successful exploitation yields code execution within the browser sandbox, which is then often chained with a sandbox escape or elevation-of-privilege bug to achieve full system compromise.

No verified public proof-of-concept code is included in this advisory data. Refer to the Microsoft Security Advisory CVE-2021-26411 for vendor technical details.

Detection Methods for CVE-2021-26411

Indicators of Compromise

• Unexpected child processes spawned by iexplore.exe or legacy MicrosoftEdge.exe, particularly cmd.exe, powershell.exe, rundll32.exe, or regsvr32.exe.
• Browser process crashes followed by suspicious file writes in %TEMP%, %APPDATA%, or %LOCALAPPDATA%\Microsoft\Windows\INetCache.
• Outbound network connections from iexplore.exe to newly registered or low-reputation domains following a page load.
• DLL loads from user-writable paths into the Internet Explorer or Edge legacy process address space.

Detection Strategies

• Hunt for process lineage anomalies where Internet Explorer or legacy Edge spawns scripting interpreters or LOLBins.
• Monitor Windows Error Reporting (WerFault.exe) and crash dumps tied to mshtml.dll or jscript9.dll for repeated failures indicative of exploitation attempts.
• Inspect proxy and DNS telemetry for browser-initiated traffic to known exploit kit infrastructure or recently observed campaign domains tied to this CVE.

Monitoring Recommendations

• Enable Microsoft Defender Application Guard and Attack Surface Reduction rules that block Office applications from creating child processes and from injecting code.
• Forward Sysmon Event IDs 1, 7, 10, and 11 from browser processes into your SIEM or data lake for retrospective hunting on mshtml-related activity.
• Alert on any execution of Internet Explorer in environments where it has been formally retired, since legitimate usage should be near zero.

How to Mitigate CVE-2021-26411

Immediate Actions Required

• Apply the March 2021 Microsoft security updates that address CVE-2021-26411 across all listed Windows and browser versions.
• Disable or uninstall Internet Explorer 11 and legacy Edge where business requirements allow, and route any remaining IE-mode workloads through current Microsoft Edge with policy controls.
• Restrict execution of iexplore.exe for users who do not require it, using AppLocker or Windows Defender Application Control.
• Treat assets that browsed unknown sites prior to patching as suspect, and perform forensic triage for the indicators listed above.

Patch Information

Microsoft released cumulative security updates for all supported Windows versions on March 9, 2021. Patch identifiers vary by operating system; consult the Microsoft Security Advisory CVE-2021-26411 for the KB article matching each affected build. CISA mandates remediation under the CISA Known Exploited Vulnerability CVE-2021-26411 directive for federal agencies.

Workarounds

• Configure Microsoft Edge to disable IE Mode for non-essential sites and enforce an allowlist for legacy applications.
• Set the Internet and Restricted Sites zones to disable Active Scripting and ActiveX controls until patches are deployed.
• Deploy Microsoft Defender Exploit Guard with hardware-enforced stack protection and Control Flow Guard to raise the cost of use-after-free exploitation.
• Block known exploit-hosting domains and apply web filtering policies that prevent rendering of untrusted HTML in legacy engines.

# Disable Internet Explorer 11 as a standalone browser via Group Policy registry key
reg add "HKLM\SOFTWARE\Policies\Microsoft\Internet Explorer\Main" /v DisableInternetExplorerApp /t REG_DWORD /d 1 /f

# Verify the March 2021 cumulative update is installed (example KB for Windows 10 20H2)
wmic qfe list brief | findstr /i "KB5000802"