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

CVE-2021-26411 Overview

CVE-2021-26411 is a critical memory corruption vulnerability affecting Microsoft Internet Explorer and Microsoft Edge (EdgeHTML-based). This Use After Free (UAF) vulnerability allows remote attackers to execute arbitrary code on vulnerable systems by convincing users to visit specially crafted web pages. The vulnerability has been confirmed as actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog.

Critical Impact

This vulnerability enables remote code execution through memory corruption in Internet Explorer's scripting engine, allowing attackers to gain the same privileges as the current user. Active exploitation has been confirmed in targeted attacks.

Affected Products

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

Discovery Timeline

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

Technical Details for CVE-2021-26411

Vulnerability Analysis

CVE-2021-26411 is classified as a Use After Free (CWE-416) vulnerability in Internet Explorer's scripting engine. The vulnerability occurs when the browser improperly handles objects in memory during JavaScript processing. When exploited, this memory corruption allows an attacker to execute arbitrary code in the context of the current user, potentially gaining complete control over the affected system.

The attack requires user interaction—specifically, the victim must be lured to a malicious website or open a specially crafted document that embeds Internet Explorer content. Once triggered, the vulnerability corrupts memory in a way that allows attackers to redirect code execution to attacker-controlled instructions.

Root Cause

The root cause of this vulnerability lies in improper memory management within the Internet Explorer scripting engine. Specifically, the browser fails to properly track object lifecycles during certain JavaScript operations, leading to a condition where memory is freed while still being referenced. This Use After Free condition creates an exploitable state where the freed memory can be reallocated with attacker-controlled data, subsequently hijacking program execution flow when the dangling reference is dereferenced.

Attack Vector

The attack vector for CVE-2021-26411 is network-based and requires user interaction. An attacker can exploit this vulnerability through several methods:

1. Malicious Website: Hosting a specially crafted webpage that triggers the vulnerability when visited by a user running a vulnerable version of Internet Explorer
2. Phishing Campaigns: Embedding malicious content in emails or documents that render using Internet Explorer's engine
3. Watering Hole Attacks: Compromising legitimate websites frequented by targeted users and injecting malicious code

The exploitation mechanism involves crafting JavaScript that manipulates object references in a specific sequence to trigger the use-after-free condition. The attacker then leverages heap spray or similar techniques to place controlled data at predictable memory locations, enabling arbitrary code execution when the freed memory is accessed.

Detection Methods for CVE-2021-26411

Indicators of Compromise

• Unusual child processes spawned by iexplore.exe or MicrosoftEdge.exe, particularly command shells or PowerShell instances
• Anomalous memory allocation patterns in Internet Explorer processes indicative of heap spray techniques
• Network connections to known malicious domains from browser processes
• Suspicious JavaScript files or HTML documents with obfuscated code targeting IE-specific functions

Detection Strategies

• Monitor for behavioral indicators of browser-based exploitation, including unexpected process creation chains from Internet Explorer
• Deploy network-based intrusion detection rules to identify exploit kit traffic patterns associated with CVE-2021-26411
• Implement endpoint detection rules targeting JavaScript-based memory corruption attempts in browser contexts
• Leverage SentinelOne's behavioral AI to detect post-exploitation activities following successful browser compromise

Monitoring Recommendations

• Enable enhanced logging for Internet Explorer and Edge processes to capture detailed execution telemetry
• Configure SIEM alerting for anomalous browser process behaviors, particularly child process creation and network activity
• Monitor for indicators of heap spray attacks and memory corruption attempts using EDR solutions
• Track access to known IOCs and malicious URLs associated with exploitation campaigns

How to Mitigate CVE-2021-26411

Immediate Actions Required

• Apply Microsoft's security update for CVE-2021-26411 immediately, as this vulnerability is actively exploited in the wild
• Disable Internet Explorer where possible and migrate users to modern browsers such as Microsoft Edge (Chromium-based) or Chrome
• Implement network-level blocking for known malicious domains associated with exploitation attempts
• Restrict user privileges to limit the impact of successful exploitation

Patch Information

Microsoft released security updates addressing CVE-2021-26411 as part of their March 2021 Patch Tuesday release. The patch corrects how the scripting engine handles objects in memory, preventing the use-after-free condition. Organizations should consult the Microsoft Security Advisory for CVE-2021-26411 for specific patch details and affected product versions.

This vulnerability is listed in CISA's Known Exploited Vulnerabilities Catalog, which mandates federal agencies to remediate within specified timeframes.

Workarounds

• Restrict access to Internet Explorer through Group Policy and guide users to use alternative browsers
• Implement application whitelisting to prevent unauthorized code execution from browser processes
• Deploy network segmentation to limit lateral movement in case of successful exploitation
• Configure Enhanced Protected Mode in Internet Explorer where disabling the browser entirely is not feasible

# Disable Internet Explorer via DISM (Windows 10/Server)
dism /online /Disable-Feature /FeatureName:Internet-Explorer-Optional-amd64 /NoRestart

# Alternatively, restrict IE execution via Group Policy or AppLocker
# Set-ExecutionPolicy restrictions for browser-spawned scripts