CVE-2021-26419: Internet Explorer Buffer Overflow Flaw

CVE-2021-26419 Overview

CVE-2021-26419 is a Scripting Engine Memory Corruption Vulnerability affecting Microsoft Internet Explorer. This vulnerability exists in the JScript scripting engine (jscript9.dll) and can be exploited when a user visits a specially crafted webpage. Successful exploitation allows an attacker to execute arbitrary code in the context of the current user, potentially gaining the same privileges as the logged-in user.

Critical Impact

This memory corruption vulnerability in Internet Explorer's scripting engine enables remote code execution through malicious web content, affecting a wide range of Windows operating systems including legacy server deployments.

Affected Products

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

Discovery Timeline

• 2021-05-11 - CVE-2021-26419 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-26419

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), a memory corruption issue within Internet Explorer's JScript9 scripting engine. The vulnerability occurs when the scripting engine improperly handles objects in memory during JavaScript execution. When a user navigates to a malicious webpage containing specially crafted JavaScript code, the scripting engine can be manipulated to write data beyond the boundaries of allocated memory buffers.

The attack requires user interaction—specifically, a victim must be enticed to visit a compromised or attacker-controlled website using Internet Explorer. While the attack complexity is considered high due to the need for specific conditions to be met, successful exploitation results in complete compromise of confidentiality, integrity, and availability within the user's context.

Root Cause

The root cause of CVE-2021-26419 lies in improper memory handling within the jscript9.dll component. The JScript9 engine fails to properly validate memory boundaries when processing certain JavaScript constructs, leading to an out-of-bounds write condition. This type of memory corruption can be leveraged to overwrite critical memory structures, potentially allowing an attacker to redirect code execution flow to attacker-controlled shellcode.

Attack Vector

The attack vector for this vulnerability is network-based, requiring the attacker to host malicious content on a web server or compromise an existing legitimate website. The attack scenario typically involves:

1. An attacker crafts a malicious webpage containing JavaScript designed to trigger the memory corruption in jscript9.dll
2. The victim is lured to visit the malicious page using Internet Explorer (via phishing emails, malicious advertisements, or watering hole attacks)
3. Upon rendering the page, the scripting engine processes the malicious JavaScript
4. The memory corruption occurs, allowing the attacker to execute arbitrary code with the privileges of the current user

The vulnerability mechanism involves improper object handling in the JScript9 scripting engine. When specific JavaScript operations are performed, the engine fails to properly validate memory boundaries, resulting in an out-of-bounds write condition. For detailed technical analysis, refer to the Packet Storm Memory Corruption Exploit documentation.

Detection Methods for CVE-2021-26419

Indicators of Compromise

• Unusual crashes or instability in Internet Explorer processes (iexplore.exe)
• Unexpected child processes spawned by iexplore.exe indicating potential code execution
• Network connections to suspicious or unknown domains originating from Internet Explorer
• Memory access violations or application errors related to jscript9.dll

Detection Strategies

• Monitor for anomalous behavior in Internet Explorer processes using endpoint detection and response (EDR) solutions
• Implement browser isolation technologies to contain potential exploitation attempts
• Deploy network intrusion detection systems (IDS) with signatures for known exploitation patterns
• Enable Windows Defender Exploit Guard memory protection features

Monitoring Recommendations

• Configure Windows Event Logs to capture application crashes and memory-related errors for IE processes
• Implement SentinelOne's behavioral AI to detect memory corruption exploitation techniques
• Monitor for suspicious JavaScript execution patterns and unusual DOM manipulation
• Track process creation events where iexplore.exe is the parent process

How to Mitigate CVE-2021-26419

Immediate Actions Required

• Apply the Microsoft security update released in May 2021 to all affected systems immediately
• Consider migrating from Internet Explorer to a modern browser such as Microsoft Edge
• Restrict Internet Explorer usage through Group Policy where feasible
• Enable Enhanced Protected Mode in Internet Explorer to reduce the impact of potential exploitation

Patch Information

Microsoft has released security updates to address this vulnerability as part of the May 2021 Patch Tuesday release. The patches are available through Windows Update, Microsoft Update Catalog, and Windows Server Update Services (WSUS). Organizations should prioritize patching systems that still require Internet Explorer for legacy application compatibility. Refer to the Microsoft Security Advisory CVE-2021-26419 for complete patch information and download links.

Workarounds

• Restrict access to jscript9.dll by modifying ACLs to deny execute permissions for non-essential users
• Configure Internet Explorer to prompt before running Active Scripting in the Internet and Local intranet security zones
• Use Software Restriction Policies or AppLocker to limit Internet Explorer execution to essential personnel
• Implement network-level filtering to block access to known malicious domains

# Restrict JScript9.dll permissions (run as Administrator)
# This workaround may impact functionality of sites requiring JavaScript
takeown /f "%windir%\system32\jscript9.dll"
cacls "%windir%\system32\jscript9.dll" /E /P everyone:N