CVE-2020-0674: Internet Explorer RCE Vulnerability

CVE-2020-0674 Overview

CVE-2020-0674 is a critical remote code execution vulnerability affecting Microsoft Internet Explorer's scripting engine. The vulnerability exists in the way the JScript scripting engine handles objects in memory, specifically manifesting as a Use-After-Free (UAF) memory corruption flaw. When successfully exploited, an attacker can execute arbitrary code in the context of the current user, potentially gaining complete control over the affected system.

This vulnerability is particularly dangerous because it can be exploited through drive-by attacks where victims simply browse to a malicious website. The flaw affects multiple versions of Internet Explorer (IE 9, 10, and 11) across a wide range of Windows operating systems, from Windows 7 through Windows 10 and Windows Server editions.

Critical Impact

This vulnerability has been actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog. Successful exploitation allows remote attackers to execute arbitrary code with the privileges of the current user, potentially leading to full system compromise.

Affected Products

• Microsoft Internet Explorer 9
• Microsoft Internet Explorer 10
• Microsoft Internet Explorer 11
• Microsoft Windows 7 SP1
• Microsoft Windows 8.1
• Microsoft Windows RT 8.1
• Microsoft Windows 10 (versions 1507, 1607, 1709, 1803, 1809, 1903, 1909)
• Microsoft Windows Server 2008 SP2 and R2 SP1
• Microsoft Windows Server 2012 and R2
• Microsoft Windows Server 2016
• Microsoft Windows Server 2019

Discovery Timeline

• February 11, 2020 - CVE-2020-0674 published to NVD
• October 29, 2025 - Last updated in NVD database

Technical Details for CVE-2020-0674

Vulnerability Analysis

CVE-2020-0674 is a Use-After-Free vulnerability in the JScript scripting engine (jscript.dll) used by Internet Explorer. The vulnerability occurs when the scripting engine improperly handles objects in memory during JavaScript execution. Specifically, the flaw involves the improper management of object lifetimes, where a memory region is accessed after it has been freed, leading to potential memory corruption.

The vulnerability is classified under CWE-416 (Use After Free), which describes scenarios where a program continues to use a pointer after it has been freed. In this case, the JScript engine fails to properly track object references, allowing an attacker to craft malicious JavaScript that triggers the use of freed memory. By carefully manipulating memory layouts and object allocations, an attacker can achieve arbitrary code execution.

The attack requires user interaction—victims must navigate to a malicious webpage or open a specially crafted document containing the exploit code. While this adds some complexity to exploitation, the prevalence of phishing and watering hole attacks makes this a highly viable attack vector in real-world scenarios.

Root Cause

The root cause of CVE-2020-0674 lies in the JScript scripting engine's memory management implementation. The vulnerability stems from improper reference counting or object lifecycle management within jscript.dll. When certain JavaScript operations are performed, the engine fails to maintain accurate tracking of object references, resulting in a dangling pointer that references freed memory.

The Use-After-Free condition occurs when:

1. An object is allocated in memory by the JScript engine
2. The object is freed due to garbage collection or explicit deallocation
3. The freed memory is reallocated for a different purpose
4. The original dangling pointer is dereferenced, accessing the now-repurposed memory

This flaw in the legacy JScript engine (as opposed to the newer Chakra JavaScript engine) affects systems that still have JScript enabled, which includes default configurations of Internet Explorer.

Attack Vector

The attack vector for CVE-2020-0674 is network-based and requires user interaction. An attacker can exploit this vulnerability through several methods:

Web-Based Attack: The most common exploitation scenario involves hosting malicious JavaScript on a website controlled by the attacker. When a victim navigates to the compromised site using Internet Explorer, the exploit code executes automatically within the browser's scripting engine.

Email-Based Attack: Attackers can embed malicious content in HTML emails or include links to exploit pages. When opened in a mail client that renders HTML using the IE engine, or when users click through to the malicious page, the exploit triggers.

Document-Based Attack: Microsoft Office documents that embed browser controls or ActiveX components using the IE rendering engine can serve as exploit delivery mechanisms.

The exploitation process involves carefully crafted JavaScript that manipulates object allocations to achieve a precise heap state, triggers the Use-After-Free condition, and then leverages the memory corruption to redirect code execution to attacker-controlled shellcode.

Detection Methods for CVE-2020-0674

Indicators of Compromise

• Suspicious iexplore.exe processes spawning unexpected child processes (cmd.exe, powershell.exe, wscript.exe)
• Abnormal memory access patterns or crashes in jscript.dll module
• Network connections from Internet Explorer to known malicious domains or IP addresses
• Unexpected file creation or modification in user temporary directories following IE usage
• Event logs showing application crashes in Internet Explorer with faulting module jscript.dll

Detection Strategies

• Deploy endpoint detection rules to monitor for Internet Explorer spawning shell processes or making suspicious system calls
• Implement network-based detection for known exploit signatures and malicious JavaScript patterns
• Enable Windows Defender Exploit Guard to detect and block exploitation attempts targeting memory corruption vulnerabilities
• Monitor for attempts to disable or bypass JScript restrictions as an indicator of reconnaissance activity
• Utilize behavioral analysis to detect post-exploitation activities such as credential dumping or lateral movement following IE usage

Monitoring Recommendations

• Configure SIEM rules to alert on jscript.dll crash events in Windows Event Logs (Application Log, Event ID 1000)
• Enable Process Monitor or Sysmon to track process creation chains originating from iexplore.exe
• Monitor for registry modifications to Internet Explorer security settings or zone configurations
• Implement web proxy logging to capture and analyze JavaScript content delivered to endpoints
• Review endpoint telemetry for unusual DLL loads or memory allocations within Internet Explorer processes

How to Mitigate CVE-2020-0674

Immediate Actions Required

• Apply the official Microsoft security update immediately through Windows Update or WSUS
• Restrict access to jscript.dll using the workaround commands if patching is not immediately possible
• Migrate users to Microsoft Edge or alternative modern browsers that do not use the legacy JScript engine
• Implement network-based blocking for known exploit domains and malicious IP addresses
• Enable Enhanced Security Configuration on Windows Server installations running Internet Explorer

Patch Information

Microsoft has released security updates to address CVE-2020-0674 through the February 2020 Patch Tuesday release cycle. Organizations should apply the appropriate cumulative update for their Windows and Internet Explorer version. The official security advisory and patch information is available through the Microsoft Security Response Center.

For enterprise environments, patches should be tested in a staging environment before deployment to production systems. Given the active exploitation status of this vulnerability, organizations should prioritize this update and consider emergency patching procedures.

Workarounds

• Restrict access to JScript by taking ownership of jscript.dll and denying access to the Everyone group
• Configure Internet Explorer to prompt before running Active Scripting in the Internet and Local Intranet security zones
• Use Group Policy to disable Internet Explorer or restrict its usage to trusted intranet sites only
• Deploy Software Restriction Policies to prevent jscript.dll from loading in Internet Explorer
• Consider implementing application whitelisting to control which scripts can execute in the browser

# Restrict JScript access workaround (run as Administrator)
# For 32-bit systems:
takeown /f %windir%\system32\jscript.dll
cacls %windir%\system32\jscript.dll /E /P everyone:N

# For 64-bit systems (also restrict 32-bit component):
takeown /f %windir%\syswow64\jscript.dll
cacls %windir%\syswow64\jscript.dll /E /P everyone:N
takeown /f %windir%\system32\jscript.dll
cacls %windir%\system32\jscript.dll /E /P everyone:N

# To undo the workaround after patching:
# cacls %windir%\system32\jscript.dll /E /R everyone
# cacls %windir%\syswow64\jscript.dll /E /R everyone