CVE-2020-1555: Microsoft Edge Scripting Engine RCE Flaw

CVE-2020-1555 Overview

CVE-2020-1555 is a remote code execution vulnerability in the way the scripting engine handles objects in memory in Microsoft Edge (HTML-based). The vulnerability could corrupt memory in such a way that an attacker could execute arbitrary code in the context of the current user. If successfully exploited, an attacker could gain the same user rights as the current user, potentially taking complete control of an affected system if the user has administrative privileges.

Critical Impact

Successful exploitation allows remote code execution with current user privileges, potentially enabling complete system compromise including program installation, data manipulation, and account creation with full user rights.

Affected Products

• Microsoft Edge (HTML-based)
• Microsoft Windows 10 (versions 1709, 1803, 1809, 1903, 1909, 2004)
• Microsoft Windows Server 2019
• Microsoft ChakraCore

Discovery Timeline

• August 17, 2020 - CVE-2020-1555 published to NVD
• February 23, 2026 - Last updated in NVD database

Technical Details for CVE-2020-1555

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), indicating that the scripting engine in Microsoft Edge improperly handles objects in memory. The flaw exists in the ChakraCore JavaScript engine, which is responsible for executing JavaScript code in the HTML-based version of Microsoft Edge.

When processing specially crafted content, the scripting engine fails to properly validate memory operations, leading to memory corruption. This memory corruption can be leveraged by an attacker to execute arbitrary code within the security context of the currently logged-in user. The impact escalates significantly if the victim has administrative privileges, as the attacker would inherit those same elevated permissions.

Root Cause

The root cause lies in improper memory handling within the ChakraCore scripting engine when processing JavaScript objects. The engine fails to correctly manage object lifetimes or memory boundaries during certain operations, resulting in an out-of-bounds write condition. This type of vulnerability typically occurs when array bounds or buffer sizes are not properly validated before write operations.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker could exploit this vulnerability through several scenarios:

1. Malicious Website: The attacker hosts a specially crafted website designed to exploit the vulnerability and convinces the user to visit it through phishing or social engineering.

2. Compromised Website: The attacker compromises a legitimate website or leverages websites that accept user-provided content or advertisements to inject malicious JavaScript.

3. Drive-by Download: When a user navigates to the malicious page using the affected Microsoft Edge browser, the crafted JavaScript triggers the memory corruption, enabling code execution.

The vulnerability requires no authentication and can be triggered simply by viewing malicious content in the browser, making it particularly dangerous for end users.

Detection Methods for CVE-2020-1555

Indicators of Compromise

• Unexpected child processes spawning from MicrosoftEdge.exe or MicrosoftEdgeCP.exe
• Abnormal memory consumption or crashes in Microsoft Edge processes
• JavaScript execution attempting to access or manipulate memory outside expected boundaries
• Network connections to suspicious domains initiated by Edge browser processes

Detection Strategies

• Monitor for suspicious process behavior originating from Microsoft Edge, particularly shell spawning or code injection attempts
• Implement browser telemetry to detect JavaScript execution anomalies and heap spray patterns
• Deploy endpoint detection rules to identify exploitation techniques targeting the ChakraCore engine
• Enable Windows Defender Exploit Guard to detect memory corruption attacks

Monitoring Recommendations

• Enable detailed logging for Microsoft Edge and Windows security events
• Monitor for Event ID 1000 (Application Error) involving MicrosoftEdge.exe or chakra.dll
• Track network traffic patterns from browser processes for unusual outbound connections
• Implement memory integrity monitoring on endpoints running vulnerable versions

How to Mitigate CVE-2020-1555

Immediate Actions Required

• Apply the August 2020 Microsoft security updates immediately to all affected systems
• Consider migrating to Microsoft Edge (Chromium-based) which is not affected by this vulnerability
• Restrict access to untrusted websites through web filtering solutions
• Ensure users are not browsing with administrative privileges

Patch Information

Microsoft has released a security update that addresses this vulnerability by modifying how the scripting engine handles objects in memory. The patch is available through Windows Update and the Microsoft Security Response Center advisory. Organizations should prioritize deployment of this update, especially for systems where Microsoft Edge (HTML-based) is actively used.

Workarounds

• Migrate users to Microsoft Edge (Chromium-based) or alternative browsers not affected by this vulnerability
• Implement network-level protections to block access to known malicious websites
• Run Microsoft Edge in isolated or sandboxed environments where possible
• Restrict JavaScript execution on untrusted sites using browser security policies

# Verify Microsoft Edge version and patch status
# Check Windows Update history for KB related to August 2020 updates
wmic qfe list brief /format:table | findstr /i "KB4565349 KB4566782 KB4571756"

# Alternatively, verify via PowerShell
Get-HotFix | Where-Object {$_.HotFixID -match "KB4565349|KB4566782|KB4571756"}