CVE-2021-26701: Microsoft .NET Core RCE Vulnerability

CVE-2021-26701 Overview

CVE-2021-26701 is a critical remote code execution vulnerability affecting Microsoft .NET Core runtime. This vulnerability allows unauthenticated remote attackers to execute arbitrary code on vulnerable systems without requiring any user interaction. The flaw exists within the .NET Core framework and can be exploited over the network, making it particularly dangerous for internet-facing applications built on affected .NET versions.

Critical Impact

This remote code execution vulnerability enables attackers to gain complete control over affected systems without authentication, potentially leading to full system compromise, data exfiltration, and lateral movement within enterprise networks.

Affected Products

• Microsoft .NET (multiple versions)
• Microsoft .NET Core (multiple versions)
• Microsoft PowerShell Core 7.0 and 7.1
• Microsoft Visual Studio 2019 (Windows and macOS)
• Fedora 32, 33, and 34

Discovery Timeline

• February 25, 2021 - CVE-2021-26701 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-26701

Vulnerability Analysis

This remote code execution vulnerability in .NET Core allows attackers to execute arbitrary code in the context of the current user. The vulnerability can be exploited remotely over the network without requiring authentication or user interaction. When successfully exploited, an attacker could install programs, view, change, or delete data, or create new accounts with full user rights on the compromised system.

The attack requires no privileges and no user interaction, making it highly exploitable in real-world scenarios. Applications and services built on vulnerable .NET Core versions that process untrusted input are at particular risk, as the vulnerability can be triggered through maliciously crafted data.

Root Cause

The vulnerability stems from improper handling of certain operations within the .NET Core runtime. While Microsoft has not disclosed specific technical details about the root cause (classified as NVD-CWE-noinfo), the nature of the vulnerability as a remote code execution flaw suggests issues with memory safety or input validation within core .NET components.

Attack Vector

The attack vector is network-based, requiring no authentication and no user interaction. An attacker can exploit this vulnerability by sending specially crafted requests to an application running on a vulnerable .NET Core version. The exploitation path involves:

1. Identifying a target application running a vulnerable .NET Core version
2. Crafting malicious input designed to trigger the vulnerability
3. Sending the payload to the target application over the network
4. Achieving code execution in the context of the application process

The vulnerability affects applications across all platforms where .NET Core is supported, including Windows, Linux, and macOS environments.

Detection Methods for CVE-2021-26701

Indicators of Compromise

• Unexpected child processes spawned from .NET Core application processes
• Anomalous network connections originating from .NET application processes
• Unusual memory allocation patterns or crashes in .NET runtime components
• Unauthorized system changes or new user accounts created by application service accounts

Detection Strategies

• Monitor .NET Core application logs for unexpected exceptions or crashes that may indicate exploitation attempts
• Implement network intrusion detection rules to identify malicious payloads targeting .NET applications
• Deploy endpoint detection and response (EDR) solutions to detect post-exploitation activities
• Audit installed .NET Core runtime versions across the enterprise to identify vulnerable systems

Monitoring Recommendations

• Enable detailed logging for all .NET Core applications in production environments
• Configure SIEM rules to alert on unusual process execution chains involving dotnet.exe or .NET application processes
• Monitor for changes to application directories and configuration files
• Track outbound network connections from application servers for signs of data exfiltration

How to Mitigate CVE-2021-26701

Immediate Actions Required

• Immediately update all .NET Core installations to the latest patched versions
• Update Microsoft Visual Studio 2019 to the latest security release
• Update PowerShell Core 7.0 and 7.1 to patched versions
• Review and rebuild applications using updated .NET SDK versions
• Conduct a thorough inventory of all .NET Core deployments across the organization

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should apply patches available through the following resources:

• Microsoft Security Advisory CVE-2021-26701 - Official Microsoft guidance and patch downloads
• Fedora users should apply updates available through their package manager as announced in the Fedora Package Announcements

For Visual Studio 2019, updates are available through the Visual Studio Installer. For .NET Core runtime and SDK, updates can be obtained from the official Microsoft .NET download page.

Workarounds

• Implement network segmentation to limit exposure of vulnerable .NET applications
• Deploy web application firewalls (WAF) with custom rules to filter potentially malicious requests
• Restrict network access to .NET applications to trusted IP ranges where possible
• Consider temporarily disabling non-critical applications until patches can be applied
• Implement strict input validation at the application layer as a defense-in-depth measure

# Verify installed .NET Core versions
dotnet --list-runtimes

# Update .NET Core runtime (example for SDK)
# Download latest version from https://dotnet.microsoft.com/download
# For package manager updates on supported Linux distributions:
sudo apt update && sudo apt upgrade dotnet-runtime-5.0
# Or for Fedora:
sudo dnf update dotnet