CVE-2021-24111: Microsoft .NET Framework DOS Vulnerability

CVE-2021-24111 Overview

CVE-2021-24111 is a Denial of Service vulnerability affecting the Microsoft .NET Framework. This vulnerability allows remote attackers to cause a denial of service condition in applications built on vulnerable versions of the .NET Framework. The vulnerability can be exploited over the network without requiring authentication or user interaction, making it particularly concerning for internet-facing applications.

Critical Impact

Remote attackers can cause service disruption in .NET Framework applications, potentially affecting availability of critical business services and web applications running on affected Windows systems.

Affected Products

• Microsoft .NET Framework 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, and 4.8
• Microsoft Windows Server 2008 SP2, 2008 R2 SP1, 2012, 2012 R2, 2016, and 2019
• Microsoft Windows 7 SP1, 8.1, RT 8.1, and Windows 10 (multiple versions including 1607, 1803, 1809, 1909, 2004, 20H2)

Discovery Timeline

• 2021-02-25 - CVE-2021-24111 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-24111

Vulnerability Analysis

This Denial of Service vulnerability exists within the .NET Framework's processing capabilities. The flaw allows an attacker to remotely trigger a condition that exhausts system resources or causes the application to crash, resulting in service unavailability. The vulnerability is particularly impactful because it requires no privileges or user interaction to exploit, and can be triggered remotely over the network.

The attack does not compromise data confidentiality or integrity but directly impacts the availability of affected systems. Applications and services built on vulnerable .NET Framework versions are susceptible to becoming unresponsive when targeted by an attacker exploiting this vulnerability.

Root Cause

The vulnerability stems from improper handling of specific input within the .NET Framework. While Microsoft has not disclosed the specific technical details publicly to prevent exploitation, the weakness allows remote attackers to craft requests that cause resource exhaustion or application crashes in the .NET runtime environment.

Attack Vector

The attack vector is network-based, meaning exploitation can occur remotely without requiring local access to the target system. An attacker can send specially crafted requests to a .NET Framework application that processes network input, triggering the denial of service condition. The attack requires:

• Network accessibility to the target application
• No authentication required
• No user interaction needed
• Low attack complexity

The vulnerability mechanism involves sending malicious input that the .NET Framework improperly handles, leading to resource exhaustion or an unrecoverable application state. Refer to the Microsoft Security Advisory for additional technical details.

Detection Methods for CVE-2021-24111

Indicators of Compromise

• Unexpected application crashes or service restarts in .NET Framework applications
• High CPU or memory utilization spikes in w3wp.exe or other .NET application processes
• Increased error rates in application event logs related to .NET runtime exceptions
• Abnormal network traffic patterns targeting .NET-based web services

Detection Strategies

• Monitor .NET Framework application event logs for crash dumps and unhandled exception patterns
• Implement application performance monitoring to detect sudden availability degradations
• Configure IIS and web server logging to capture requests that precede application failures
• Deploy network intrusion detection rules to identify potential DoS attack patterns against .NET services

Monitoring Recommendations

• Enable Windows Event Log monitoring for .NET Runtime errors (Event ID 1026) and Application Errors
• Configure alerting thresholds for application pool recycling frequency in IIS environments
• Implement real-time service availability monitoring for critical .NET Framework applications
• Review application crash telemetry and correlate with incoming network request patterns

How to Mitigate CVE-2021-24111

Immediate Actions Required

• Apply the Microsoft security update for CVE-2021-24111 from the February 2021 Patch Tuesday release
• Prioritize patching internet-facing .NET Framework applications and services
• Implement rate limiting and request filtering at the network perimeter for critical applications
• Consider deploying web application firewalls to filter potentially malicious requests

Patch Information

Microsoft has released security updates addressing this vulnerability as part of the February 2021 security updates. The patches are available through Windows Update, Microsoft Update Catalog, and Windows Server Update Services (WSUS). Organizations should consult the Microsoft Security Advisory for CVE-2021-24111 to identify the specific KB articles applicable to their .NET Framework version and operating system combination.

Workarounds

• Implement network-level access controls to restrict access to .NET Framework services from untrusted networks
• Deploy load balancers or reverse proxies with DoS protection capabilities in front of vulnerable applications
• Configure application pool settings in IIS to enable rapid recovery from crashes with automatic recycling
• Implement connection throttling and request queuing limits to mitigate resource exhaustion attempts

# Example: Configure IIS application pool rapid failure protection
# This helps maintain availability during potential DoS attempts
%windir%\system32\inetsrv\appcmd set apppool /apppool.name:"DefaultAppPool" /failure.rapidFailProtectionInterval:00:05:00
%windir%\system32\inetsrv\appcmd set apppool /apppool.name:"DefaultAppPool" /failure.rapidFailProtectionMaxCrashes:10