CVE-2021-26423
📋 TL;DR
CVE-2021-26423 is a denial-of-service vulnerability in .NET Core and Visual Studio where an attacker can cause the application to crash by sending specially crafted requests. This affects applications built with vulnerable .NET Core versions and developers using affected Visual Studio versions. The vulnerability requires the attacker to be able to send requests to the vulnerable application.
💻 Affected Systems
- .NET Core
- Visual Studio
📦 What is this software?
.net by Microsoft
.net Core by Microsoft
.net Core by Microsoft
Powershell Core by Microsoft
Powershell Core by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete service unavailability for .NET Core applications, disrupting business operations and requiring manual restart.
Likely Case
Application crashes leading to temporary service disruption until automatic or manual restart occurs.
If Mitigated
Minimal impact with proper network segmentation, rate limiting, and monitoring in place to detect and respond to attacks.
🎯 Exploit Status
Microsoft has not disclosed specific exploitation details, but the vulnerability allows unauthenticated attackers to trigger denial of service.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: .NET Core 5.0.5, 3.1.14, 2.1.27; Visual Studio 2019 version 16.9.4
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26423
Restart Required: Yes
Instructions:
1. Update .NET Core runtime/SDK to patched versions. 2. Update Visual Studio to version 16.9.4 or later. 3. Rebuild and redeploy applications with updated .NET Core. 4. Restart affected services and applications.
🔧 Temporary Workarounds
Network Segmentation and Filtering
allRestrict network access to vulnerable applications using firewalls or network security groups
Rate Limiting
allImplement rate limiting on application endpoints to reduce impact of DoS attempts
🧯 If You Can't Patch
- Implement Web Application Firewall (WAF) with DoS protection rules
- Monitor application health and implement automatic restart mechanisms for crashes
🔍 How to Verify
Check if Vulnerable:
Check .NET Core version with 'dotnet --info' or 'dotnet --version'. Check Visual Studio version via Help > About Microsoft Visual Studio.Check Version:
dotnet --versionVerify Fix Applied:
Verify installed .NET Core version is 5.0.5+, 3.1.14+, or 2.1.27+. Verify Visual Studio is version 16.9.4+.📡 Detection & Monitoring
Log Indicators:
- Application crash logs
- Unexpected process termination
- High error rates in application logs
Network Indicators:
- Unusual traffic patterns to .NET Core applications
- Multiple connection attempts from single sources
SIEM Query:
source="application_logs" AND ("crash" OR "terminated" OR "unhandled exception") AND process="dotnet"