CVE-2021-26701
📋 TL;DR
CVE-2021-26701 is a remote code execution vulnerability in .NET Core that allows attackers to execute arbitrary code on affected systems. This affects applications running vulnerable versions of .NET Core, potentially compromising server integrity and data security.
💻 Affected Systems
- .NET Core
- .NET 5.0
📦 What is this software?
.net by Microsoft
.net Core by Microsoft
.net Core by Microsoft
Fedora by Fedoraproject
Fedora by Fedoraproject
Fedora by Fedoraproject
Powershell Core by Microsoft
Powershell Core by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attacker to install malware, exfiltrate sensitive data, pivot to other systems, and maintain persistent access.
Likely Case
Application compromise leading to data theft, service disruption, and potential lateral movement within the network.
If Mitigated
Limited impact with proper network segmentation, least privilege, and monitoring in place, potentially only affecting isolated application instances.
🎯 Exploit Status
Exploitation requires specific conditions but has been demonstrated. The vulnerability is in the .NET runtime and can be triggered through various application inputs.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: .NET Core 2.1.26, 3.1.13, 5.0.4 or later
Vendor Advisory: https://github.com/dotnet/announcements/issues/194
Restart Required: Yes
Instructions:
1. Identify affected .NET Core versions. 2. Download and install updated .NET Core runtime from Microsoft. 3. Restart all applications using .NET Core. 4. Rebuild and redeploy applications if using self-contained deployments.
🔧 Temporary Workarounds
Network Segmentation
allIsolate .NET Core applications from untrusted networks and implement strict firewall rules.
Application Whitelisting
allImplement application control to prevent execution of unauthorized code.
🧯 If You Can't Patch
- Implement strict network access controls to limit exposure to trusted sources only
- Deploy additional monitoring and intrusion detection focused on .NET Core application behavior
🔍 How to Verify
Check if Vulnerable:
Run 'dotnet --info' to check installed .NET Core version. Compare against affected versions: 2.1.0-2.1.25, 3.1.0-3.1.12, 5.0.0-5.0.3.Check Version:
dotnet --info | findstr "Version"Verify Fix Applied:
Verify installed version is 2.1.26+, 3.1.13+, or 5.0.4+ using 'dotnet --info'. Test application functionality after update.📡 Detection & Monitoring
Log Indicators:
- Unusual process creation from .NET applications
- Abnormal network connections from .NET runtime processes
- Security event logs showing code injection attempts
Network Indicators:
- Unexpected outbound connections from .NET applications
- Traffic patterns inconsistent with normal application behavior
SIEM Query:
Process Creation where (Image contains "dotnet" OR ParentImage contains "dotnet") AND CommandLine contains unusual patterns🔗 References
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/S2AZOUKMCHT2WBHR7MYDTYXWOBHZW5P5/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TW3ZSJTTMZAFKGW7NJWTVVFZUYYU2SJZ/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/UBOSSX7U6BSHV5RI74FCOW4ITJ5RRJR5/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WA5WQJVHUL5C4XMJTLY3C67R4WP35EF4/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/XPUKFHIGP5YNJRRFWKDJ2XRS4WTFJNNK/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/YLFATXASXW4OV2ZBSRP4G55HJH73QPBP/
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26701
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/S2AZOUKMCHT2WBHR7MYDTYXWOBHZW5P5/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TW3ZSJTTMZAFKGW7NJWTVVFZUYYU2SJZ/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/UBOSSX7U6BSHV5RI74FCOW4ITJ5RRJR5/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WA5WQJVHUL5C4XMJTLY3C67R4WP35EF4/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/XPUKFHIGP5YNJRRFWKDJ2XRS4WTFJNNK/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/YLFATXASXW4OV2ZBSRP4G55HJH73QPBP/
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26701
