How to Safeguard Your ASP.NET Core Apps from the CVE-2026-40372 Vulnerability

Introduction

On Tuesday evening, Microsoft released an emergency patch for ASP.NET Core to address a high-severity vulnerability (CVE-2026-40372) affecting Linux and macOS environments. The flaw resides in the Microsoft.AspNetCore.DataProtection NuGet package versions 10.0.0 through 10.0.6. It allows unauthenticated attackers to forge authentication payloads during HMAC validation, potentially gaining SYSTEM privileges and compromising the entire system. Critically, even after patching, any forged credentials created by an attacker remain valid unless explicitly purged. This guide walks you through identifying vulnerable installations, applying the patch, and ensuring your systems are fully secured.

What You Need

• Administrative or root access to the affected Linux or macOS machines where ASP.NET Core apps run.
• NuGet package manager or CLI access (e.g., dotnet CLI) to update packages.
• List of your deployed ASP.NET Core applications and their dependency versions.
• A backup or snapshot of critical systems before making changes.
• Awareness of any custom certificates or data protection keys that may need regeneration (see tips).

Step-by-Step Guide

Step 1: Identify Affected Versions

Start by checking all your ASP.NET Core projects for the Microsoft.AspNetCore.DataProtection NuGet package. Use the dotnet list package command in your project directory:

dotnet list package --include-transitive

Look for Microsoft.AspNetCore.DataProtection with version between 10.0.0 and 10.0.6. If found, the application is vulnerable.

Step 2: Update the Package to a Patched Version

Microsoft has released version 10.0.7 which fixes the cryptographic signature verification flaw. Update the package using NuGet package manager or the dotnet CLI:

dotnet add package Microsoft.AspNetCore.DataProtection --version 10.0.7

Alternatively, update your project’s .csproj file directly to reference version 10.0.7 and run dotnet restore.

Step 3: Rebuild and Redeploy Your Application

After updating the package, rebuild your application to ensure the new binary is linked:

dotnet build --configuration Release

If running in a production environment, deploy the updated build following your usual release process. Restart the application to load the patched code.

Step 4: Purge Any Forged Authentication Credentials

This step is critical. Even after patching, any forged authentication payloads created by an attacker before the update remain valid. You must clear the data protection key ring and force re-creation:

1. Locate the data protection key store. By default, it resides in the %LOCALAPPDATA%\ASP.NET\DataProtection-Keys on Windows, and on Linux/macOS it is typically $HOME/.aspnet/DataProtection-Keys or an Azure Blob/Redis store if configured.
2. Delete all key files (or purge entries in your persistent store). Warning: This will invalidate all existing, valid user sessions and require users to log in again. Plan for downtime or communicate maintenance.
3. Restart the application to allow ASP.NET Core to generate a fresh key ring.
4. Test that authentication works correctly with new keys.

Step 5: Verify the Patch is Applied

Confirm that the vulnerability has been addressed:

• Run dotnet list package --outdated to ensure no vulnerable packages remain.
• Check the version of Microsoft.AspNetCore.DataProtection in your deployed binaries (e.g., using strings or dependency inspection). It should be 10.0.7 or higher.
• If possible, simulate an attack using a test environment to confirm that HMAC forgery is no longer possible.

Step 6: Monitor for Signs of Compromise

Since the vulnerability allows SYSTEM-level access, an attacker may have already breached your system. After patching, perform security checks:

• Review system logs for unauthorized access or privilege escalation attempts around the time the vulnerability was unpatched.
• Check for any new user accounts or suspicious processes.
• Rotate all passwords and tokens that may have been exposed.

Tips for a Smooth Recovery

• Back up keys before deletion: If you have applications relying on persistent data protection (e.g., encrypted cookies), backing up old keys allows you to decode existing data temporarily. However, for security, it’s best to expire all old keys.
• Plan for user disruption: Purging keys forces all users to reauthenticate. Communicate maintenance windows in advance.
• Consider key storage rotation: If you store keys in Azure Key Vault or a centralized store, rotate secrets and update permissions.
• Test in a staging environment: Run the steps first on a non-production system to avoid unexpected issues.
• Stay updated: Subscribe to Microsoft’s security advisories to receive future patches promptly.