Microsoft released an emergency patch for its ASP.NET Core to fix a high-severity vulnerability that allows unauthenticated attackers to gain SYSTEM privileges on devices that use the Web development framework to run Linux or macOS apps.

The software maker said Tuesday evening that the vulnerability, tracked as CVE-2026-40372, affects versions 10.0.0 through 10.0.6 of the Microsoft.AspNetCore.DataProtection NuGet, a package that’s part of the framework. The critical flaw stems from a faulty verification of cryptographic signatures. It can be exploited to allow unauthenticated attackers to forge authentication payloads during the HMAC validation process, which is used to verify the integrity and authenticity of data exchanged between a client and a server.

Beware: Forged credentials survive patching

During the time users ran a vulnerable version of the package, they were left open to an attack that would allow unauthenticated people to gain sensitive SYSTEM privileges that would allow full compromise of…