Microsoft has released a critical security update to address what is being described as the most severe vulnerability ever discovered in ASP.NET Core. The flaw, identified as CVE-2025-55315, is an HTTP request smuggling vulnerability found in the Kestrel web server. It allows authenticated attackers to inject unauthorized HTTP requests, potentially hijacking user sessions or bypassing front-end security mechanisms. According to Microsoft, exploitation of this flaw could result in the exposure of sensitive information, such as user credentials, unauthorized modification of server files, or even denial-of-service conditions by crashing the server. To mitigate these risks, Microsoft has issued patches for multiple platforms, including Visual Studio 2022, ASP.NET Core versions 2.3, 8.0, and 9.0, along with updates to the Microsoft.AspNetCore.Server.Kestrel.Core package for legacy ASP.NET Core 2.x applications. To secure affected systems, Microsoft advises developers using .NET 8 or newer to apply the latest updates via Microsoft Update and restart their applications or systems. For those on .NET 2.3, the recommended action is to update the Kestrel package to version 2.3.6, then recompile and redeploy the application. Developers using self-contained or single-file deployments should follow similar steps: apply the update, recompile, and redeploy. Barry Dorrans, .NET security technical program manager, emphasized that the potential impact depends heavily on the specific application logic. While the flaw could enable privilege escalation, server-side request forgery, or injection attacks, these outcomes are more likely in applications that skip critical security checks. Despite the variability in risk, Microsoft strongly urges all users to apply the necessary updates without delay.
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