ASP.NET Security Feature Bypass Vulnerability

How could an attacker exploit the vulnerability?
An authenticated attacker could exploit the vulnerability by sending a malicious http request to the web server.

According to the CVSS metrics, successful exploitation of this vulnerability could lead to major loss of confidentiality (C:H), and integrity (I:H), and some loss of availability (A:L). What does that mean for this vulnerability?
An attacker who successfully exploited this vulnerability could view sensitive information such as other user's credentials (Confidentiality) and make changes to file contents on the target server (Integrity), and they might be able to force a crash within the server (Availability).

What should I do to make sure my ASP.NET Core application is protected?
If you are running .NET 8 or later install the .NET update from Microsoft Update, then restart your application or reboot the machine., If you are running .NET 2.3 you must update the package reference for Microsoft.AspNet.Server.Kestrel.Core to 2.3.6, then recompile your application and redeploy., If you are running a self-contained/single-file application, install the .NET update, recompile your application and redeploy.

According to the CVSS metric, a successful exploitation could lead to a scope change (S:C). What does this mean for this vulnerability?
An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.

What kind of security feature could be bypassed by successfully exploiting this vulnerability?
An attacker who successfully exploited this vulnerability could smuggle another HTTP request and bypass front-end security controls or hijack other users' credentials.

This vulnerability is rated as an Important, Security Feature Bypass that is less likely to be exploited. Why is the CVSS score 9.9 out of 10?
ASP.NET Core is a framework. CVSS scores applications. This mismatch makes scoring ASP.NET challenging. In situations like this, it is Microsoft's standard practice to score the worst possible case scenario for any application written using ASP.NET Core. "Exploitation less likely" refers to a more typical application which doesn't go outside the typical ASP.NET Core uses.
The vulnerability in ASP.NET Core allows HTTP request smuggling. This is a security feature bypass that can affect how your application enforces authentication, authorization, and other critical security checks. The vulnerability allows attackers to craft a request that hides another request inside it. Depending on how your app processes requests, this could lead to:
Elevation of privilege (EOP) — impersonating another user, Server-side request forgery (SSRF) — making internal requests, Cross-site request forgery (CSRF) bypass, Injection attacks — bypassing input validation
These are serious risks, especially for applications handling sensitive data or operating in regulated environments.
While not every app is vulnerable, the scoring reflects the worst-case scenario—a standard practice in CVSS to ensure customers take action.

Security updates
8.0.21 — https://support.microsoft.com/help/5068331
9.0.10 — https://support.microsoft.com/help/5068332
2.3.6 — https://github.com/dotnet/aspnetcore/issues/64033
17.12.13 — https://learn.microsoft.com/en-us/visualstudio/releases/2022/release-notes