FAQ about CVE-2025-55315, the 9.9-rated CVE in ASP.NET Core

What is CVE-2025-55315?

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CVE-2025-55315 is a critical HTTP request smuggling and security feature bypass vulnerability that affects ASP.NET Core and Microsoft.AspNetCore.Server.Kestrel.Core. Microsoft rates the issue with a CVSS of 9.9 because, while the parser bug itself enables request smuggling, the worst-case impacts on applications, including authentication bypass, server-side request forgery, injection attacks, privilege escalation can be catastrophic depending on your application design.

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Which versions of .NET are affected?

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All ASP.NET Core versions 6 and above are vulnerable to the exploit detailed in CVE-2025-55315. 

Specific runtime and package versions listed in Microsoft’s advisory are affected and have corresponding patched versions for ASP.NET Core 8, 9 and 10.

ASP.NET Core 6 is also vulnerable, but Microsoft did not publish a patch for .NET 6 because that release is end-of-life (EOL). Microsoft does not publish CVEs or patches for EOL versions of .NET.

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Which .NET components are affected by this CVE?

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If your app references Microsoft.AspNetCore.Server.Kestrel.Core, runs an affected runtime listed by Microsoft, or is on an EOL version of .NET, such as .NET 6, you are exposed.

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How does the vulnerability work?

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The bug is an inconsistent interpretation of HTTP requests between components that parse chunked transfer encoding and newline handling. An attacker can smuggle an malicious request inside another request, the hidden request can then bypass checks or trigger sensitive actions depending on how the application processes the incoming request stream.

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How can I verify if I am vulnerable to CVE-2025-55315?

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Check runtime and package versions with dotnet --info and inspect package references; then compare them to Microsoft’s affected-version lists. 

The absence of .NET 6 from Microsoft’s advisories does not imply safety. HeroDevs provides a reproducible test tool you can run locally against .NET 6 builds to confirm vulnerability status. If the two TCP-based tests in the repro tool pass, your build is not vulnerable; if they fail, it indicates vulnerability.

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How do I remediate the threat posed by this vulnerability?

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1. Upgrade to patched runtimes or packages where possible. Install the patched runtime/SDK versions Microsoft published for ASP.NET Core 8, 9, or 10, or update Microsoft.AspNetCore.Server.Kestrel.Core to version 2.3.6 or above. 
2. Restart your application processes after installing runtime or package updates so the patches take effect.
3. For self-contained deployments, recompile and redeploy after updating the runtime or Kestrel package.
4. If you are running unpatched .NET 6:
   
   1. Use the local reproduction test tool to verify your exposure risk.
   2. Apply a post-EOL mitigation such as HeroDevs’ NES for .NET 6, which contains a patch for this and other post-EOL vulnerabilities.

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What if I cannot migrate from .NET 6 immediately?

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While some of the remediations above may limit your exposure, the best option is to consider post-EOL support for .NET from HeroDevs, via NES for .NET 6, which patches .NET 6 for CVE-2025-55315 as well as other critical and high CVEs in .NET 6.

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Are there other steps I can take in addressing the risk of CVE-2025-55315?

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1. Review middleware and pipeline code for places where request parsing influences authentication, server-side calls (HTTP client use), or database updates. Those are the highest-impact places for HTTP request-smuggling exploitation.
2. Apply firewall rules and ingress-layer protections to block malformed request framing. This is not a full replacement for patching but a useful, temporary layered defense.
3. Increase logging and alerting around unusual request boundary behavior, chunked-encoding anomalies, or unexpected rapid sequences of requests. This does not remediate or mitigate the risk, but can alert you to an attack in progress.

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Secure Your .NET 6 Applications

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CVE-2025-55315 poses a significant threat to unpatched ASP.NET Core 6 deployments. While immediate migration may not always be feasible, HeroDevs offers a solution. Contact HeroDevs to learn more about NES for .NET 6, and ensure your applications are protected against this and other critical post-EOL vulnerabilities. Don't leave your .NET 6 applications exposed, act now to secure your systems and remain compliant.

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