A Practical Decisionmakers Guide To Responding to CVE-2025-55315

CVE-2025-55315 is a critical-impact HTTP request smuggling and security feature bypass in ASP.NET Core that enables authentication bypass, server-side request forgery, and other serious downstream impacts when an attacker successfully smuggles requests into an application’s request stream.

This CVE’s CVSS score of 9.9 underscores how a parser-level bug can become an application‑level crisis for network‑facing services. Microsoft rated CVE-2025-55315 as a 9.9 due to its broad exploitation potential, depending on how applications parse HTTP requests.

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Microsoft’s Response

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Microsoft published patched builds for supported .NET releases (8, 9, and 10) but not for end-of-life versions like .NET 6, which it also omitted from the CVE.

Our research determined that .NET 6 is vulnerable to the same HTTP request smuggling and security feature bypass as .NET 8+. The absence of .NET 6 from the CVE and related security advisory does not imply safety, quite the opposite. It is also probable that CVE security scanners you may have deployed will not detect vulnerable .NET 6 versions in production.

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The Obvious Fix: Upgrade Your .NET

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Patch-first is the simplest and lowest‑risk long‑term strategy for most teams. Immediately upgrading to the patched releases Microsoft published for .NET 8, 9, or 10, and update Microsoft.AspNetCore.Server.Kestrel.Core to version 2.3.6 or higher . Restart processes and rebuild/redeploy self‑contained apps after you update so the patched runtime/package actually takes effect. 

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The .NET 6 Quandary: Still Vulnerable After EOL

Many organizations still run .NET 6 in production. Because .NET 6 is EOL, Microsoft will not backport a patch for this CVE, leaving your applications built on .NET 6 vulnerable, and your organization potentially out of compliance. The absence of a Microsoft patch or CVE entry for EOL .NET 6 does not mean the vulnerability is not present. You can reproduce and test the attack locally against .NET 6 builds using  this guidance and tool from HeroDevs.

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Migration Versus Post-EOL Support for .NET

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Key decision factors in determining whether to simply mitigate or obtain post-EOL support for .NET include:

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1. Your organization’s security posture: Upgrading to a supported runtime gives continuing vendor patches and eliminates the systemic risk of running EOL platforms.
2. Time and effort: Migration effort varies by codebase complexity, CI systems, reliance on third‑party libraries, and custom middleware. When migrating, plan for testing, dependency upgrades (or replacements), and rollout windows.
3. Short‑term urgency: If you can’t complete migration quickly and host internet‑facing or critical apps on .NET 6, post‑EOL support can provide immediate, targeted remediation for this and other vulnerabilities present in .NET 6 while you plan migration.
4. Cost and compliance: Migration incurs one‑time engineering and QA costs and reduces recurring vendor dependency. Paid post-EOL support adds recurring licensing but can be far cheaper than an urgent, resource‑heavy migration in the short term.

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Practical Next Steps For Decision makers

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If your organization has internet‑exposed, customer‑facing, or compliance‑sensitive apps, consider prioritizing a migration to a supported, patched runtime ASAP. If you can’t migrate easily before your next operational window or audit, then HeroDevs is here for you. NES for .NET 6 will help to close the immediate gap.

For internal, low‑risk apps with well‑controlled ingress, if your organization can tolerate interim risk (and not risk being out of compliance), apply compensating controls (some of which are discussed below), and schedule migration on a firm timeline. Consider immediate post-EOL support if compliance or regulatory requirements dictate.

If your organization has a mixed fleet of external and internal facing .NET 6 applications, use phased migration to move the most exposed workloads to a supported version of .NET first, protect remaining .NET 6 workloads with monitoring and compensating controls, and consider immediate EOL support if timelines are tight or for compliance requirements.

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Short-Term Hardening Checklist

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In the short term, while migrating to a supported .NET version or deploying post-EOL support for .NET 6, you can take the following steps to reduce (but not eliminate) exposure to CVE-2025-55315.

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1. Use the public CVE‑2025‑55315 repro tool to confirm whether a given .NET 6 or other runtime build is vulnerable.
2. Deploy ingress firewall rules to block malformed chunked requests and unusual request framing where possible. Note: this is a temporary layer, not a substitute for a migration or patch.
3. Audit middleware and endpoints where parsing differences could change authentication, authorization, or server‑side request forwarding as these are highest‑impact targets for exploitation.
4. Enhance logging and alerts, such as adding telemetry to detect anomalous chunk boundaries, repeated fragmented HTTP requests, or spikes in internal requests that could indicate smuggling attempts.
5. Lock down internal APIs and reduce trust between services so a successful smuggle cannot easily trigger internal SSRF or lateral movement.
6. If migration isn’t immediately feasible, introduces breaking changes to your app, or will take too long, our post-EOL support for your .NET applications is your best bet.

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An ROI Framework For Migration vs. Post-EOL Support

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1. Triage your applications by external exposure, sensitivity of operations, and impact of compromise.
2. Patch supported runtimes first. For exposed .NET 6 apps, run the repro tool, temporarily harden your .NET 6 applications.
3. Allocate engineering sprints and budget to upgrade libraries, CI, and staging pipelines to support .NET 8+. If none of the above are doable in a timely manner, or introduce an intolerable amount of risk, post EOL .NET support from HeroDevs is likely your best and fastest route to closing this critical vulnerability.

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Final Take

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CVE‑2025‑55315 is a reminder that parser‑level bugs can translate into catastrophic application impact. The safest course is straightforward: apply Microsoft’s patches on supported runtimes and migrate off EOL versions of .NET. When migration isn’t immediately possible, use objective testing, strong perimeter controls, increased telemetry, and post-EOL support that delivers verifiable fixes while you execute a measured migration plan.

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