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CVE-2025-55315 represents a critical HTTP request smuggling vulnerability residing within the Kestrel web server component of ASP.NET Core.
The flaw originates from discrepancies in how incoming HTTP requests are parsed and interpreted by front-end proxies versus the back-end Kestrel server.
Specifically, ambiguities involving `Content-Length` and `Transfer-Encoding` header fields allow malicious clients to construct malformed payloads.
When a proxy normalizes or overlooks these conflicting headers differently than Kestrel, the boundary of a single request becomes blurred.
This permits an attacker to smuggle a second, hidden HTTP request piggybacked on the first legitimate connection stream.
The Kestrel server processes this smuggled request independently, attributing it to a trusted internal user session or context.
Because web applications rely heavily on strict request isolation for authorization and routing, bypassing these boundaries enables severe security breaches.
Attackers can leverage this capability to bypass front-end security controls, poison web caches, or directly access unauthorized administrative endpoints.
In multi-tenant cloud deployments, request smuggling can lead to cross-tenant data access, exposing sensitive user tokens and confidential records.
The vulnerability carries a maximum CVSS score of 9.9 due to its potential for unauthenticated remote code execution and systemic compromise.
Mitigating this issue requires updating the underlying .NET runtime libraries to versions containing official patches issued by Microsoft.
Developers must also ensure that front-end reverse proxies like Azure Front Door or Nginx are correctly configured to reject ambiguous requests.
DailyCVE Form:
Platform: Microsoft ASP.NET Core
Version: Multiple .NET versions
Vulnerability : Request Smuggling
Severity: Critical
date: October 14, 2025
Prediction: Already patched
What Undercode Say:
Bash Commands and Code
dotnet --version dotnet add package Microsoft.AspNetCore.App --version 9.0.0 dotnet restore dotnet build --configuration Release
Exploit: (Educational Purposes!)
HTTP request smuggling is tested by sending a crafted request where both `Content-Length` and `Transfer-Encoding: chunked` headers are supplied simultaneously:
POST / HTTP/1.1 Host: vulnerable-app.local Content-Length: 6 Transfer-Encoding: chunked 0 GET /admin HTTP/1.1 Host: vulnerable-app.local
If the front-end processes `Content-Length` and Kestrel processes Transfer-Encoding, the trailing `GET /admin` request is treated as a separate, unauthenticated request by the backend.
Protection: from this CVE
Update ASP.NET Core and the .NET runtime to the latest security patch releases.
Configure front-end reverse proxies (such as Nginx, IIS, or Azure Front Door) to strictly reject requests containing conflicting or ambiguous transfer headers.
Implement robust input validation and request normalization across all proxy layers.
Impact:
Complete bypass of front-end security controls and web application firewalls.
Unauthorized access to sensitive administrative functionality and internal APIs.
Potential cache poisoning affecting application availability and data integrity for other users.
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Sources:
Reported By: github.com
Extra Source Hub:
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