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Microsoft has released a security advisory to address a critical security feature bypass vulnerability affecting the Kestrel web server component within .NET projects. Specifically, the vulnerability resides in Microsoft.AspNetCore.Server.Kestrel.Core, where the server incorrectly accepts newline characters embedded within HTTP/2 request trailers and values retrieved from the dynamic HPACK table. Under strict HTTP/2 protocol specifications, these characters should be immediately recognized and rejected as malformed input to prevent request smuggling or header injection scenarios. Because Kestrel fails to properly validate and sanitize these header streams during HPACK decompression and trailer processing, it introduces inconsistent interpretation risks between front-end proxies and backend ASP.NET Core applications. Malicious clients can leverage this flaw to smuggle unauthorized requests or bypass security boundaries implemented at the proxy layer, posing a severe threat to enterprise application integrity and confidentiality across targeted Windows, Linux, and macOS environments.
DailyCVE Form:
Platform: Microsoft .NET
Version: 8.0 to 11.0
Vulnerability : HTTP/2 Request Smuggling
Severity: Critical
date: 10/13/2026
Prediction: October 13, 2026
What Undercode Say:
To thoroughly analyze this vulnerability, engineers must inspect how Kestrel handles HTTP/2 HPACK decompression and trailer parsing logic. The underlying code flaw allows newline characters to pass through validation checks in Microsoft.AspNetCore.Server.Kestrel.Core. Reviewing the runtime configuration and network traces can help pinpoint anomalous traffic patterns exploiting header parsing discrepancies.
dotnet --info dotnet add package Microsoft.AspNetCore.App.Runtime --version 8.0.32
Exploit: (Educational Purposes!)
Attackers construct malicious HTTP/2 requests embedding carriage return or newline characters (\r\n) inside HPACK compressed dynamic table entries or request trailers. When the upstream proxy normalizes or rejects the characters differently than the downstream Kestrel server, inconsistent message parsing occurs, allowing the attacker to bypass authentication filters or smuggle secondary payloads directly into the application pipeline.
Protection: from this CVE
Developers must immediately update their .NET SDK and runtime packages to the patched versions (such as version 8.0.32, 9.0.21, 10.0.13, or 11.0.0-rc.2 depending on the major release branch). After updating package references or installing the latest .NET SDK, applications must be fully restarted, and self-contained deployments must be recompiled and redeployed to ensure the vulnerable Kestrel binaries are completely replaced.
Impact:
Successful exploitation of this vulnerability can lead to a critical security feature bypass, enabling unauthorized actors to bypass front-end proxy security controls, poison web caches, or achieve HTTP request smuggling. This compromises end-to-end data confidentiality and application integrity across exposed multi-platform enterprise deployments.
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Sources:
Reported By: github.com
Extra Source Hub:
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