Andrew Lock provides an in-depth technical breakdown of the ASP.NET Core CVE-2025-55315 vulnerability, explaining request smuggling, exploitation avenues, and steps for developers to remain secure.

Understanding the Worst .NET Vulnerability Ever: Request Smuggling and CVE-2025-55315

Andrew Lock explores the details of the CVE-2025-55315 request smuggling vulnerability in ASP.NET Core, assigned a record-high CVSS score of 9.9 by Microsoft. This post clarifies how request smuggling works, the specifics of the discovered flaw, its impact, and the required mitigation steps.

What Is CVE-2025-55315?

On October 14th, 2025, Microsoft patched all supported versions of .NET, releasing a security advisory about a bypass vulnerability in ASP.NET Core: CVE-2025-55315. The issue involves inconsistent HTTP request interpretation leading to possible security feature bypass via request smuggling.

Key points from the advisory:

The bug enables HTTP Request Smuggling in ASP.NET Core.
Attackers could log in as other users (escalation of privilege), make internal requests (SSRF), bypass CSRF, or conduct injection attacks, depending on app specifics.
The severity was rated at 9.9 to maximize awareness and drive patch adoption due to the broad consequences.

Request Smuggling, Explained

Request smuggling exploits the differences in how proxies and backend servers parse ambiguous or malformed HTTP requests—allowing an attacker to “smuggle” a malicious request past a front-end server.

Example scenario:

An HTTP request with two different Content-Length headers causes the proxy and backend to interpret the end of the request differently.
The backend parses a hidden second request not seen by the proxy, potentially bypassing authentication or other controls.

The post provides code-level examples and references PortSwigger’s documentation for more practical attack variations.

Exploitation Potential

If exploited, request smuggling could allow attackers to:

Steal other users’ credentials or session data
Poison caches or reflect malicious data
Access endpoints that should be blocked by proxies
Inject malicious headers

Severity depends on an application’s structure and any custom handling of HTTP requests.

The ASP.NET Core / CVE-2025-55315 Vulnerability

This CVE leverages ambiguity in parsing chunked transfer encoding with chunk extensions.

Older ASP.NET Core versions were lenient, skipping lines ending with just \n.
Different handling between proxies and Kestrel could allow request smuggling, bypassing proxy security rules.

The post details the vulnerable implementation and how malformed chunk extensions could trigger the vulnerability.

Who Is Affected? What Should You Do?

Affected: All unpatched versions of .NET 8 and above, plus earlier versions (no official fixes for versions <8, except via HeroDevs support).
Mitigation: Update ASP.NET Core to patched releases (see official issue for version specifics).
If you cannot patch: Use a patched proxy (e.g., Azure App Services have applied mitigations); consider enforcing use of HTTP/2/3.
Testing: Check your runtime version with dotnet --info and review HeroDevs’ testing repo for exploit scripts.

Technical Details

The fix enforces strict parsing of chunk extensions, rejecting standalone line feeds (\n) in chunk headers.
Applications manipulating HttpRequest.Body or acting as custom proxies may be at increased risk.
The vulnerability does not affect HTTP/2 or HTTP/3.

Action Items for Developers

Patch immediately to the latest supported .NET/ASP.NET Core release.
If unsupported, consider HeroDevs for extended patching, or apply a secure, patched proxy.
Review custom code that processes HTTP requests or acts as a proxy within ASP.NET Core.
Stay alert for further advisories by watching the GitHub issue.

References

Andrew Lock’s article breaks down the complex security context to provide actionable guidance for technical practitioners.

This post appeared first on “Andrew Lock’s Blog”. Read the entire article here