If you’ve been following cybersecurity news lately, you’ve probably heard about Microsoft HTTP request smuggling vulnerability CVE-2025-55315 making headlines. But what exactly is HTTP request smuggling, and why should you care? Let’s break it down in plain English.

What Is HTTP Request Smuggling?

HTTP request smuggling is a technique where an attacker crafts a single HTTP request that gets interpreted as two different requests by different systems in the processing chain. Think of it like someone is passing through a guarded gate, but he hides another person under his coat.

Why Does This Happen?

Modern web applications aren’t just a single server sitting in a closet somewhere. They typically have multiple layers working together:

Front-end systems: Reverse proxies, load balancers, firewalls, Content Delivery Networks (CDNs), Web Application Firewalls (WAFs).
Back-end systems: The actual web application servers running your code.

These systems need to communicate efficiently, and they do so by reusing HTTP connections. But to reuse a connection, each system needs to know exactly where one request ends and the next one begins. HTTP provides two main ways to specify request boundaries:

Content-Length: Specifies exactly how many bytes the request body contains.
Transfer-Encoding: chunked: Sends the body in chunks with size indicators, ending with a zero-length chunk

The most common classic Request Smuggling Attack:

The most well-known HTTP request smuggling technique exploits disagreements between Content-Length (CL) and Transfer-Encoding (TE).
And here an example:

Here’s what an attack might look like:

1POST / HTTP/1.1
2Host: vulnerable-website.com
3Content-Length: 13
4Transfer-Encoding: chunked
5
60
7
8GET /Admin/Users HTTP/1.1
9Host: vulnerable-website.com

In the previous example:

In this scenario:

The front-end server processes Content-Length and thinks the body is 13 bytes long.
The back-end server processes Transfer-Encoding: chunked and sees the 0, interpreting it as the end of the request, and then interpret GET /Admin/Users as a second valid request.

Beyond Content-Length and Transfer-Encoding

While CL/TE conflicts get the most attention, HTTP request smuggling isn’t limited to just those techniques. The vulnerability landscape is much broader. For example:

Header Parsing Discrepancies
HTTP/2 Downgrade Attack
Connection Header Manipulation

But they all share the same core issue:

When two systems in a processing chain disagree about where one request ends and another begins, security boundaries collapse.

Why Microsoft CVE-2025-55315 is a big deal.

Microsoft on 15th of October 2025 announce about CVE-2025-55315.
CVE-2025-55315 carries a CVSS score of 9.9 Cyber Press.

In Plain English how they score vulnerability?

Before we dive into Microsoft, let us explain some techie words.

CVSS:

To score how bad vulnerability is, the industry came up with a standard called Common Vulnerability Scoring System, and it is a scale from 0 to 10.

CVE:

CVE stands for Common Vulnerabilities and Exposures, and it is a system to track all vulnerabilities in all systems, and it is managed by Home land security.

Why ASP.NET vulnerability has Score 9.9

Back to ASP.NET vulnerability CVE-2025-55315, with CVSS score 9.9 is nearly the maximum possible severity rating of 10. This is ASP.NET’s highest CVSS score to date.

It has that higher score because smuggling HTTP request will open Pandora box, and can lead to the following chain attacks:

Elevation of privilege (EOP) — impersonating another user
Server-side request forgery (SSRF) — making internal requests
Cross-site request forgery (CSRF) bypass
Injection attacks — bypassing input validation

Understanding HTTP Request Smuggling and latest ASP.NET vulnerability