CLTE-SMUGGLED-GET-CL-PLUS

What it does

This is a variant of SMUG-CLTE-SMUGGLED-GET that uses a malformed Content-Length header (+N) while also sending Transfer-Encoding: chunked.

Some HTTP stacks reject Content-Length: +N as invalid; others parse it leniently. In a proxy chain, these disagreements can reintroduce classic CL.TE smuggling.

What it sends

The request body begins with a valid chunked terminator (0\r\n\r\n) and then immediately contains an entire GET / request.

POST / HTTP/1.1\r\n
Host: localhost:8080\r\n
Content-Length: +45\r\n
Transfer-Encoding: chunked\r\n
\r\n
0\r\n
\r\n
GET / HTTP/1.1\r\n
Host: localhost:8080\r\n
\r\n

What the RFC says

“A server MAY reject a request that contains both Content-Length and Transfer-Encoding… Regardless, the server MUST close the connection after responding to such a request.” — RFC 9112 §6.1

Even if a server chooses to accept and process the message, it must close the connection afterward to prevent request boundary confusion and smuggling.

Why it matters

Malformed framing headers are a common source of front-end/back-end parsing disagreements. If one hop accepts Content-Length: +N while another rejects it (or ignores it in favor of chunked framing), the embedded GET / can be interpreted as a separate request.

This test looks for concrete evidence of request-boundary confusion by checking whether the server emits multiple HTTP status lines (multiple responses) after a single client send. In a proxy chain, this same behavior is what enables request smuggling.

Verdicts

• Pass: The server rejects with 400, or closes the connection.
• Fail: Multiple HTTP responses are observed (embedded GET likely executed).
• Fail: The server accepts and keeps the connection open (MUST-close violated), even if no extra response is observed.

Sources

• RFC 9112 §6.1