Package Instance

Netty vulnerable to request smuggling due to incorrect parsing of chunk extensions
## Summary
A flaw in netty's parsing of chunk extensions in HTTP/1.1 messages with chunked encoding can lead to request smuggling issues with some reverse proxies.

## Details
When encountering a newline character (LF) while parsing a chunk extension, netty interprets the newline as the end of the chunk-size line regardless of whether a preceding carriage return (CR) was found. This is in violation of the HTTP 1.1 standard which specifies that the chunk extension is terminated by a CRLF sequence (see the [RFC](https://datatracker.ietf.org/doc/html/rfc9112#name-chunked-transfer-coding)).

This is by itself harmless, but consider an intermediary with a similar parsing flaw: while parsing a chunk extension, the intermediary interprets an LF without a preceding CR as simply part of the chunk extension (this is also in violation of the RFC, because whitespace characters are not allowed in chunk extensions). We can use this discrepancy to construct an HTTP request that the intermediary will interpret as one request but netty will interpret as two (all lines ending with CRLF, notice the LFs in the chunk extension):

```
POST /one HTTP/1.1
Host: localhost:8080
Transfer-Encoding: chunked

48;\nAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\n0

POST /two HTTP/1.1
Host: localhost:8080
Transfer-Encoding: chunked

0

```

The intermediary will interpret this as a single request. Once forwarded to netty, netty will interpret it as two separate requests. This is a problem, because attackers can then the intermediary, as well as perform standard request smuggling attacks against other live users (see [this Portswigger article](https://portswigger.net/web-security/request-smuggling/exploiting)).

## Impact
This is a request smuggling issue which can be exploited for bypassing front-end access control rules as well as corrupting the responses served to other live clients.

The impact is high, but it only affects setups that use a front-end which:
1. Interprets LF characters (without preceding CR) in chunk extensions as part of the chunk extension.
2. Forwards chunk extensions without normalization.

## Disclosure

 - This vulnerability was disclosed on June 18th, 2025 here: https://w4ke.info/2025/06/18/funky-chunks.html

## Discussion
Discussion for this vulnerability can be found here:
 - https://github.com/netty/netty/issues/15522
 - https://github.com/JLLeitschuh/unCVEed/issues/1

## Credit

 - Credit to @JeppW for uncovering this vulnerability.
 - Credit to @JLLeitschuh at [Socket](https://socket.dev/) for coordinating the vulnerability disclosure.

Netty's HttpPostRequestDecoder can OOM
### Summary
The `HttpPostRequestDecoder` can be tricked to accumulate data. I have spotted currently two attack vectors 

### Details
1. While the decoder can store items on the disk if configured so, there are no limits to the number of fields the form can have, an attacher can send a chunked post consisting of many small fields that will be accumulated in the `bodyListHttpData` list.
2. The decoder cumulates bytes in the `undecodedChunk` buffer until it can decode a field, this field can cumulate data without limits

### PoC

Here is a Netty branch that provides a fix + tests : https://github.com/vietj/netty/tree/post-request-decoder


Here is a reproducer with Vert.x (which uses this decoder) https://gist.github.com/vietj/f558b8ea81ec6505f1e9a6ca283c9ae3

### Impact
Any Netty based HTTP server that uses the `HttpPostRequestDecoder` to decode a form.

Netty vulnerable to HTTP Response splitting from assigning header value iterator
### Impact
When calling `DefaultHttpHeaders.set` with an _iterator_ of values (as opposed to a single given value), header value validation was not performed, allowing malicious header values in the iterator to perform [HTTP Response Splitting](https://owasp.org/www-community/attacks/HTTP_Response_Splitting).

### Patches
The necessary validation was added in Netty 4.1.86.Final.

### Workarounds
Integrators can work around the issue by changing the `DefaultHttpHeaders.set(CharSequence, Iterator<?>)` call, into a `remove()` call, and call `add()` in a loop over the iterator of values.

### References
[HTTP Response Splitting](https://owasp.org/www-community/attacks/HTTP_Response_Splitting)
[CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers](https://cwe.mitre.org/data/definitions/113.html)

### For more information
If you have any questions or comments about this advisory:
* Open an issue in [[example link to repo](https://github.com/netty/netty)](https://github.com/netty/netty)
* Email us at [netty-security@googlegroups.com](mailto:netty-security@googlegroups.com)