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The vulnerability arises from Netty’s mishandling of the HTTP/2 `SETTINGS_MAX_HEADER_LIST_SIZE` parameter. This setting is defined in the HTTP/2 specification as an advisory mechanism that informs a peer of the maximum size of the header field section it is willing to accept, measured in octets. However, when a malicious client sends this setting to a vulnerable Netty server, the server processes it in a way that leads to an exception while writing response headers. More specifically, the server reads the request, proxies it to the origin, attempts to produce a response, and then triggers an exception during the header writing phase.
This behavior is functionally similar to the HTTP/2 Rapid Reset attack, albeit with a different on‑wire signature. Instead of relying on rapid stream resets, this flaw abuses the `SETTINGS_MAX_HEADER_LIST_SIZE` advisory to cause an internal error that can be repeatedly triggered, leading to resource exhaustion and a Denial‑of‑Service (DoS) condition. The attack does not require authentication and can be performed remotely over the network, making it particularly dangerous for publicly accessible Netty-based HTTP/2 servers.
The root cause is that Netty treats the client‑supplied `SETTINGS_MAX_HEADER_LIST_SIZE` as an enforceable limit rather than an advisory. According to the RFC 9113, a server is not obligated to honor this setting, and indeed many other HTTP/2 implementations ignore it when acting as a server. Netty’s adherence to the client’s value creates an asymmetric situation where the client can dictate a tiny header list size, causing the server to fail when it attempts to write a larger response header. This failure manifests as an exception, and repeated exploitation can overwhelm the server, degrading or denying service to legitimate clients.
The vulnerability affects Netty versions 4.1.134.Final and earlier, as well as versions from 4.2.0.Final up to 4.2.14.Final. The issue is addressed in versions 4.1.135.Final and 4.2.15.Final, where the `SETTINGS_MAX_HEADER_LIST_SIZE` setting is correctly ignored as advisory. Users are strongly advised to upgrade to these patched versions immediately to mitigate the risk of DoS attacks.
DailyCVE Form:
Platform: Netty
Version: <=4.1.134.Final,4.2.0-14.Final
Vulnerability : HTTP/2 Rapid Reset
Severity: Moderate (CVSS 5.3)
date: 2026-06-12
Prediction: 2026-06-12
What Undercode Say:
Check installed Netty version via Maven (example) mvn dependency:tree | grep netty-codec-http2 Test for vulnerability using a custom Python script python3 exploit_cve-2026-50560.py --target https://example.com --port 443
exploit_cve-2026-50560.py – Conceptual exploit
import h2.connection
import h2.config
import socket
target = ("example.com", 443)
config = h2.config.H2Configuration(client_side=True)
conn = h2.connection.H2Connection(config=config)
conn.initiate_connection()
Send malicious SETTINGS_MAX_HEADER_LIST_SIZE (e.g., 0 bytes)
conn.update_settings({h2.settings.SettingCodes.MAX_HEADER_LIST_SIZE: 0})
sock = socket.create_connection(target)
sock.sendall(conn.data_to_send())
... further requests will cause Netty to throw an exception
Exploit:
- Establish an HTTP/2 connection to a vulnerable Netty server.
- Send a `SETTINGS` frame with `SETTINGS_MAX_HEADER_LIST_SIZE` set to an extremely small value (e.g., 0).
- Send a regular HTTP/2 request that would result in a response header larger than the advertised limit.
- Netty will attempt to write the response, exceed the client‑imposed limit, and throw an exception, effectively aborting the request.
- Repeating this process rapidly consumes server resources, leading to DoS.
Protection:
- Upgrade to Netty versions 4.1.135.Final or 4.2.15.Final immediately.
- If upgrading is not possible, apply the patch that ignores `SETTINGS_MAX_HEADER_LIST_SIZE` when acting as a server (Netty’s security advisory GHSA-563q-j3cm-6jxm).
- Use a reverse proxy or WAF to filter or normalize HTTP/2 settings frames before they reach the backend Netty server.
- Monitor for anomalous spikes in `RST_STREAM` frames or exception logs related to header size violations.
Impact:
- Denial of Service (DoS): An unauthenticated remote attacker can crash or render a Netty‑based HTTP/2 server unresponsive by repeatedly exploiting the header size exception.
- Resource Exhaustion: Each attack forces the server to allocate resources for processing the request and then handle an exception, leading to CPU and memory exhaustion.
- High Feasibility: The attack is easy to execute, requires no special privileges, and can be distributed (DDoS) to amplify the effect.
- Wide Exposure: Any application using Netty’s HTTP/2 codec and exposing it to untrusted clients is potentially affected.
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Sources:
Reported By: github.com
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