Summary
Netty: Security Control Bypass via CORS Short-Circuit Failure
Netty's CorsHandler provides a shortCircuit() configuration designed to reject unauthorized cross-origin requests immediately, acting as a security control before requests reach the application. However, due to a logical operator error in the origin evaluation process, this protection can be entirely bypassed. An attacker can bypass the short-circuit mechanism by sending a request with an Origin: null header. This failure forwards unauthorized requests to the backend application, bypassing intended access controls.
Details
In io.netty.handler.codec.http.cors.CorsHandler#channelRead, the short-circuit logic relies on the configuration returned by getForOrigin(origin) to determine if an origin is authorized. If getForOrigin returns a configuration object, the short-circuit check (!(origin == null || config != null)) is bypassed, and the request proceeds to the backend.
The vulnerability is located in the getForOrigin method:
if (corsConfig.isNullOriginAllowed() || NULL_ORIGIN.equals(requestOrigin)) {
return corsConfig;
}
If an attacker sends Origin: null, NULL_ORIGIN.equals(requestOrigin) evaluates to true. The method returns the configuration object regardless of whether isNullOriginAllowed() was configured by the developer. The short-circuit is bypassed.
Impact
Applications relying on CorsHandler's short-circuit feature to prevent unauthorized cross-origin requests from reaching their backend logic are completely exposed. The framework fails to enforce the developer's intended access controls, allowing unauthorized requests to be processed.
CVE-2026-56746 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and user interaction required. 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 (4.2.16.Final, 4.1.136.Final); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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io.netty:netty-codec-http to 4.2.16.Final or later; io.netty:netty-codec-http to 4.1.136.Final or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-56746? CVE-2026-56746 is a medium-severity security vulnerability in io.netty:netty-codec-http (maven), affecting versions >= 4.2.0.Final, < 4.2.16.Final. It is fixed in 4.2.16.Final, 4.1.136.Final.
- How severe is CVE-2026-56746? CVE-2026-56746 has a CVSS score of 6.5 (Medium). 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.
- Which versions of io.netty:netty-codec-http are affected by CVE-2026-56746? io.netty:netty-codec-http (maven) versions >= 4.2.0.Final, < 4.2.16.Final is affected.
- Is there a fix for CVE-2026-56746? Yes. CVE-2026-56746 is fixed in 4.2.16.Final, 4.1.136.Final. Upgrade to this version or later.
- Is CVE-2026-56746 exploitable, and should I be worried? Whether CVE-2026-56746 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
- What actually determines whether CVE-2026-56746 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.
- How do I fix CVE-2026-56746?
- Upgrade
io.netty:netty-codec-httpto 4.2.16.Final or later - Upgrade
io.netty:netty-codec-httpto 4.1.136.Final or later
- Upgrade