Summary
Netty Vulnerable to Cache Poisoning and Information Disclosure via CORS Vary Header Overwrite
Netty's CorsHandler silently overwrites existing Vary headers, enabling cache poisoning and sensitive information disclosure.
Details
io.netty.handler.codec.http.cors.CorsHandler#setVaryHeader overwrites any existing Vary headers set by backend applications.
private static void setVaryHeader(final HttpResponse response) {
response.headers().set(HttpHeaderNames.VARY, HttpHeaderNames.ORIGIN);
}
Because set() replaces all existing values for the header, if a backend application sets a Vary header (such as Vary: Authorization or Vary: Cookie) to ensure that intermediate caches (like CDNs) cache responses separately per user, the CorsHandler will overwrite it with Vary: origin. This causes the caching proxy to ignore the authorization context and cache the response based solely on the URL and Origin, allowing an attacker to retrieve another user's cached sensitive data.
Impact
This is a Cache Poisoning vulnerability that leads to Information Disclosure. It impacts any Netty-based web application that uses the CorsHandler, sets its own Vary headers to manage caching of authenticated or user-specific responses (e.g., Vary: Authorization), and is deployed behind a caching proxy or CDN. The end-users of these applications are impacted, as their sensitive data may be leaked to unauthorized actors.
CVE-2026-59903 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. 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 (4.2.17.Final, 4.1.137.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 4.2.17.Final or later; io.netty:netty-codec-http to 4.1.137.Final or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-59903? CVE-2026-59903 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 4.2.17.Final, 4.1.137.Final.
- How severe is CVE-2026-59903? CVE-2026-59903 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-2026-59903? io.netty:netty-codec-http (maven) versions >= 4.2.0.Final, <= 4.2.16.Final is affected.
- Is there a fix for CVE-2026-59903? Yes. CVE-2026-59903 is fixed in 4.2.17.Final, 4.1.137.Final. Upgrade to this version or later.
- Is CVE-2026-59903 exploitable, and should I be worried? Whether CVE-2026-59903 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-2026-59903 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-2026-59903?
- Upgrade
io.netty:netty-codec-httpto 4.2.17.Final or later - Upgrade
io.netty:netty-codec-httpto 4.1.137.Final or later
- Upgrade