CVE-2026-59903 is a medium-severity (CVSS 6.5) CWE-524 vulnerability in io.netty:netty-codec-http. A fix is available for io.netty:netty-codec-http — see the affected versions and patch details below.
Netty: Cache Poisoning and Information Disclosure via CORS Vary Header Overwrite
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-59903.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-59903 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 384,189 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
io.netty:netty-codec-http☕io.netty:netty-codec-httpReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
Summary
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.netty:netty-codec-http | ≥ 4.2.0.Final&&< 4.2.17.Final | 4.2.17.Finalio.netty:netty-codec-http:4.2.17.Final |
| ☕Maven | io.netty:netty-codec-http | all versions | 4.1.137.Finalio.netty:netty-codec-http:4.1.137.Final |
Affected Products
nettynettyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-codec-http, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.netty:netty-codec-http to 4.2.17.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-59903 is resolved across your whole dependency graph.
Workarounds
Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Moderate impact information disclosure flaw in Netty arises from the `CorsHandler` incorrectly overwriting application `Vary` headers with `Origin`. This could allow caching proxies or CDNs to reuse authenticated responses across different users, potentially exposing sensitive data. Exploitation requires the…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-59903 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-59903 in your dependencies?
Find it across Maven, including transitive dependencies.