CVE-2026-42581 is a medium-severity security vulnerability in io.netty:netty-codec-http (maven), affecting versions >= 4.2.0.Alpha1, <= 4.2.12.Final. It is fixed in 4.2.13.Final, 4.1.133.Final.
NETTY HTTP/1.0 TE+CL Coexistence Bypasses Smuggling Sanitization | Field | Value | |-----------|-------| | Library | io.netty:netty-codec-http | | Component | codec-http, HttpObjectDecoder | | Severity | HIGH | | Affects | HEAD, commit 4f3533ae confirmed | Summary HttpObjectDecoder strips a conflicting Content-Length header when a request carries both Transfer-Encoding: chunked and Content-Length, but only for HTTP/1.1 messages. The guard is absent for HTTP/1.0. An attacker that sends an HTTP/1.0 request with both headers causes Netty to decode the body as chunked while leaving Content-Length intact in the forwarded HttpMessage. Any downstream proxy or handler that trusts Content-Length over Transfer-Encoding will disagree on message boundaries, enabling request smuggling. Root Cause The conflict-resolution path is gated on message.protocolVersion() == HttpVersion.HTTP11. When the request declares HTTP/1.0, the condition is false, handleTransferEncodingChunkedWithContentLength is never called, and the Content-Length header survives into the forwarded message. Netty still processes the body as chunked; a downstream component that is CL-first interprets the same bytes as a separate request. Proof of Concept Netty consumes the full chunked body (5 bytes + terminator). A downstream CL-first proxy reads Content-Length: 0, considers the request complete at the blank line, and treats 5\r\nGPOST\r\n0\r\n\r\n as the start of a second request. Conditions Required Netty is deployed behind a reverse proxy or load balancer that is Content-Length-first (nginx, some HAProxy configs, AWS ALB in certain modes). Attacker can send HTTP/1.0 requests (either directly or by downgrading via connection manipulation). No additional HTTP/1.0 stripping layer between attacker and Netty. Impact Request smuggling at the Netty edge. Allows cache poisoning, session fixation against other users, unauthorized access to internal endpoints, and bypassing of WAF or authentication layers that inspect only the first logical request. Confirmed PoC Test Verified against HEAD (4f3533ae) using EmbeddedChannel. Both tests pass, confirming the vulnerability and the HTTP/1.1 contrast. Fix Guidance Remove the message.protocolVersion() == HttpVersion.HTTP11 guard in HttpObjectDecoder, applying handleTransferEncodingChunkedWithContentLength unconditionally whenever both Transfer-Encoding: chunked and Content-Length are present, regardless of protocol version.
CVE-2026-42581 has a CVSS score of 5.8 (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 (4.2.13.Final, 4.1.133.Final). Upgrading removes the vulnerable code path.
maven
io.netty:netty-codec-http (>= 4.2.0.Alpha1, <= 4.2.12.Final)io.netty:netty-codec-http (<= 4.1.132.Final)io.netty:netty-codec-http → 4.2.13.Final (maven)io.netty:netty-codec-http → 4.1.133.Final (maven)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 instead of chasing every advisory.
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Upgrade the following packages to resolve this vulnerability:
io.netty:netty-codec-http to 4.2.13.Final or laterio.netty:netty-codec-http to 4.1.133.Final or laterKodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
CVE-2026-42581 is a medium-severity security vulnerability in io.netty:netty-codec-http (maven), affecting versions >= 4.2.0.Alpha1, <= 4.2.12.Final. It is fixed in 4.2.13.Final, 4.1.133.Final.
CVE-2026-42581 has a CVSS score of 5.8 (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.
io.netty:netty-codec-http (maven) versions >= 4.2.0.Alpha1, <= 4.2.12.Final is affected.
Yes. CVE-2026-42581 is fixed in 4.2.13.Final, 4.1.133.Final. Upgrade to this version or later.
Whether CVE-2026-42581 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
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.
io.netty:netty-codec-http to 4.2.13.Final or laterio.netty:netty-codec-http to 4.1.133.Final or later