CVE-2026-42581: Netty: HTTP/1.0 TE+CL Coexistence Bypasses Smuggling Sanitization

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-05-13 · updated 2026-09-18
Weakness: CWE-444

Description

Netty is an asynchronous, event-driven network application framework. Prior to 4.2.13.Final and 4.1.133.Final, 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. This vulnerability is fixed in 4.2.13.Final and 4.1.133.Final.

Affected products

netty: before 4.1.133; from 4.2.0 before 4.2.13 — fixed in 4.1.133, 4.2.13

How to fix

Upgrade to a fixed version: netty 4.1.133, 4.2.13. Check the vendor advisory for mitigations.

References

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