CVE-2026-50020: Netty's HttpObjectDecoder skips arbitrary initial control characters when only initial CRLF characters are permitted

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-12 · updated 2026-06-17
Weakness: CWE-444

Description

Netty is a network application framework for development of protocol servers and clients. Prior to versions 4.1.135.Final and 4.2.15.Final, before reading the first request-line, `HttpObjectDecoder` skips every byte for which `Character.isISOControl(b)` is `true` (0x00–0x1F and 0x7F) as well as all whitespace. RFC 9112 §2.2 only asks servers to ignore empty CRLF lines preceding the request-line — a carefully scoped robustness allowance intended to handle HTTP/1.0 POST workarounds. Silently absorbing NUL bytes, SOH, STX, and other non-CRLF control characters goes significantly beyond this, and can be exploited for request-boundary confusion in pipelined or multiplexed transports where a front-end component treats those bytes differently. Versions 4.1.135.Final and 4.2.15.Final patch the issue.

Affected products

netty: before 4.1.135; from 4.2.0 before 4.2.15 — fixed in 4.1.135, 4.2.15

How to fix

Upgrade to a fixed version: netty 4.1.135, 4.2.15. Check the vendor advisory for mitigations.

References

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