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CVE-2026-59900 affects Netty's HTTP/2-to-HTTP/1.x translation layer before versions 4.1.136.Final and 4.2.16.Final in their respective release branches. Http2StreamFrameToHttpObjectCodec and InboundHttp2ToHttpAdapter fail to deduplicate or validate authority information when an HTTP/2 HEADERS frame contains both the :authority pseudo-header and a literal host header. The translator converts :authority to Host while separately copying the literal host header, producing an HttpRequest with two Host headers containing attacker-controlled, differing values. This ambiguity can enable request-routing or host-based security-control bypass.
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A header-validation flaw in Netty's HTTP/2-to-HTTP/1.x translation components, Http2StreamFrameToHttpObjectCodec and InboundHttp2ToHttpAdapter, produces an HttpRequest containing two Host headers with attacker-controlled, differing values. The issue affects versions before the respective fixes in 4.1.136.Final and 4.2.16.Final. The advisory rates it medium severity, with CVSS v3 5.3 and CVSS v4 6.9.
A vulnerability identified as CVE-2026-59900, tagged in the content as involving request smuggling and Netty. No technical impact, affected versions, or remediation details are provided.
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