Vulnerability in Netty
CVE-2026-59900
Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, Netty's HTTP/2-to-HTTP/1.x translation layer (`Http2StreamFrameToHttpObjectCodec` and `InboundHttp2ToHttpAdapter`) fails to deduplicate or validate `Host` headers when an HTTP/2 client supplies both the `:authority` pseudo-header and a literal `host` header in a single HEADERS frame. The translator maps `:authority` to `Host` and separately copies the literal `host` header, producing an `HttpRequest` object containing two `Host` headers with attacker-controlled differing values. This issue has been fixed in versions 4.1.136.Final and 4.2.16.Final.
Vulnerability class: HTTP Request Smuggling
EPSS: 0.002 (14.1th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N.
Affected products
- Netty — versions >= 4.2.0.Final, < 4.2.16.Final, < 4.1.136.Final
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_MISC, Release Notes)
- security-advisories@github.com (x_refsource_MISC, Release Notes)
- security-advisories@github.com (x_refsource_CONFIRM, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-59900?
- CVE-2026-59900 is a medium-severity vulnerability in Netty, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 5.3/10. Published 2026-07-29.
- How severe is CVE-2026-59900?
- Medium severity. CVSS v3 base score is 5.3 out of 10.