CVE-2026-59900 Netty codec-http2: Lack of Host Header Deduplication in HTTP/2→HTTP/1.x Translation Leads to Request Routing Bypass
Attackers can bypass request routing controls by sending duplicate Host headers during HTTP/2 to HTTP/1.x translation in Netty.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59900?
CVE-2026-59900 is a medium-severity vulnerability in Netty Netty, published on 29 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-444
Which versions are affected by CVE-2026-59900?
4 open source packages ship an affected version, including io.netty:netty-codec-http2. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
io.netty:netty-codec-http2 Maven | All versions | 4.1.136.Final |
io.netty:netty-codec-http2 Maven | All versions | 4.1.136.Final |
io.netty:netty-codec-http2 Maven | From 4.2.0.Final | 4.2.16.Final |
io.netty:netty-codec-http2 Maven | From 4.2.0.Final | 4.2.16.Final |
Products include Netty Netty.
How do I fix CVE-2026-59900?
Upgrade io.netty:netty-codec-http2 to a fixed version for your release line: 4.1.136.Final or 4.2.16.Final.
Is CVE-2026-59900 being exploited?
There is no known exploitation of CVE-2026-59900 in the wild and no public exploit code.
Other vulnerabilities in Netty Netty
- CVE-2026-75595 Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext
- CVE-2026-75596 Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
- CVE-2026-59901 Netty Bzip2Decoder: Infinite Loop in RLE State Machine Leads to Event-Loop Thread Hang
- CVE-2026-55851 Netty codec-haproxy: Signed-Byte Sentinel Collision in HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion
- CVE-2026-56745 Netty SpdyHttpDecoder: ByteBuf Reference Leak on RST_STREAM Leads to Native Memory Exhaustion
Sources
Also known as CLEANSTART-2026-JF33356, CLEANSTART-2026-RR39735, CLEANSTART-2026-SR05479, CLEANSTART-2026-VS28854, CLEANSTART-2026-ZF06834, DEBIAN-CVE-2026-59900 and 89 more.