CVE-2026-59903 Netty: Cache Poisoning and Information Disclosure via CORS Vary Header Overwrite
Netty's CORS handler overwrites Vary headers, enabling cache poisoning and disclosure of authenticated responses to other users.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59903?
CVE-2026-59903 is a medium-severity vulnerability in Netty Netty, published on 17 Aug 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 4.1.137.Final and 4.2.17.Final, io.netty.handler.codec.http.cors.CorsHandler setVaryHeader replaces application Vary headers such as Authorization or Cookie with Origin, allowing a caching proxy or CDN to reuse authenticated responses across users and disclose sensitive information. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.
Weakness: CWE-524
Which versions are affected by CVE-2026-59903?
4 open source packages ship an affected version, including io.netty:netty-codec-http. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
io.netty:netty-codec-http Maven | All versions | 4.1.137.Final |
io.netty:netty-codec-http Maven | All versions | 4.1.137.Final |
io.netty:netty-codec-http Maven | From 4.2.0.Final | 4.2.17.Final |
io.netty:netty-codec-http Maven | From 4.2.0.Final | 4.2.17.Final |
Products include Netty Netty.
How do I fix CVE-2026-59903?
Upgrade io.netty:netty-codec-http to a fixed version for your release line: 4.1.137.Final or 4.2.17.Final.
Is CVE-2026-59903 being exploited?
There is no known exploitation of CVE-2026-59903 in the wild and no public exploit code.
Public exploit code: 1 proofs of concept on GitHub.
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-FH63085, CLEANSTART-2026-HJ08355, CLEANSTART-2026-HK72539, CLEANSTART-2026-PK10148, CLEANSTART-2026-UV29003, CLEANSTART-2026-VW00414 and 108 more.