CVE-2026-59901 Netty Bzip2Decoder: Infinite Loop in RLE State Machine Leads to Event-Loop Thread Hang
Attackers can cause a denial of service by sending malformed bzip2 streams that hang the event-loop thread in Netty's Bzip2Decoder.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59901?
CVE-2026-59901 is a high-severity denial of service 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, the `Bzip2Decoder` handler in Netty's compression codec pipeline is vulnerable to a denial-of-service attack through a malformed bzip2 stream that permanently captures the event-loop thread in an infinite loop. The vulnerability exists in the run-length encoding (RLE) state machine within [`Bzip2BlockDecompressor.read()`]. This issue has been fixed in versions 4.1.136.Final and 4.2.16.Final.
Weakness: CWE-835
Which versions are affected by CVE-2026-59901?
4 open source packages ship an affected version, including io.netty:netty-codec and io.netty:netty-codec-compression. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
io.netty:netty-codec Maven | All versions | 4.1.136.Final |
io.netty:netty-codec Maven | All versions | 4.1.136.Final |
io.netty:netty-codec-compression Maven | From 4.2.0.Final | 4.2.16.Final |
io.netty:netty-codec-compression Maven | From 4.2.0.Final | 4.2.16.Final |
Products include Netty Netty.
How do I fix CVE-2026-59901?
Upgrade io.netty:netty-codec to 4.1.136.Final or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-59901 being exploited?
There is no known exploitation of CVE-2026-59901 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-2026-55851 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
- CVE-2026-45674 Netty Vulnerable to DNS Cache Poisoning via Missing Bailiwick Checks in CNAME Records
Sources
Also known as CLEANSTART-2026-DK41558, CLEANSTART-2026-FG43911, CLEANSTART-2026-NQ53265, CLEANSTART-2026-OC34252, CLEANSTART-2026-TO73483, CLEANSTART-2026-XJ77136 and 120 more.