CVE-2026-55833 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in io.netty:netty-codec-http. A fix is available for io.netty:netty-codec-http — see the affected versions and patch details below.
Netty SPDY zlib header block continues decoded expansion after maxHeaderSize truncation
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55833.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-55833 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 391,411 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
io.netty:netty-codec-http☕io.netty:netty-codec-httpReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
Summary
Netty SPDY header decoding continues inflating zlib-compressed header blocks after the raw header parser has already exceeded maxHeaderSize and marked the frame truncated. At commit b2d2137c4404af425bf9d5d601a62576f5c06925, a 12,253-byte compressed SPDY header block can declare and inflate a 12 MiB header-name field with maxHeaderSize=16, forcing compression-amplified decode and skip work in a reachable SpdyFrameCodec pipeline.
PoC
run with:
bash ./poc/run.sh
expected output:
NETTY_SPDY_ZLIB_DECODED_AFTER_LIMIT_TRIGGERED compressed_bytes=12253 declared_name_length=12582912 max_header_size=16 truncated=true invalid=false
The fingerprint means the compressed input was fully consumed while the raw header parser ended with truncated=true and invalid=false after processing the oversized decoded name. That specific state distinguishes this bug from a generic setup failure: the maxHeaderSize guard fired, but the zlib/raw decode path still inflated and skipped the full 12 MiB declared name.
Impact
A remote unauthenticated peer that can speak SPDY to a Netty pipeline containing SpdyFrameCodec can send a small compressed HEADERS block that expands into much larger raw header data after the configured maxHeaderSize limit has already been exceeded. The attack requires a reachable SPDY codec, ordinary transport setup such as TCP and optional TLS, and no independent compressed-frame-size or connection-rate limit ahead of SpdyFrameCodec. The satisfied protocol guards are straightforward: the HEADERS frame uses a nonzero stream id and length >= 4, the decoder factory selects the zlib decoder, the payload uses the SPDY dictionary, and the raw block appends a zero-length value so the already-truncated frame reaches END_HEADER_BLOCK. The user-visible effect is denial of service through compression-amplified CPU and allocation churn.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.netty:netty-codec-http | ≥ 4.2.0.Final&&< 4.2.16.Final | 4.2.16.Finalio.netty:netty-codec-http:4.2.16.Final |
| ☕Maven | io.netty:netty-codec-http | ≥ 4.1.0.Final&&< 4.1.136.Final | 4.1.136.Finalio.netty:netty-codec-http:4.1.136.Final |
Affected Products
nettynettyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-codec-http, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.netty:netty-codec-http to 4.2.16.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55833 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This is an Important denial of service vulnerability in Netty's SPDY header decoding. A remote, unauthenticated attacker can send a small, compressed SPDY header block that, during decompression, expands significantly beyond configured limits. This leads to excessive CPU and memory consumption, potentially causing…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-55833 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Cryostat 4 on RHEL 9 | cryostat/cryostat-reports-rhel9:4.2.0-14 | RHSA-2026:68333 |
| Red Hat AMQ Broker 7.13.6 | netty-codec-http | RHSA-2026:66545 |
| Red Hat AMQ Broker 7.14.1 | netty-codec-http | RHSA-2026:66488 |
| Red Hat build of Apache Camel 4.18.3 for Spring Boot 3.5.16 | netty-codec-http | RHSA-2026:54622 |
| Red Hat build of Apicurio Registry 3.3.3 | netty-codec-http | RHSA-2026:76637 |
| Red Hat build of Quarkus 3.27.4.SP3 | netty-codec-http | RHSA-2026:47189 |
| Red Hat build of Quarkus 3.33.2.SP3 | netty-codec-http | RHSA-2026:47172 |
| Red Hat Data Grid 8.6.3 | netty-codec-http | RHSA-2026:69296 |
Frequently Asked Questions
Is CVE-2026-55833 in your dependencies?
Find it across Maven, including transitive dependencies.