GHSA-4grm-h2qv-h6w6 is a high-severity (CVSS 7.5) CWE-770 vulnerability in io.netty:netty-codec-http3. A fix is available for io.netty:netty-codec-http3 — see the affected versions and patch details below.
Netty HTTP/3 QPACK Blocked Streams Memory Exhaustion
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 GHSA-4grm-h2qv-h6w6.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
GHSA-4grm-h2qv-h6w6 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 379,842 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-http3Real-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
A memory exhaustion vulnerability in the Netty HTTP/3 codec allows the creation of an infinite number of blocked streams, which can cause OOM error.
Details
The vulnerability exists in io.netty.handler.codec.http3.QpackDecoder#shouldWaitForDynamicTableUpdates:
If a client sends a header referencing a table entry that the server hasn't received yet, the server must pause that stream and wait for the missing entry to arrive. To prevent attackers from exhausting resources by intentionally sending missing references, Netty limits the number of streams that can be blocked at the same time.
However, the check is implemented as:
if (blockedStreamsCount == maxBlockedStreams - 1) {
If the server enables QPACK dynamic tables (by setting HTTP3_SETTINGS_QPACK_MAX_TABLE_CAPACITY > 0) but does not explicitly configure HTTP3_SETTINGS_QPACK_BLOCKED_STREAMS, it defaults to 0.
When maxBlockedStreams is 0, the condition evaluates to blockedStreamsCount == -1. Since blockedStreamsCount starts at 0 and only increments, it never equals -1. This bypasses the limit, allowing an attacker to open an infinite number of streams that block indefinitely. Additionally, the QpackDecoder never removes unblocked streams from the blockedStreams map or decrements the counter, meaning the ReadResumptionListener for each blocked stream is kept in memory for the entire lifetime of the connection. This exhausts server memory and crashes the JVM.
Impact
Denial of Service. Any server using netty-codec-http3 with QPACK dynamic tables enabled and maxBlockedStreams defaulting to 0 is impacted.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.netty:netty-codec-http3 | ≥ 4.2.0.Final&&< 4.2.15.Final | 4.2.15.Finalio.netty:netty-codec-http3:4.2.15.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-http3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.netty:netty-codec-http3 to 4.2.15.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-4grm-h2qv-h6w6 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.
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 GHSA-4grm-h2qv-h6w6 (CC BY 4.0)
Frequently Asked Questions
Is GHSA-4grm-h2qv-h6w6 in your dependencies?
Find it across Maven, including transitive dependencies.