GHSA-4grm-h2qv-h6w6 is a high-severity (CVSS 7.5) CWE-770 vulnerability in io.netty:netty-codec-http3. O3 Security confirms whether GHSA-4grm-h2qv-h6w6 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-4grm-h2qv-h6w6 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 357,322 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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.Final |
Detection & 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-4grm-h2qv-h6w6 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-4grm-h2qv-h6w6. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-4grm-h2qv-h6w6 in your dependencies?
O3 detects GHSA-4grm-h2qv-h6w6 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.