Networks
CVE-2026-100663
Netty HTTP/3 codec mistranslates HTTP/1 CONNECT authority, allowing tunnel authority tampering.
Major vulnerability. Netty is a widely used Java networking library; this flaw lets clients tamper CONNECT :authority and bypass proxy egress/allow-lists across many services.
- CVSS
- 8.7Highscored by the publisher (CNA)
- Exploited in the wild
- Not knownpublic proof of concept per CISA SSVC; not in CISA KEV
- EPSS
- 0.30%chance of exploitation in the next 30 daystop 77% of scored CVEsFIRST score of 12 Nov 2026
- Fix
- Fixed in 4.2.18.Finalper the vendor
- Published
- 26 Sept 2026analysed here 29 Sept 2026
Do first. Upgrade to a fixed release: 4.2.18.Final.
Summary
Impact
Remote attacker can control HTTP/3 CONNECT :authority via mismatched Host, bypassing tunnel allow-lists, egress policy, backend selection, and audit controls.
Triage
The summary, impact and action text on this page are written by our AI analysis pipeline, or taken from the NVD description when a CVE was imported from NVD. They are not the vendor's wording: check them against the sources. A longer AI executive summary and technical analysis are generated for signed-in accounts on the Max plan.
Affected products
| Product | Affected versions | Vendor says fixed in |
|---|---|---|
| netty | >= 4.2.2.Final and < 4.2.18.FinalVendor says fixed in: 4.2.18.Final |
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.
Products named in our analysis
- io.netty:netty-codec-http3 4.2.2.Final - 4.2.17.Final
- Netty-based HTTP/1-to-HTTP/3 proxies and gateways
- Applications using Netty HTTP/3 codec for CONNECT tunneling
Fix and mitigation
Action required
Patch when possible
WAF and other mitigations
Can a WAF block it? Partly. The bug can be reached over the network, so generic WAF rules may stop some attacks. Its weakness type is unknown, so we cannot be more precise.
Also consider: patch
Based on the weakness type only; no CVE-specific evidence yet. We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.
Remediation steps (AI-written)
Step-by-step remediation is generated for accounts on the Max plan, for CVEs that match a technology in their stack. Add this technology to your stack to see the fix checklist.
Exploitation evidence
- CISA KEV catalog
- Not listed as of our last sync.
- Public exploit code
- None on record in Exploit-DB, Metasploit, Nuclei templates or PoC-in-GitHub.
- CISA SSVC
- exploitation poc, automatable no, technical impact partialAssessed by CISA 28 Sept 2026 (SSVC 2.0.3). Exploitation is none, poc (public proof of concept) or active.
Timeline
- 26 Sept 2026Published
- 28 Sept 2026CISA SSVC assessment: exploitation poc2 days later
- 28 Sept 2026CVE record last updatedsame day later
- 29 Sept 2026Analysed by vulneraibility.com1 day later
Common questions
Is CVE-2026-100663 being exploited?
Not confirmed. CVE-2026-100663 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.30% probability of exploitation in the next 30 days.
How severe is CVE-2026-100663?
CVE-2026-100663 is rated High with a CVSS score of 8.7. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-100663?
Yes. A fix has been recorded for CVE-2026-100663. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-100663 affect?
CVE-2026-100663 affects io.netty:netty-codec-http3 4.2.2.Final - 4.2.17.Final, Netty-based HTTP/1-to-HTTP/3 proxies and gateways, Applications using Netty HTTP/3 codec for CONNECT tunneling. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-100663?
Upgrade to io.netty:netty-codec-http3 4.2.18.Final; audit proxy CONNECT handling and allow-lists.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
References
2 references in the CVE record, grouped by the tags the record gives them.
Advisories (2)
- github.com /netty/netty/security/advisories/GHSA-jgph-cgq3-c627
- vulncheck.com /advisories/netty-http-1-connect-authority-form-mistranslated-to-malformed-http-
Also tracked as GHSA-jgph-cgq3-c627 (OSV).
Where the data on this page comes from
- CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
- NVD: CVSS and CPE data
- CISA Vulnrichment (ADP): SSVC decision points
- FIRST EPSS: exploit probability and percentile
- OSV: advisory aliases and downstream package versions
Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.
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