Netty is a widely used Java networking library; this flaw lets clients tamper CONNECT :authority and bypass proxy egress/allow-lists across many services.
CVE-2026-100663
Netty HTTP/3 codec mistranslates HTTP/1 CONNECT authority, allowing tunnel authority tampering.
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.
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.
Remote attacker can control HTTP/3 CONNECT :authority via mismatched Host, bypassing tunnel allow-lists, egress policy, backend selection, and audit controls.
Patch when possible
- affected>= 4.2.2.Final and < 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.