CVE-2026-55851
Netty HAProxy decoder bug allows remote memory exhaustion (DoS) via crafted PROXY v2.
Is CVE-2026-55851 being exploited?
Not confirmed. CVE-2026-55851 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 0.61% probability of exploitation in the next 30 days.
How severe is CVE-2026-55851?
CVE-2026-55851 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-2026-55851?
Yes. A fix has been recorded for CVE-2026-55851. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-55851 affect?
CVE-2026-55851 affects Netty codec-haproxy module, Netty 4.2.0.Final up to <4.2.16.Final, Netty 4.1.0.Final up to <4.1.135. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-55851?
Upgrade to Netty 4.2.16.Final or 4.1.136.Final; block malformed PROXY v2 traffic if possible.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote attacker can send crafted PROXY v2 packets causing unbounded cumulation buffer growth and OOM, leading to service crash/DoS.
Patch when possible
- affected>= 4.2.0.Final and < 4.2.16.Final
- affected>= 4.1.0.Final and < 4.1.136.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.