Affects widely used HAProxy load balancer; DoS could disrupt services across many orgs.
CVE-2026-55204
HAProxy HPACK defrag NPE under memory pressure; worker crash.
Is CVE-2026-55204 being exploited?
Not confirmed. CVE-2026-55204 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 0.48% probability of exploitation in the next 30 days.
How severe is CVE-2026-55204?
CVE-2026-55204 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-2026-55204?
Yes. A fix has been recorded for CVE-2026-55204. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-55204 affect?
CVE-2026-55204 affects HAProxy <= 3.4.0. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-55204?
Upgrade to patched HAProxy (post-commit 9a6d1fe) or newer
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
DoS via NULL pointer dereference during HPACK defrag under memory pressure; HAProxy workers crash.
Patch when possible
- affected<= 3.4.0
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.