CVE-2026-66731
Unauthenticated HTTP chunked parser bug allows remote crash via negative chunk size
Is CVE-2026-66731 being exploited?
Not confirmed. CVE-2026-66731 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 0.60% probability of exploitation in the next 30 days.
How severe is CVE-2026-66731?
CVE-2026-66731 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-2026-66731?
Not known from our data. No patch reference has been recorded for CVE-2026-66731, which is not the same as no patch existing — a fix may have shipped without a tagged reference, or after this record was written. The vendor advisory is the only authority on whether a fix exists, and mitigations may be available regardless.
What does CVE-2026-66731 affect?
CVE-2026-66731 affects facil.io 0.7.5 - 0.7.6, facil.io http1_parser (http1_parser.h). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-66731?
Block/validate chunked Transfer-Encoding (reject '-' chunk sizes) and apply vendor patch when released.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Unauthenticated remote DoS: attacker can crash servers by sending a chunked POST with a negative chunk-size, causing memory corruption and a fault.
Immediate action required
- affected>= 0.7.5 and <= 0.7.6
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.