CVE-2026-66731: Boazsegev Facil.io

High severity, CVSS 7.5. EPSS: 0.8% chance of exploitation in the next 30 days.

facil.io 0.7.5 through 0.7.6 contains a denial-of-service vulnerability in the HTTP/1.1 chunked transfer encoding parser that allows unauthenticated remote attackers to crash the server by sending a negative chunk size value. Attackers can send a single POST request with a Transfer-Encoding: chunked header containing a leading minus sign in the chunk size field, causing the parser in http1_parser.h to compute a large positive integer from the negated value, corrupting internal state and moving the read pointer into unmapped memory resulting in a fault.

Affected products

• Boazsegev Facil.io: from 0.7.5, up to and including 0.7.6

Published 2026-07-27. Last modified 2026-07-31.