CVE-2026-66731
HIGH 8.7EPSS 0.6%
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.
- CVSS v4.0
- 8.7 HIGH
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - CVSS v3.1
- 7.5 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - EPSS
- 0.60% chance of exploitation in the next 30 days, 46th percentile
- Published
- 2026-07-27
- Updated
- 2026-07-31
Proof-of-concept exploits (2)
- https://tpaidakis.com/writeups/facilio-parser-dos/
- theopaid/Negative-Chunk-Size-Parsing-Causes-Memory-Corruption-in-facil.io-leading-to-Serv…0★ · 2026-07-28