Vulnerability · Published Jul 27, 2026 · Assigned by VulnCheck

CVE-2026-66731facil.io 0.7.5 - 0.7.6 HTTP/1.1 Chunked Transfer Encoding Parser Crash DoS

High 7.5

In plain English

A high-severity out-of-bounds read in facil.io from boazsegev. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.8% (higher than 56% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code.

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.8%top 43.8% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack tooling1Public PoC

Official description

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 (as reported)

Vendor	Product	Versions
boazsegev	facil.io	0.7.5 – through 0.7.6

Weakness: CWE-125 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

Researcher Blog technical-description, exploit
Researcher Disclosure technical-description, exploit
https://www.vulncheck.com/advisories/facil-io-http-chunked-transfer-encoding-parser-crash-dos third-party-advisory