PT-2026-65067 · Boazsegev · Facil.Io

Theodosis Paidakis · CVE-2026-66731 · Published 2026-07-27 · Updated 2026-07-27

Mitre

CVSS v3.1

7.5

High

Base vector

AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

Exploit

Fix

Out of bounds Read

Weakness Enumeration

CWE-125

Related Identifiers

CVE-2026-66731

Affected Products

Facil.Io

References · 3

• 🔥 https://github.com/theopaid/Negative-Chunk-Size-Parsing-Causes-Memory-Corruption-in-facil.io-leading-to-Server-Crash · Exploit
• https://nvd.nist.gov/vuln/detail/CVE-2026-66731 · Security Note
• https://vulncheck.com/advisories/facil-io-http-chunked-transfer-encoding-parser-crash-dos · Note