Denial-of-Service Vulnerability in facil.io HTTP/1.1 Parser
CVE-2026-66731

8.7HIGH

Key Information:

Vendor

Boazsegev

Status
Facil.io
Vendor
CVE Published:
27 July 2026

Badges

👾 Exploit Exists🟡 Public PoC

What is CVE-2026-66731?

facil.io versions 0.7.5 and 0.7.6 contain a vulnerability in the HTTP/1.1 chunked transfer encoding parser. An unauthenticated remote attacker can exploit this flaw by sending a specially crafted POST request with a negative chunk size value in the 'Transfer-Encoding: chunked' header. This manipulation leads to the parser calculating an erroneous large positive integer, which corrupts the server's internal state and redirects the read pointer into unmapped memory. Consequently, this results in a server crash, thus impacting service availability.

Affected Version(s)

facil.io 0.7.5 <= 0.7.6

Exploit Proof of Concept (PoC)

PoC code is written by security researchers to demonstrate the vulnerability can be exploited. PoC code is also a key component for weaponization which could lead to ransomware.

CVE-2026-66731 - Proof of Concept

References

https://github.com/theopaid/Negative-Chunk-Size-Parsing-C...
technical-descriptionexploit
https://www.vulncheck.com/advisories/facil-io-http-chunke...
third-party-advisory

CVSS V4

Score:
8.7
Severity:
HIGH
Confidentiality:
None
Integrity:
None
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Attack Required:
None
Privileges Required:
Undefined
User Interaction:
None

Timeline

  • 🟡

    Public PoC available

  • 👾

    Exploit known to exist

  • Vulnerability published

  • Vulnerability Reserved

Credit

Theodosis Paidakis
.