Integer Underflow Vulnerability in facil.io Product
CVE-2026-66729

8.7HIGH

Key Information:

Vendor

Boazsegev

Status
Facil.io
Vendor
CVE Published:
27 July 2026

Badges

👾 Exploit Exists🟡 Public PoC

What is CVE-2026-66729?

The facil.io product through version 0.7.6 is susceptible to an integer underflow vulnerability within its multipart MIME body parser. This flaw allows unauthenticated remote attackers to exploit the server by sending a specially crafted Content-Disposition header with an empty field name. Such an attack can trigger a wraparound in the uint32_t data type, leading to an out-of-bounds memory read past the name pointer. This results in a bus fault and subsequent crash of the handling worker, potentially causing significant service disruptions with a single malicious POST request.

Affected Version(s)

facil.io 0.6.0 <= 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-2026-66729 - Proof of Concept

References

https://github.com/theopaid/Out-of-Bounds-Read-in-facil.i...
technical-descriptionexploit
https://www.vulncheck.com/advisories/facil-io-integer-und...
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
.